Khovanov-Rozansky homology for infinite multi-colored braids
Michael Willis

TL;DR
This paper introduces a limiting version of Khovanov-Rozansky homology for semi-infinite multi-colored braids, extending categorification results to infinite braid structures and broadening the understanding of their algebraic properties.
Contribution
It defines a new limiting homology for infinite multi-colored braids and demonstrates its role as a categorification of a highest-weight projector, extending prior work on infinite twist braids.
Findings
Limiting homology categorifies highest-weight projectors.
Extension of categorification to semi-infinite and bi-infinite braids.
Generalization of previous results on infinite twist braids.
Abstract
We define a limiting Khovanov-Rozansky homology for semi-infinite positive multi-colored braids, and we show that this limiting homology categorifies a highest-weight projector for a large class of such braids. This effectively completes the extension of Cautis' similar result for infinite twist braids, begun in our earlier papers with Islambouli and Abel. We also present several similar results for other families of semi-infinite and bi-infinite multi-colored braids.
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Khovanov-Rozansky homology for infinite multi-colored braids
Michael Willis
Department of Mathematics, UCLA
Abstract
We define a limiting Khovanov-Rozansky homology for semi-infinite positive multi-colored braids, and we show that this limiting homology categorifies a highest-weight projector for a large class of such braids. This effectively completes the extension of Cautis’ similar result for infinite twist braids, begun in our earlier papers with Islambouli and Abel. We also present several similar results for other families of semi-infinite and bi-infinite multi-colored braids.
1 Introduction
The Jones-Wenzl projector [Wen87] is a special idempotent element of the Temperley-Lieb algebra representing a highest-weight projector in the representation theory of , used in particular to define WRT-invariants for 3-manifolds (see, for example, [KL94]). Similarly we have analogous highest-weight projectors in the representation theory of for all . A sequence of papers by various authors [Ros14, Cau15, Hog15] showed that, in all of these cases, such highest-weight projectors could be categorified via infinite chain complexes associated to the stable limiting Khovanov-Rozansky complex of infinite full twists (including the case as corresponding to Khovanov-Rozansky HOMFLY-PT homology). Indeed similar statements hold when we consider infinite full twists where we allow ourselves to color the strands with any natural number less than , corresponding to irreducible anti-symmetric representations of .
A natural question for all of these cases one might ask would be what happens if we consider the limiting Khovanov-Rozansky complex of some other infinite braid. Together with Islambouli in [IW18] and Abel in [AW], the author has shown that, for all (semi-)infinite uni-colored braids that are both positive and complete (a braid is complete if each braid group generator appears infinitely many times; intuitively, this means that contains all the crossings necessary to build the infinite full twist ), the limiting Khovanov-Rozansky complex is chain homotopy equivalent to , the limiting complex of the infinite full twist. Thus we have the following imprecise theorem (see the original papers for the precise versions).
Theorem 1.1** ([IW18] Theorem 1.1, [AW] Theorem 1.1).**
All positive complete semi-infinite uni-colored braids categorify corresponding highest weight projectors.
The goal of this paper is to remove the “uni-colored” restriction from Theorem 1.1. Doing so will require a new restriction on the types of braids that are considered. The imprecise version is presented here (the precise version will be stated in Section 4.5).
Theorem 1.2**.**
All positive color-complete semi-infinite braids categorify corresponding highest weight projectors.
We will define color-completeness precisely in Section 4.4, but intuitively this will mean that the braid will contain all of the ‘properly colored’ crossings necessary to build the ‘properly colored’ infinite full twist . Figure 1 shows an example of a braid which is complete but not color-complete, and it can be seen why such a restriction might be expected. Since the strands colored and in that example never interact, it seems plausible that the resulting limiting complex may not be related to the complex of the infinite full twist in which the strands colored and must twist around each other infinitely often.
From Theorem 1.2 we will be able to deduce corollaries for a variety of situations. We will have results for semi-infinite braids with finitely many negative crossings (Corollary 5.4) allowing for a version of Theorem 1.2 taking Reidemeister II moves into account (Corollary 5.5). We also have results for certain non-color-complete braids (Corollary 5.6). All of these are similar to our previous results in [AW, IW18]. We will also discuss bi-infinite braids, for which defining a coloring and a corresponding limiting complex requires some more care.
Theorem 1.3**.**
All positive color-complete bi-infinite braids have limiting complexes of the form where and indicate categorified projectors for two (possibly different) sequences of colors depending on the coloring of , while is a complex categorifying certain maps between the representations determined by the two color sequences.
The precise version of Theorem 1.3 will be presented as Corollary 5.10.
The proof of Theorem 1.2 effectively generalizes the earlier proofs for different versions of Theorem 1.1 in [IW18, AW]. We present a short outline here. Let be a semi-infinite braid word representing the infinite braid , and let denote the finite ‘partial’ braid word consisting of the first crossings in . We view as a limit of as the length of the words . Unlike in previous versions, this limit will be taken along a subsequence of lengths such that each represents a so-called ‘color-pure’ braid (where the sequences of colors at the top and bottom of the braid must match). The color-completeness assumption will ensure we have maps roughly of the form where is some non-decreasing function of such that as along the necessary subsequence. Our goal then is to show that is an isomorphism in homological degrees below some bound that grows arbitrarily large as . This is accomplished by keeping track of homological shifts in the cone of (actually in a larger complex containing ) resulting from pulling ‘rungs’ through crossings and full twists. Despite the structural similarity to the earlier versions of the proof, there are significant differences in the details of this version of the construction which we will focus on: the color-completeness and color-purity requirements for and respectively; the resulting care needed to properly construct the system maps, the map , and its cone; and the slightly different combinatorics encountered when computing the necessary homological shifts.
Remark 1.4*.*
The local nature of the arguments in this paper potentially lend themselves well to similar homology theories for links in other 3-manifolds. For instance, we expect all of our results to hold for the link homologies defined in the recent papers [Que15, QW] when applied to an infinite braid within a 3-ball. We leave precise statements and proofs about such generalizations for future consideration.
This paper is arranged as follows. Section 2 will present a short review of the necessary homological algebra, focused on defining and comparing limits of inverse systems of complexes as well as manipulating complexes defined as multicones. Section 3 will review the relevant definitions and manipulations of colored Khovanov-Rozansky homology for braids. Section 4 is the technical heart of the paper, providing the proof of Theorem 1.2. Section 5 will explore a variety of corollaries, including the handling of negative crossings and bi-infinite braids.
1.1 Acknowledgments
The author would like to thank Gabriel Islambouli and Michael Abel for their help with earlier versions of the arguments; Paul Wedrich for some helpful conversations; Sucharit Sarkar and Ciprian Manolescu for various advice; and Matt Hogancamp for many very patient conversations on handling some of the more technical homological algebra results. The author was partially funded by NSF grant DMS-1563615 while preparing this manuscript.
2 Homological algebra background
In order to properly state and prove our theorems about semi-infinite chain complexes coming from semi-infinite braids, we will need the following ideas from homological algebra. Throughout this section, all complexes are taken over some additive category, and differentials are taken to increase homological grading by one.
Definition 2.1**.**
We employ the following basic notations.
- •
Chain complexes shall be denoted by capital boldface letters, as in , with differential (the subscript will be omitted if no confusion is possible).
- •
Superscripts on capital letters will indicate homological degree. Thus will denote the objects of in homological degree , while will denote the subcomplex of consisting of all objects in homological degrees greater than or equal to .
- •
Single terms within a complex shall often be denoted by lowercase greek letters; the notation indicates that is a term in the complex .
