Admissible colourings of 3-manifold triangulations for Turaev-Viro type invariants

TL;DR

This paper introduces a new combinatorial approach to compute Turaev-Viro invariants for 3-manifolds, simplifying calculations and significantly reducing the number of colourings needed, thus improving computational efficiency.

Contribution

It characterizes embedded surface intersections in triangulations, introduces a new coordinate system for edge colourings, and develops algorithms that efficiently compute invariants, especially for degree four.

Findings

Reduced the set of colourings needed by a factor of 2^n

Developed an algorithm for degree four invariants, a #P-hard problem

Successfully distinguished most Z-homology spheres up to complexity 11

Abstract

Turaev Viro invariants are amongst the most powerful tools to distinguish 3-manifolds: They are implemented in mathematical software, and allow practical computations. The invariants can be computed purely combinatorially by enumerating colourings on the edges of a triangulation T. These edge colourings can be interpreted as embeddings of surfaces in T. We give a characterisation of how these embedded surfaces intersect with the tetrahedra of T. This is done by characterising isotopy classes of simple closed loops in the 3-punctured disk. As a direct result we obtain a new system of coordinates for edge colourings which allows for simpler definitions of the tetrahedron weights incorporated in the Turaev-Viro invariants. Moreover, building on a detailed analysis of the colourings, as well as classical work due to Kirby and Melvin, Matveev, and others, we show that considering a much smaller set of colourings suffices to compute Turaev-Viro invariants in certain significant cases. This results in a substantial improvement of running times to compute the invariants, reducing the number of colourings to consider by a factor of $2^n$. In addition, we present an algorithm to compute Turaev-Viro invariants of degree four -- a problem known to be #P-hard -- which capitalises on the combinatorial structure of the input. The improved algorithms are shown to be optimal in the following sense: There exist triangulations admitting all colourings the algorithms consider. Furthermore, we demonstrate that our new algorithms to compute Turaev-Viro invariants are able to distinguish the majority of $\mathbb{Z}$-homology spheres with complexity up to $11$ in $O(2^n)$ operations in $\mathbb{Q}$.