Differential graded algebras for trivalent plane graphs and their representations

TL;DR

This paper generalizes a dg-algebra associated with trivalent plane graphs to non-commutative coefficients, establishing a correspondence between algebra representations and face colorings, linking Legendrian contact geometry with combinatorial graph theory.

Contribution

It extends the Casals--Murphy dg-algebra to non-commutative settings and proves a new correspondence between dg-algebra representations and face colorings in Grassmannians.

Findings

Generalized dg-algebra to non-commutative coefficients

Established functoriality properties in the non-commutative setting

Linked dg-algebra representations to Grassmannian face colorings

Abstract

To any trivalent plane graph embedded in the sphere, Casals and Murphy associate a differential graded algebra (dg-algebra), in which the underlying graded algebra is free associative over a commutative ring. Our first result is the construction of a generalization of the Casals--Murphy dg-algebra to non-commutative coefficients, for which we prove various functoriality properties not previously verified in the commutative setting. Our second result is to prove that rank $r$ representations of this dg-algebra, over a field $\mathbb{F}$, correspond to colorings of the faces of the graph by elements of the Grassmannian $\operatorname{Gr}(r,2r;\mathbb{F})$ so that bordering faces are transverse, up to the natural action of $\operatorname{PGL}_{2r}(\mathbb{F})$. Underlying the combinatorics, the dg-algebra is a computation of the fully non-commutative Legendrian contact dg-algebra for Legendrian satellites of Legendrian 2-weaves, though we do not prove as such in this paper. The graph coloring problem verifies that for Legendrian 2-weaves, rank $r$ representations of the Legendrian contact dg-algebra correspond to constructible sheaves of microlocal rank $r$. This is the first explicit such computation of the bijection between the moduli spaces of representations and sheaves for an infinite family of Legendrian surfaces.