- •
For , shall denote the homological degree of the term within .
- •
will also denote the homological shift functor, so that indicates the complex where all terms have been shifted upwards in homological degree by .
- •
The notation will indicate that two chain maps and are homotopic. The notation will indicate that two complexes and are chain homotopy equivalent.
2.1 Inverse systems and limits
The entirety of this section is based on definitions in [Roz14], and is taken nearly verbatim from the similar sections in [IW18, AW].
Definition 2.2**.**
Let and be chain complexes and suppose is a chain map. Define the homological order of , which we denote by , to be the maximal degree for which the cone is chain homotopy equivalent to a complex that is contractible below homological degree .
Roughly speaking, denotes the maximal homological degree through which gives a homotopy equivalence on the truncated complexes of and up to degree .
Definition 2.3**.**
An inverse system of chain complexes is a sequence of chain complexes equipped with chain maps
[TABLE]
An inverse system is called Cauchy if as .
Definition 2.4**.**
An inverse system has a limit (or inverse limit), which we denote by or , if there exist maps such that
- •
for all , and
- •
as .
Theorem 2.5** ([Roz14] Propositions 3.7 and 3.12).**
An inverse system of chain complexes has a limit if and only if it is Cauchy.
Based on Definition 2.4, it is easy to see (see [Roz14]) that limits to inverse systems are unique if they exist. From this it is also easy to prove the following lemma, which will be used mainly for concatenations of finite and infinite braid complexes.
Lemma 2.6**.**
Given an inverse system with limit and a complex , there is a corresponding inverse system with inverse limit satisfying
[TABLE]
In other words, the limiting process commutes with the tensor product.
Proof.
Apply the functor to the diagram comprised of the inverse system together with and the maps . Note that homological orders of maps are preserved, and then appeal to the uniqueness of the limit. ∎
We conclude this section with a result from [IW18] which allows us to prove two inverse systems have equivalent limits.
Proposition 2.7** ([IW18] Proposition 2.13).**
Suppose and are two Cauchy inverse systems with limits and respectively. Let be a nondecreasing function on such that . Suppose there are maps forming a commuting diagram with the system maps and for appropriate and . If as then .
In Figure 2 we include a diagram to better explain the situation in Proposition 2.7. In principal we could allow cases where , necessitating the use of multiple system maps in the commutation required; in the cases of interest in this paper, however, this will never be the case and will only ever increase by one or not at all.
2.2 Multicone complexes
Many of the complexes that we will be concerned with in this paper are most easily understood and manipulated using a generalization of the usual cone construction as follows (a large portion of this section is taken nearly verbatim from [Wil]).
Definition 2.8**.**
Suppose we are given the following data in a fixed category of chain complexes over some additive category:
- •
A finite index set with a -grading .
- •
For all , a chain complex with differential .
- •
For all , a map (not necessarily a chain map) satisfying
- –
,
- –
for all in with , , and
- –
for all , .
Then we can form the multicone
[TABLE]
which is a chain complex whose terms are the direct sum of all of the terms of the complexes
[TABLE]
and whose differential is the sum of all of the maps
[TABLE]
For a term , we determine the homological grading as the sum of the contributions of viewing in and viewing in
[TABLE]
The reader may verify that this definition gives a well defined chain complex. When , the maps assemble to define chain maps; when , the maps assemble to form null-homotopies for the compositions of any two of these chain maps; and so on. Because the original system was finite, this process must eventually end, and so of course the sum in the definition of is finite. When and we have the single chain map , this construction recovers the usual cone on .
Remark 2.9*.*
We employ the term ‘multicone’ in Definition 2.8 following [Roz]. A complex built in this manner is also often referred to as a totalization or convolution of a twisted complex. See for instance [BK91].
Note that any finite chain complex can be represented as a multicone by declaring that is indexed by the terms in while the maps are given by the differentials of . Meanwhile all of the maps with are zero maps (ie no homotopies are needed).
The following proposition gives us our main tool for manipulation of multicone complexes.
Proposition 2.10**.**
Given a chain complex presented as a multicone as in Equation (1), and given chain homotopy equivalences for each , there exist maps such that we can form the multicone
[TABLE]
that is chain homotopy equivalent to :
[TABLE]
Proof.
This is a standard result generalizing the fact that the homotopy category of complexes over an additive category is triangulated (Proposition 2 in [BK91]). ∎
3 Colored link homology background
3.1 The foam category and colored Khovanov-Rozansky homology
Colored Khovanov-Rozansky homology was first constructed independently by Wu [Wu14] and Yonezawa [Yon11]. This homology theory generalizes the original construction of Khovanov and Rozansky [KR08] and categorifies the colored polynomial when coloring components by fundamental representations. Queffelec and Rose gave a combinatorial/geometric construction of colored Khovanov-Rozansky homology in terms of ‘webs’ and ‘foams’ [QR16]. It is this construction which we will briefly recall here (again, a large portion of this review is taken nearly verbatim from [AW]).
We begin with the category , as described by Cautis, Kamnitzer, and Morrison in [CKM14]. The objects of are given by sequences where and called colorings, together with a zero object. The 1-morphisms are formal sums of upward oriented trivalent graphs with edges labeled by integers in the same coloring set . At any vertex, the labels of the two edges of similar orientation (incoming or outgoing) are required to sum up to give the label of the third edge. Such graphs are generated by the basic graphs in Figure 3. There is also a set of local relations for such trivalent graphs; we list a few of the most important ones in Figure 4, and refer the reader to [CKM14] for a complete list.
We interpret any such graph as a mapping between the coloring at the bottom to the coloring at the top. These graphs are called -webs due to their relation to the representation theory of . Sometimes we will omit edges labeled by [math] and , but allowing these labels in the definition will make later definitions easier to write. By convention, we will also allow edges labeled by integers larger than for the sake of later definitions. However, any web with such an edge will be set equal to the zero web (that is, the unique morphism factoring through the zero object).
We now move on to the -category . The objects and 1-morphisms of are the same as those for , except that the 1-morphisms are considered as direct sums rather than genuine sums, and the relations between such 1-morphisms are discarded for the moment. The 2-morphisms are matrices of labeled singular cobordisms between -webs, called -foams. These cobordisms are generated by the basic cobordisms in Figure 5.
Similar to the convention for -webs, we will interpret -foams as mapping from the bottom boundary to the top boundary. Each facet of an -foam is labeled with an element of . Any facet whose boundary is shared with an edge of a web must share the same label as that edge of the web. We allow decorations on the facets of the foams where is a symmetric polynomial in a number of variables equal to the label of the facet. There also exists a set of local relations for these -foams which allow for a lifting of the web relations in (such as those in Figure 4) to 2-isomorphisms between the corresponding 1-morphisms in . The reader should consult §3 of [QR16] for more details.
The 2-morphisms in satisfy an adjunction equality in the following sense. If and are two -webs (1-morphisms in ), then we have an isomorphism of 2-morphism spaces
[TABLE]
Here denotes the 2-morphism spaces, denotes the relevant colored identity diagram, and denotes the dual of obtained from by reflection about a horizontal line. The notation in Equation (3) indicates contatenation of -webs, which plays the role of (monoidal) tensor product. An illustration of this isomorphism will be provided in Figure 7 for the more general setting of complexes over .
The category also admits a grading. We will note the gradings in cases that it is necessary, but will once again refer the reader to [QR16] for more complete information. All chain complexes of foams are assumed to have degree 0 differentials. We will denote grading shifts in with the notation for a grading shift upwards by .
To any tangle diagram whose components are labeled by elements of , we can associate a chain complex in , which we will denote by . The homotopy equivalence type of is an isotopy invariant of the tangle . In diagrams and figures, we will often omit the notation and simply draw the corresponding tangle or web unless there is a chance for confusion. In this text we will exclusively focus on the case that the tangle is actually a braid.
To build a chain complex in for a colored braid (by convention we orient all of the strands upwards), we construct basic chain complexes for each crossing. Suppose that , then
[TABLE]
[TABLE]
Trivalent graphs of the form illustrated on the right hand side of Equations (4) and (5) will be referred to as ladders; the nearly horizontal slanted edges will be referred to as rungs. The term furthest to the left is taken to have homological degree zero, and the differential has homological degree . The symbol is used to denote a shift in the internal quantum grading; we will often omit this shift as it will have no bearing on our arguments. We remark that the labeled edges determine all edges in each web, and that certain webs may be zero webs if they have a label larger than .
The maps are degree [math] foams as specified in [QR16]. The maps are the same foams, but reflected to switch the source and target webs. Finally, if , then we reflect each web around a vertical axis and perform the analogous transformation to the foams and .
We note that our conventions differ from those of [QR16]. In particular, our $C_{N}\left(\leavevmode\hbox to18.51pt{\vbox to22.22pt{\pgfpicture\makeatletter\hbox{\hskip 3.97475pt\lower-11.40001pt\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\pgfsys@setlinewidth{0.4pt}\pgfsys@invoke{ }\nullfont\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }{}{} {}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@setlinewidth{0.8pt}\pgfsys@invoke{ }{}\pgfsys@moveto{0.0pt}{0.0pt}\pgfsys@curveto{0.0pt}{-2.0302pt}{2.23105pt}{-2.23105pt}{3.66661pt}{-3.66661pt}\pgfsys@moveto{7.33339pt}{-7.33339pt}\pgfsys@curveto{8.76895pt}{-8.76895pt}{11.00002pt}{-8.96982pt}{11.00002pt}{-11.00002pt}\pgfsys@moveto{11.00002pt}{0.0pt}\pgfsys@curveto{11.00002pt}{-6.09082pt}{0.0pt}{-4.9092pt}{0.0pt}{-11.00002pt}\pgfsys@stroke\pgfsys@invoke{ } \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope \par
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.0 {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{-1.64166pt}{3.87222pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{9.79424pt}{3.19168pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope{{ {}{}{}{}{}}{{{}}{{}}}}{}{}\hss}\pgfsys@discardpath\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope\hss}}\lxSVG@closescope\endpgfpicture}}\right)\mathrm{h}^{i}\mathrm{q}^{i}i\leq j). However, our convention makes studying stabilization behavior more straightforward. We remark that under our convention, Reidemeister I and II moves hold only up to a shift (see [[AW](#bib.bibx1)]). In particular, a Reidemeister II move incurs a shift of \mathrm{h}^{t}t$ is the minimum between the two colors involved. Reidemeister I shifts will not be relevant for us in this paper.
In order to complete the definition of for a positive braid , we take the planar tensor product of the various $C_{N}\left(\leavevmode\hbox to18.51pt{\vbox to22.22pt{\pgfpicture\makeatletter\hbox{\hskip 3.97475pt\lower-11.40001pt\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\pgfsys@setlinewidth{0.4pt}\pgfsys@invoke{ }\nullfont\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }{}{} {}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@setlinewidth{0.8pt}\pgfsys@invoke{ }{}\pgfsys@moveto{0.0pt}{0.0pt}\pgfsys@curveto{0.0pt}{-2.0302pt}{2.23105pt}{-2.23105pt}{3.66661pt}{-3.66661pt}\pgfsys@moveto{7.33339pt}{-7.33339pt}\pgfsys@curveto{8.76895pt}{-8.76895pt}{11.00002pt}{-8.96982pt}{11.00002pt}{-11.00002pt}\pgfsys@moveto{11.00002pt}{0.0pt}\pgfsys@curveto{11.00002pt}{-6.09082pt}{0.0pt}{-4.9092pt}{0.0pt}{-11.00002pt}\pgfsys@stroke\pgfsys@invoke{ } \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope \par
.0 {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{-1.64166pt}{3.87222pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{9.79424pt}{3.19168pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope{{ {}{}{}{}{}}{{{}}{{}}}}{}{}\hss}\pgfsys@discardpath\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope\hss}}\lxSVG@closescope\endpgfpicture}}\right)C_{N}\left(\leavevmode\hbox to18.51pt{\vbox to22.22pt{\pgfpicture\makeatletter\hbox{\hskip 3.97475pt\lower-11.40001pt\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\pgfsys@setlinewidth{0.4pt}\pgfsys@invoke{ }\nullfont\hbox to0.0pt{\pgfsys@beginscope\pgfsys@invoke{ }{}{} {}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}{}{{}}{} {{}{}}{{}} {{{}}{{}}}{{}}{{{}}{{}}}{}{{}}{}{}{}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@setlinewidth{0.8pt}\pgfsys@invoke{ }{}\pgfsys@moveto{0.0pt}{0.0pt}\pgfsys@curveto{0.0pt}{-6.09082pt}{11.00002pt}{-4.9092pt}{11.00002pt}{-11.00002pt}\pgfsys@moveto{11.00002pt}{0.0pt}\pgfsys@curveto{11.00002pt}{-2.0302pt}{8.76895pt}{-2.23105pt}{7.33339pt}{-3.66661pt}\pgfsys@moveto{3.66661pt}{-7.33339pt}\pgfsys@curveto{2.23105pt}{-8.76895pt}{0.0pt}{-8.96982pt}{0.0pt}{-11.00002pt}\pgfsys@stroke\pgfsys@invoke{ } \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope \par
.0 {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{-1.64166pt}{3.87222pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} {{}}{{}}\hbox{\hbox{{\pgfsys@beginscope\pgfsys@invoke{ }{{}{}{{ {}{}}}{ {}{}} {{}{{}}}{{}{}}{}{{}{}}{}{}{}{}{} { }{{{{}}\pgfsys@beginscope\pgfsys@invoke{ }\pgfsys@transformcm{0.7}{0.0}{0.0}{0.7}{9.79424pt}{3.19168pt}\pgfsys@invoke{ }\hbox{{\definecolor{pgfstrokecolor}{rgb}{0,0,0}\pgfsys@color@rgb@stroke{0}{0}{0}\pgfsys@invoke{ }\pgfsys@color@rgb@fill{0}{0}{0}\pgfsys@invoke{ }\hbox{{}} }}\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope}}} \pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope{{ {}{}{}{}{}}{{{}}{{}}}}{}{}\hss}\pgfsys@discardpath\pgfsys@invoke{\lxSVG@closescope }\pgfsys@endscope\hss}}\lxSVG@closescope\endpgfpicture}}\right)T in the usual sense of Bar-Natan’s planar algebras [[BN05](#bib.bibx3)]; that is, we stitch together the various webs and foams while taking the tensor product of the corresponding complexes. Thus, if we choose to apply Equations ([4](#S3.E4)) and ([5](#S3.E5)) to only certain crossings within a given braid, we will construct multicone complexes where each term involves ladder diagrams within a larger braid-like diagram (see Figure [6](#S3.F6) for an example illustrating this notion and our notation). We refer to such diagrams as *web-braid diagrams* (\mathfrak{sl}_{N}nn$ outgoing strands, projected to the plane with no turnbacks present).
The category of complexes over admits an adjunction equality much like that of Equation (3). Given two tangles (or more generally, two web-braid diagrams) and , the space of chain maps between their corresponding complexes is itself a complex, and satisfies
[TABLE]
where once again is obtained from by a horizontal reflection, and is the relevant (colored) identity tangle. We illustrate this diagrammatically in Figure 7.
3.2 Manipulating some basic complexes in
Given a web-braid diagram, we may ‘slide’ and/or ‘twist’ rungs past various crossings (see Figures 8 and 9). In such cases, the colors involved at the crossings that have been passed by will change and we will need to understand how this affects the resulting complex. The following proposition is proved in [AW].
Proposition 3.1** ([AW] Proposition 2.19).**
Let and be any two colored positive web-braid diagrams such that we can transform into via a sequence of braid-like Reidemeister moves, rung slides (see Figure 8) and rung twists (see Figure 9). Then where
[TABLE]
Here each sum is taken over all crossings in each diagram. The notation denotes the minimal color amongst the two strands at the crossing .
If we look at Equation (4), the homological size of the complex is dependent on the minimum of the two colors at hand, and so Equation (7) is really just saying that, during such an isotopy of web-braids, the right-most homological grading remains fixed regardless of the overall size of the complex. In any case, if we want to use Proposition 3.1 to estimate homological shifts, we have to keep track of possible color-minimums of crossings for a given coloring. This motivates the following definition.
Definition 3.2**.**
Given a coloring , we define the color size of , denoted by , to be
[TABLE]
We end this short section with a color size computation that we will require later. Let denote the braid group on strands and denote its standard (positive) generators. We let denote the full twist braid on strands (see Figure 10 for a specific example).
Proposition 3.3**.**
Given a coloring , let denote the full twist on strands colored according to . Then in the notation of Equation (7), we have
[TABLE]
Proof.
A full twist involves each pair of strands crossing each other precisely twice. Meanwhile, computes the sum of the minimum color between each pair of colors. ∎
3.3 The complex for a uni-colored crossing
In order to prove Theorem 1.2, we will need to understand the complexes associated to certain special colored braids. The first and easiest scenario is the one utilized in [AW]. Consider the case of Equation (4) when . In other words, consider the complex associated to a uni-colored positive crossing. In this case, the left-most term on the right-hand side of Equation (4) has a zero on both rungs, and so the diagram is in fact the identity diagram on two strands colored . We shall denote this 2-strand identity diagram by .
Lemma 3.4**.**
Let denote a single positive crossing between two strands colored by . Then is equal to a cone
[TABLE]
where is a complex such that, for any diagram ,
- •
, and
- •
* is a ladder diagram containing an intermediate coloring with .*
Proof.
This is a direct translation of Equation (4) in the case when ; the complex is precisely and the intermediate colorings are of the form . Visually we see
[TABLE]
where intermediate colorings can be seen at the vertical midpoints of ladder diagrams. As usual, we are ignoring the -degree shifts. ∎
3.4 The complex for a two-colored clasp
The author learned the following argument from conversations with Matt Hogancamp. We begin with a simple lemma from homological algebra.
Lemma 3.5**.**
Consider a chain complex of the form
[TABLE]
over some additive category, with no terms in negative homological grading. Let be the projection map onto the one-term complex sitting in homological degree zero. Suppose that is chain-homotopic to the zero map. Then the complex is chain homotopy equivalent to one of the form
[TABLE]
where the term has been removed, and the complex is a direct summand of the original .
Proof.
In this setting, a chain homotopy between and [math] is precisely a map that inverts the corresponding component of the differential, which in turn allows for a Gaussian elimination argument (perhaps after passing to the Karoubi envelope of our category). The details are left to the reader. ∎
In the statement of the following lemma, we use the notation to indicate the two-strand identity diagram with strands colored and .
Lemma 3.6**.**
Let denote a pair of adjacent positive crossings (called a clasp) between two strands colored by . Then is chain homotopy equivalent to a cone
[TABLE]
where is a direct summand of a complex satisfying, for any diagram ,
- •
, and
- •
* is a ladder diagram containing an intermediate coloring with .*
Proof.
We will consider the case where (the case of is similar). We begin by expanding along both crossings into a complex of various web diagrams. It is not hard to see from Equation (4) that the only term in homological degree zero is the trapezoid
[TABLE]
while every other diagram, sitting in , contains intermediate colorings of lesser color-size. We will declare to be the complex . If , we are done (letting , trivially a direct summand as desired). Otherwise, we use the 2-isomorphism which lifts the second equation of Figure 4 to replace this trapezoid with a large direct sum of terms (all still in homological degree zero). A careful inspection of the right-hand side of Figure 4 indicates that precisely one of these summands will be the identity diagram , while the other summands will contain (-shifted) trapezoids of the form
[TABLE]
for .
Now we fix any such trapezoidal diagram, calling it . As in Lemma 3.5, we consider the projection map
[TABLE]
to the corresponding one-term complex in homological degree zero. Using the duality equivalence of Figure 7, together with the fact that any such trapezoid is symmetric about its central horizontal axis, we deduce the following:
[TABLE]
The positive homological shift in the first diagram of the last hom-space is derived from Proposition 3.1. This shift ensures that this hom-space is actually the zero-space, and thus we must have chain homotopic to the zero map. Lemma 3.5 allows us to conclude that there is a Gaussian elimination argument which removes any such diagram from the complex , replacing with some direct summand of itself. Iterating this process to remove all such trapezoids from the homological degree zero term leaves us with a complex of the desired form. ∎
4 Proving Theorem 1.2
Theorem 1.2 is about limiting complexes associated with certain semi-infinite braids. Such braids will be represented by semi-infinite braid words up to finitely many Reidemeister moves. In order to prove Theorem 1.2, we begin by proving a version for semi-infinite braid words; this will take up the majority of this section. Once we have this result (Section 4.5), it will be relatively simple to lift the result to semi-infinite braids.
The overall strategy for the semi-infinite braid words is similar to that of [IW18, AW], and we summarize it here. We will construct inverse systems for the infinite twist, as well as for other semi-infinite words, where the system maps are quotient maps corresponding to resolving certain crossings as vertical identity braids (Proposition 4.7). In the case of the infinite twist, it will be relatively simple to show that the system is Cauchy and thus has a limit categorifying a projector as in Cautis’ results (Theorem 4.13). For the systems coming from other semi-infinite words, we will construct maps to the system for the infinite twist that are also quotient maps corresponding to identity resolutions (Section 4.5.1); this will guarantee that these maps commute with all system maps. We will then seek to apply Proposition 2.7 by estimating the homological orders of these maps; this will be done by expanding as a multicone, simplifying the multicone using Proposition 2.10, and estimating the resulting homological orders of elements using Propositions 3.1 and 3.3 (Section 4.5.2).
Throughout this section, will denote the identity braid on strands.
4.1 Color-pure braids
If is a braid on strands, we will use the notation to indicate that has been colored so that the sequence of colors at the top (respectively bottom) of is given by (respectively ). Of course, since is a braid, must be some permutation of determined by .
Definition 4.1**.**
A colored braid is called color-pure (with respect to ) if . In such a case we will omit the superscript and simply write for the color-pure braid.
Clearly all pure braids are color-pure with respect to any . If denotes a uni-coloring, then all colored braids are color-pure. Meanwhile, if denotes a coloring using all distinct colors, then a colored braid is color-pure if and only if it is pure. Note that the full twist is pure, and so our earlier notation agrees with the convention of omitting the superscript for Definition 4.1.
Our first task is to combine Lemmas 3.4 and 3.6 into a more general statement about color-pure braids. We begin with a helpful lemma that will help us find clasps.
Lemma 4.2**.**
Any non-trivial positive color-pure braid that does not have any uni-colored crossings must be braid isotopic to another positive color-pure braid that contains some clasp .
Proof.
We begin by noting that a color-pure braid that does not have any uni-colored crossings must in fact be pure. Indeed if we have a color-pure braid that is not pure, there must be some strand that begins at one colored end-point and ends at a different end-point of the same color. If this is the case, then the strand that begins at must end at another end-point of the same color without crossing the first strand. It is not hard to see that this must eventually lead to a contradiction due to having only finitely many strands.
Therefore, if we let denote the usual map from the braid group to the symmetric group , we must have , the identity permutation. It is well known that and share the same generators (transpositions) and relations, with having the additional relation for any generator. Since was positive and non-trivial, but , we can conclude that can be altered via braid relations into some isotopic (positive) braid that contains a clasp as desired. ∎
We now state and prove the key proposition about the structure of the complex associated to a positive color-pure braid.
Proposition 4.3**.**
Given a positive color-pure braid on strands, the complex is chain homotopy equivalent to a cone
[TABLE]
where is a direct summand of a complex satisfying, for any diagram ,
- •
, and
- •
* is a ladder diagram containing an intermediate coloring with .*
The simplified complex written in this way will be denoted .
Proof.
Let denote our starting braid. If there is a uni-colored crossing in , we apply Lemma 3.4 to allowing us to write as a cone as in a two-term version of Figure 6:
[TABLE]
Here the braid is derived from by deleting , while the complex is based upon concatenating with the rest of the braid. For the sake of notational convenience moving forward, we declare so that is trivially a direct summand of the complex .
Now since we have not changed any of the colors at the top or bottom of the braid, we see that is again color-pure but with one less crossing than . If contains another uni-colored crossing, we apply the same reasoning again to get
[TABLE]
We can continue in this way until we have written as a large iterated cone with initial term where is a positive color-pure braid with no uni-colored crossings (here is the number of uni-colored crossings that were in , which have now been deleted). Each of the complexes satisfy the properties of Lemma 3.4, and we can view any as a trivial direct summand of satisfying these same properties.
According to Lemma 4.2, our color-pure with no uni-colored crossings must be braid isotopic to some having a clasp . Thus we can apply Lemma 3.6 to this clasp and write as a cone again as in Figure 6:
[TABLE]
where the braid is obtained from by deleting the clasp, while is a direct summand of the complex which is based upon concatenating with the rest of the braid as in the logic of Figure 6. Since a cone is just a two-term multicone, Proposition 2.10 allows us to plug this cone in place of the complex in the iterated cone for , so we have
[TABLE]
Thus is again a color-pure braid with no uni-colored crossings, and so we can apply the same reasoning yet again. We continue in this way until we have written as a large iterated cone with initial term , the one-term complex associated to the identity braid colored by :
[TABLE]
Here describes the number of steps it took to delete all of the uni-colored crossings and two-colored clasps from to arrive at the identity braid. Every complex is a direct summand of some whose terms satisfy the desired properties. The cone operation simply takes direct sums of the objects in the complexes (with a positive homological shift), and so the statement of the proposition is satisfied by taking and .
∎
4.2 Color-pure semi-infinite braids
We begin by giving a more precise definition for our main class of braids. Let denote the standard set of multiplicative generators for the braid group , with denoting the subset consisting of only positive generators .
Definition 4.4**.**
A semi-infinite braid word on strands is a map , written as an infinite word
[TABLE]
on the generators (where each ). We call positive if (equivalently each , so we may ignore them from the notation).
In order to use the results of Section 2.1, we need to describe a semi-infinite braid word as a limit of finite braid words. First we establish notation.
Definition 4.5**.**
Let be a semi-infinite braid word and . We define the th partial braid of , denoted by , as
[TABLE]
More generally, the partial sub-braid from to , denoted by , is defined to be the braid word
[TABLE]
In this way, the partial braid is equivalent to , and in general . We will also use the notation to denote the truncated infinite braid word obtained from by deleting the first crossings.
In this text we are concerned with colored braids. In our previous papers on the subject, when all strands in a braid were colored by , the uni-coloring at the start and end of a semi-infinite braid word could easily be identified with the coloring at the start and end of any partial braid . When we allow arbitrary colorings however, more care is needed.
Definition 4.6**.**
Let be a fixed coloring. Then a semi-infinite braid word gives rise to a sequence of colorings by defining , and then defining for as the coloring at the bottom of determined by coloring the top by . We call color-pure with respect to if for infinitely many , and denote it . The sequence of all such that is called the maximal purity sequence for .
It is clear that, after fixed an ‘initial’ coloring for the ‘top’ of , any partial sub-braid within is colored as .
With the notation set up, we have the following proposition allowing us to build complexes for certain semi-infinite braid words.
Proposition 4.7**.**
If is a positive semi-infinite color-pure braid word with maximal purity sequence , then there is a corresponding inverse system of complexes
[TABLE]
where is defined to be the colored identity braid and the maps are the quotient maps implied by Proposition 4.3. Furthermore, if this inverse system is Cauchy with limit denoted and is any subsequence of the maximal purity sequence, then there is a corresponding inverse system
[TABLE]
which is also Cauchy with a chain homotopy equivalent limit
[TABLE]
Proof.
First we describe the maps in slightly more detail. Fixing , we let denote the partial sub-braid of crossings in that are not in ; by assumption is color-pure with respect to . If we use Proposition 4.3 to write the simplified complex for , and then expand along this cone as in Figure 6, we see (with a slight abuse of notation)
[TABLE]
The map is the quotient map from this cone to the complex . From this it is clear that a subsequence has its own set of quotient maps and we can construct a commuting diagram of inverse systems as in Figure 2 where all of the horizontal maps are identity maps. Being a subsequence ensures that our new inverse system is also Cauchy (details here are left to the reader), and since identity maps have infinite homological order, Proposition 2.7 gives us our desired chain homotopy equivalence. ∎
We can now present the main definitions for semi-infinite braids.
Definition 4.8**.**
A semi-infinite braid is an equivalence class of semi-infinite braid words , where is equivalent to if and only if can be arrived at from via a finite set of braid moves. We call positive if some choice of representative word is positive. We call color-pure with respect to , and denote it , if any (and thus every) word representing is color-pure (note that this condition is not affected by finitely many braid moves).
Remark 4.9*.*
The restriction to allowing only finitely many braid moves was neglected in our earlier papers [IW18, AW], but it is clearly necessary to avoid situations where a sequence of braid moves starting from the word ‘limits’ to a new word that does not share the properties of . For instance consider the following sequence of braid moves in for :
[TABLE]
which would show ‘in the limit’ that is equivalent to the infinite twist on the first two strands with no occurrence of . We wish to disallow such limiting statements.
Proposition 4.10**.**
Let be a positive color-pure semi-infinite braid, and let and be two positive semi-infinite words representing with corresponding inverse systems and via Proposition 4.7. Then there are maps commuting with the system maps such that as . In particular, if is Cauchy, then so is and their limits are chain homotopy equivalent.
Proof.
If and are related by finitely many Reidemeister moves, then there exists some and such that the partial braid words and are braid isotopic, and the truncated semi-infinite braid words and are equal.
[TABLE]
The maps , for , are the chain homotopy equivalences induced by this braid isotopy while keeping the ‘later’ crossings fixed. Since the system maps of Proposition 4.7 beyond this point rely only on resolutions of color-pure braids within and , our maps trivially commute with the system maps.
Furthermore, because each of these are chain homotopy equivalences, we have and so that is Cauchy and we are done via Proposition 2.7. (The maps for are irrelevant and can be taken to be projection maps to the identity via Proposition 4.3, trivially commuting with all system maps.) ∎
Thus for a positive semi-infinite color-pure braid , we have a well-defined inverse system up to maps connecting any two such systems arising from different (positive) words representing . If any such word provides a Cauchy inverse system, we have a well-defined limiting system up to chain homotopy equivalence. The requirement that we consider only positive representative words is only a crutch for the moment; we will see in Section 5.1 that allowing words with negative crossings produces degree shifts (as is to be expected; we’ve already remarked upon this fact for Reidemeister II moves in our normalization) but does not change the nature of our results.
4.3 The infinite full twist
Ultimately, we will use Proposition 2.7 to compare our Cauchy inverse systems to a well understood Cauchy inverse system coming from studying the infinite full twist which we now discuss.
Fix . We begin by noting that a single full twist is a positive pure braid (and thus is color pure with respect to any coloring ), so that the semi-infinite braid word is color-pure with respect to any via the sequence of partial braid words defined by . We can therefore speak of the colored infinite full twist . In [Roz14], Rozansky proved that the standard (uncolored, ) Khovanov complex of the infinite full twist was a well-defined complex and that it categorified the Jones-Wenzl projector. We now state the analogous result in colored Khovanov-Rozansky homology due to Cautis.
Theorem 4.11** (Cautis [Cau15]).**
Let denote the positive full twist braid on strands colored by . Then, taking all complexes over the ground ring ,
[TABLE]
is a well-defined idempotent chain complex categorifying a highest weight projector
[TABLE]
factoring through the unique highest weight subrepresentation of .
In this text, we will write for Cautis’ limiting complex for the sake of brevity.
Remark 4.12*.*
Strictly speaking, Theorem 4.11 is proven for Rickard complexes in the categorification of . However this category is equivalent to and thus can be viewed as a result for complexes associated to full twist braids in (See [Cau15] and [QR16] for more details).
As indicated above, Cautis’ proof assumes that the ground ring is actually the field . Furthermore, Cautis’ complex is based on taking the inverse system built from the sequence of complete full twists, which may not necessarily be the maximal purity sequence for the semi-infinite colored braid . In the theorem below we will prove the existence of this limiting complex over any ground ring using the maximal purity sequence for the given coloring , and then Proposition 4.7 will show that our maximal purity sequence version recovers in the case that the ground ring was . The proof of this theorem provides a good warm-up for the proof of Theorem 1.2 to come afterwards.
Theorem 4.13**.**
Fix a coloring and let denote the maximal purity sequence for the colored infinite full twist . Then the corresponding inverse system is Cauchy with limiting complex denoted
[TABLE]
In particular, when the ground ring is , this limiting complex is chain homotopy equivalent to the complex of Cautis in Theorem 4.11, justifying the notation.
Proof.
By definition, we must show that the homological orders of the maps grow infinite as .
Fix . As in the proof of Proposition 4.7, we apply Proposition 4.3 to the (color-pure) partial sub-braid . Then is defined as a quotient map on the corresponding cone (we will omit the notations as usual in the visual presentations):
[TABLE]
As such, we have and since is a direct summand of a complex whose terms satisfy the properties of Proposition 4.3, is likewise chain homotopy equivalent to a direct summand of the complex .
We now expand the complex as a multicone along as expressed visually below:
[TABLE]
From here, we consider terms within this multicone. First of all, since we must have
[TABLE]
for some and some color-pure braid . Note that we must have as . Meanwhile, Proposition 4.3 shows that has an intermediate coloring with . We use this fact to write as a concatenation at this intermediate coloring:
[TABLE]
where the colorings and are indicated at various points of the diagram. Thus, our multicone for is comprised entirely of complexes of the form
[TABLE]
Now we use the fact, described in Remark 3.17 of [AW], that the web-braid diagram commutes with the full twists via a braid-like isotopy that ‘pulls rungs through the interior of the torus’ as illustrated for a single rung in Figure 11 (pulling through is merely restating the well-known fact that the full twist is central in the braid group). Thus we have
[TABLE]
This isotopy produces a homological shift according to Proposition 3.1 that can be estimated with the help of Proposition 3.3. Since all of the crossings in the web-braid remain colored the same except those within the full twist, we have
[TABLE]
As such, every complex within the multicone expansion for can be replaced (up to chain homotopy equivalence, via Proposition 2.10) by one of the form , with minimum non-zero homological degree bounded below by (all crossings in the web-braid diagram are positive). Thus any diagram coming from such a complex within our multicone for has overall homological degree estimated with the help of Equation (2):
[TABLE]
where we have used Proposition 4.3 to ensure that . As such, our complex is chain homotopy equivalent to one supported in homological degrees greater than or equal to . Then since is a direct summand of this complex, we can conclude that
[TABLE]
and as mentioned above, as . This completes the proof that the inverse system is Cauchy.
In the case when the ground ring is , Cautis’ inverse system amounts to taking the limit using a subsequence of the maximal purity sequence for the infinite twist, and so Proposition 4.7 shows that our limiting complex is chain homotopy equivalent to his. ∎
4.4 Color-complete semi-infinite braid words
Informally, a color-pure semi-infinite braid word is called color-complete if one could arrive at the infinite full twist by deleting some color-pure braid words from the infinite word . In order to make this notion more precise, we need to establish some notation.
Definition 4.14**.**
Let denote a positive semi-infinite braid word that is color-pure with respect to . Given , the notation will indicate the positive semi-infinite braid word
[TABLE]
derived from by deleting the partial sub-braid . Similarly, we will use the notation to denote the word derived from by deleting multiple (possibly infinitely many) partial sub-braids. See Figure 12 for a visual illustration.
Lemma 4.15**.**
If is a positive semi-infinite braid word that is color-pure with respect to , and the partial sub-braid is color-pure with respect to , then is also a positive semi-infinite braid word that is color-pure with respect to .
Proof.
Deleting a color-pure sub-braid within an infinite braid word does not affect the colorings either before or after it. However, the maximal purity sequence may be shifted beyond the point where was deleted. See Figure 12. Details are left to the reader. ∎
Definition 4.16**.**
A positive color-pure semi-infinite braid word is called color-complete if there exists a (finite or infinite) sequence of natural numbers (occurring in pairs )
[TABLE]
such that the following conditions hold.
- •
Each partial sub-braid is color-pure with respect to (that is, ).
- •
The semi-infinite braid word is (braid move equivalent to) the infinite full twist .
This definition is preserved under finitely many allowable braid moves, and so we can consider a positive color-pure semi-infinite braid to be color-complete if any positive word representing it is color-complete.
4.5 The main result for braid words
With the language of the preceding sections in place, we can state and prove our main result on the level of braid words.
Theorem 4.17**.**
Fix , and let be any coloring in . Let be any color-complete positive semi-infinite braid word with maximal purity sequence . Then the corresponding inverse system coming from Proposition 4.7 is Cauchy, with limiting complex satisfying
[TABLE]
where is the complex for the infinite full twist defined in Theorem 4.13.
We will prove this theorem using Proposition 2.7. Having fixed , the infinite twist has some maximal purity sequence giving rise to a Cauchy inverse system such that the limiting complex satisfies (Theorem 4.13). In order to use Proposition 2.7, we must first construct ‘horizontal’ maps which commute with the system maps, and then we will need to show that as . We split these tasks into subsections for organizational clarity below.
4.5.1 Constructing the maps
The color-completeness of implies that, if we fix some , the partial braid word contains some color-pure partial sub-braids such that
[TABLE]
where for some non-decreasing function such that as (this will be precisely the indexing function required by Proposition 2.7). The index depends on as well, but this dependence will be irrelevant moving forward. In an attempt to mimic the proof of Proposition 4.3, we will use the notation
[TABLE]
which allows the following visual presentation of :
[TABLE]
where the indices depend on the exact word , but will not concern us. The important point is that can be presented in a piecewise fashion as indicated.
Now Proposition 4.3 ensures that, for each , we have
[TABLE]
where is a direct summand of a complex with specified properties. We begin by applying this fact to each of the in in an iterative fashion. If we follow along the reasoning presented in the proof of Proposition 4.3, we arrive at the following iterated cone presentation for (we have omitted the word ‘cone’ from the notation to avoid clutter):
[TABLE]
We again collapse this into a single cone
[TABLE]
where is a direct summand of the complex with the definitions of and illustrated below:
[TABLE]
This allows us to define via the quotient map implied by Equation (11). Since all of the system maps are built as similar quotient maps corresponding to deleting various partial sub-braids, it is clear that commutes with the system maps as required.
4.5.2 Estimating with a lower bound
In order to estimate , we need to understand . Mimicking the proof of Theorem 4.13, we consider the complex instead. We examine a single summand of by expanding it as a large iterated multicone along each for (recall the notation for the simplified cone complex of Proposition 4.3). We illustrate the first two steps in Figure 13.
In the end we may collapse the resulting iterated multicone into one large multicone incorporating all of the possible complexes. In order to write this cleanly, we introduce some notation.
Definition 4.18**.**
We will write as a shorthand to indicate the formal tensor product , with objects . To the formal object we associate a diagram also denoted formed by placing each in place of in the diagram for as illustrated below
[TABLE]
Maps between two formal objects then induce maps between the diagram complexes formed by stitching together cobordisms in the usual way.
With this notation we can collapse our iterated multicone into a single multicone
[TABLE]
for which we have the formula
[TABLE]
From here, we analyze the single complexes within this multicone (12) in a manner very similar to the proof of Theorem 3.3 in [AW]. We begin by fixing such a complex , and we define two quantities based on . First, we have
[TABLE]
Since any must have , Equation (13) quickly produces the bound
[TABLE]
Second, we define
[TABLE]
and we make the following claim.
Lemma 4.19**.**
Any complex of the form indicated above is chain homotopy equivalent to a complex having no terms in homological degree less than .
Proof.
Indeed if we expand as its own multicone along the complex , we see (denoting the terms in by parallel to the notation for )
[TABLE]
Recall from Proposition 4.3 that any is a ladder diagram containing an intermediate coloring with , so that we may split just as in the proof of Theorem 4.13. We also have full twists available somewhere in the diagram (independently of the choice of ) which commute with any and all web-braids (recall Figure 11), and so we can perform the following moves on any single complex within the multicone:
[TABLE]
In these diagrams, and are web-braid diagrams consisting of various partial sub-braids together with various depending on the precise format of our original diagram . The various intermediate colorings are also indicated. Since all of the came from color-pure diagrams , and sub-braids can permute colors but not change them, we have some intermediate coloring between and that is a permutation of . In particular, . Now when we commute the full twists past the other parts of the diagram, we leave all colors on all crossings unchanged except for the crossings of the full twists, and so we have an overall homological shift of again just as in the proof of Theorem 4.13.
From here, Proposition 2.10 ensures that our multicone for is equivalent to another multicone made up entirely of complexes of the form on the right-hand side of Equation (17). Using the formula (2) for homological grading of terms in a multicone, together with the fact that for any (Proposition 4.3), we can conclude that indeed has no terms in homological degree below . ∎
We now apply Proposition 2.10 to the multicone (12) to replace the complexes with the corresponding complexes from Lemma 4.19:
[TABLE]
Any non-zero term in our multicone for must be a diagram of the form coming from some in the multicone. The homological grading of is computed via Equation (2):
[TABLE]
We then invoke the bound of Equation (15) together with our homological condition on the complex (Lemma 4.19) to conclude that
[TABLE]
and in turn that the complex is supported in homological gradings greater than or equal to the bound defined as
[TABLE]
Since is a direct summand of , we can conclude the same about giving us .
4.5.3 Finishing the argument
Proof of Theorem 4.17.
We have two inverse systems and together with maps between them creating the commuting diagram of Figure 2 (Section 4.5.1). We have also produced a lower bound (18) on the homological orders (Section 4.5.2).
Just as in the proof of Theorem 3.3 in [AW], the reader can quickly verify that the color-completeness of ensures that this bound must grow infinite as . Roughly speaking, as grows, so too does (this is color-completeness), and thus we have an ever-growing number of full twists ‘available’ in any diagram involved in any . If the full twists are largely uninterrupted, must be large by definition; otherwise we have many non-identity diagrams ‘in the way’, and so must be large. In either case, their sum which defines is growing without bound as grows.
A similar (and simpler) argument shows that the system is Cauchy, and so by Proposition 2.7 we are done. ∎
Corollary 4.20**.**
If is a positive color-complete semi-infinite braid, then there is a well-defined limiting system up to chain homotopy equivalence built from the inverse system arising from any positive word representing .
Proof.
Combine Theorem 4.17 and Proposition 4.10. ∎
5 Further corollaries and general results
In this section we record some corollaries of our work that explore a variety of situations. We begin with a quick corollary that shows how our limiting complexes behave similarly to categorified highest weight projectors.
Corollary 5.1**.**
If is a positive braid color-pure with respect to some coloring , then
[TABLE]
Proof.
Since is positive and color-pure with respect to , we can view the concatenation as a single positive semi-infinite color-complete braid word with limiting complex by Theorem 4.17. The statement of the corollary then follows from Lemma 2.6. ∎
Corollary 5.2**.**
If is a positive semi-infinite color-complete braid word with maximal purity sequence , then for any . That is to say, the complex for is independent of the ‘starting point’ for the braid word, provided that this starting point is colored by .
5.1 Negative crossings
In order to deal with semi-infinite braid words involving negative crossings, we require the following proposition that can be seen as a weak generalization of Proposition 4.3.
Proposition 5.3**.**
Given a color-pure braid on strands, let
[TABLE]
denote the sum of the minimum colors at each negative crossing in . Then the complex is chain homotopy equivalent to a multicone satisfying the following properties.
- •
There is a single term corresponding to the identity diagram , and .
- •
Every other term contains some intermediate coloring with .
Proof.
This proposition is proved in the same general fashion as Proposition 4.3, but is much simpler because we are not attempting to isolate the identity diagram. As such we will be brief.
We begin by expanding as an iterated multicone along each of the uni-colored crossings as in the logic of Figure 6. The identity resolutions of such crossings sit in right-most homological degree for negative crossings, and so we have the uni-colored negative crossings contributing their part to (and all other resolutions contain intermediate colorings as required).
We then have a corresponding version of Lemma 4.2 stating that a color-pure braid with no uni-colored crossings is braid isotopic to one having a positive clasp, a negative clasp, or a Reidemeister II move available. A Reidemeister II move can be applied, creating a shift by the minimum of the two colors as necessary. Clasps can be expanded as a tensor product of two copies of Equation (4) or (5); in either case, we have a trapezoid (in either far left or far right homological degree) that can be replaced by a sum of terms including the identity diagram just as in the proof of Lemma 3.6, except this time we do not bother with Gaussian eliminations. Instead, we simply note that all of the non-identity diagrams, whether in the same homological degree or not, have intermediate colorings as required. The details of this expansion and the resulting homological placement of the identity diagram are left to the reader. ∎
Now given a color-pure semi-infinite braid word with only finitely many negative crossings, there is some minimal such that is color-pure and is both color-pure and positive. Thus has a maximal purity sequence , which we can use to define an inverse system for .
Corollary 5.4**.**
If is a color-complete semi-infinite braid with only finitely many crossings, then the inverse system assigned to has an inverse limit satisfying
[TABLE]
where is defined as in Equation (19) and is some other shift depending on the negative crossings present in .
Proof.
Let be as above; being color-complete ensures that is color-complete and positive. Combining Lemma 2.6 and Theorem 4.17 we have
[TABLE]
where we’ve used the subsequence of complete full twists rather than the maximal purity sequence for simplicity.
Now we use Proposition 5.3 to expand each as a multicone along . Every non-identity term in has intermediate colorings of lesser color-size than , which means that each such term gets shifted in homological degree by some amount that grows with just as in the proof of Theorem 4.13. Thus each is chain homotopy equivalent to a complex with in homological degree , and other terms in higher homological degrees (once is large enough). This enables one to build quotient maps from our inverse system to the system for , whose cones live in larger and larger homological orders, allowing Proposition 2.7 to finish the argument as usual. The details are left to the reader. ∎
Corollary 5.5**.**
Given a positive semi-infinite color-complete braid on strands, the corresponding limiting complex is well-defined up to a degree shift that depends on the negative crossings included in the choice of word representing . Equivalently, a Reidemeister II move on a semi-infinite braid induces the same degree shift on the limiting complex that it would induce on a finite braid (this shift depends on the colorings of the strands).
Proof.
This follows from Corollary 5.4 since we allow only finitely many Reidemeister moves. ∎
5.2 Horizontal splittings
Corollaries 5.2 and 5.4 use finite ‘vertical’ composition of color-pure braids. We also have a result utilizing ‘horizontal’ composition. Rather than set up special notation just for this case, we present a simplistic visual version of the relevant result. The reader can consult Corollary 3.8 in [AW] for a more detailed statement in the uni-colored case.
Corollary 5.6**.**
Suppose is a positive color-pure semi-infinite braid word that can be decomposed as
[TABLE]
for some finite such that
- •
* is color-pure with respect to , and*
- •
each semi-infinite (on strands) is color-complete with respect to the coloring obtained from partitioning as indicated.
Then there is a well-defined limiting complex satisfying
[TABLE]
Proof.
As in Corollary 5.4 we use our value to define . We then appeal to Lemma 2.6 and the ‘horizontal’ concatenation properties of to complete the proof. See the proof of Corollary 3.8 in [AW] for slightly more detail. ∎
5.3 Bi-infinite braids
For semi-infinite braids it was easy to consider a given coloring as applying to the strands at the ‘start’ of the braid . In order to write down a well-defined limiting complex then, it was required that the coloring at the ‘end’ of the braid was also fixed. Demanding that the ‘start’ and ‘end’ match naturally leads to the definition of color-purity for , as motivated by the purity (and hence color-purity) of the powers of the full twist.
If we wish to generalize to bi-infinite braids, we will need to alter our approach slightly.
Definition 5.7**.**
A bi-infinite braid word is a map ; the word is called positive if . Partial words and truncated words are defined in the obvious way. We say is colored from to , and denote it , if there exist two integers , called starting points, such that the following properties hold.
- •
The partial word can be colored as , and neither nor exist as intermediate colorings in .
- •
The truncated semi-infinite words and are color-pure with respect to and , respectively. (Here color-purity of is defined in the obvious way.)
In this case we have a function of induced colorings between crossings as before, with two maximal purity sequences and satisfying and for all .
Thus we visualize a colored bi-infinite braid word as having some central ‘starting word’, and ‘growing outwards’ from there:
[TABLE]
Now bi-infinite braids should be considered unchanged by finitely many Reidemeister moves as before, but also by shifts in the function since there is no well-defined ‘starting point’ for the braid.
Definition 5.8**.**
Given a bi-infinite braid word and a finite , the shifted braid word is the map . Then a bi-infinite braid is an equivalence class of bi-infinite braid words up to finitely many braid moves and shifts, and as before we consider positive if some word representing it is positive, and we can color if we can color one (and thus all) of its representatives.
It should be clear that a colored positive bi-infinite braid gives rise to inverse systems for and , which we can concatenate with to define an inverse system for . Such a system would appear to depend on the choice of starting points in general. Still, this viewpoint makes it clear what color-completeness should mean.
Definition 5.9**.**
A positive colored bi-infinite braid word is called color-complete if, for some (and thus any) choice of starting points satisfying the coloring definition, both semi-infinite braid words and are color complete with respect to and , respectively. As usual, the positive braid is color-complete if some (and thus any) representative word for is.
The following corollary provides a precise version of Theorem 1.3.
Corollary 5.10**.**
To a positive color-complete bi-infinite braid word we may assign an inverse system with limiting complex
[TABLE]
that is independent of the choice of starting points up to chain homotopy equivalence. Thus we may assign a corresponding limiting complex to any positive color-complete bi-infinite braid that is well-defined up to degree shifts allowing for Reidemeister II moves creating (or deleting) finitely many negative crossings in the representative word.
Proof.
For a fixed choice of starting points it is clear from Theorem 4.17 and Lemma 2.6 that our assumptions ensure that the inverse system has limiting complex as described. If we choose different starting points , we partition into separate pieces using all four starting points. Depending on the relative positions of the starting points, we will then have a color-pure finite braid available which can be absorbed into one of the limiting complexes on either end via Corollary 5.1. One case is illustrated below, with the notation omitted:
[TABLE]
The equivalence on the left is viewing as color-pure with respect to , while the equivalence on the right is viewing as color-pure with respect to .
With finitely many negative crossings in a word , we use a suitably modified version of Corollary 5.4 to get our result. Finite shifts are also easy to handle by shifting the starting points as well. Details are left to the reader. ∎
Remark 5.11*.*
We can apply similar (and simpler) reasoning to assign limiting complexes to positive semi-infinite braid words colored from to (that is to say, and for infinitely many ). If we let be the smallest index for which , then we can decompose and conclude
[TABLE]
We leave it to the reader to fill in the details, including the passage to positive semi-infinite braids where a Reidemeister move may change the necessary value of , but the limiting complex will remain the same up to chain homotopy equivalence (and degree shifts for Reidemeister II moves) thanks to Lemma 2.6 and Corollary 5.1.
Remark 5.12*.*
Definition 5.7 and Corollary 5.10 view bi-infinite braids as built ‘outwards’. This seems the most natural definition to us, or at least the most amenable to our methods in this paper. One could also imagine an infinite braid built ‘inwards’, perhaps by defining as a ‘limit’ of a sequence of finite words where is built by inserting various braids throughout . As long as such insertions are color-pure, this would preserve the overall coloring leading to a well-defined inverse system and the potential for a limiting complex as above. However, it is possible to have several non-color-pure insertions that, when taken together, maintain the colors at the endpoints. It seems unclear whether such a process could also produce an inverse system of maps leading to a well-defined limiting complex.
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