Restricted Space Algorithms for Isomorphism on Bounded Treewidth Graphs

TL;DR

This paper develops space-efficient algorithms for the Graph Isomorphism problem on graphs with bounded treewidth, improving complexity bounds and enabling logspace computation of canonical forms.

Contribution

It introduces restricted space algorithms for isomorphism on bounded treewidth graphs, including logspace solutions and complexity class improvements.

Findings

Isomorphism for bounded tree distance width graphs is in L.

Canonical form computation for these graphs is in logspace.

The isomorphism problem for bounded treewidth graphs is in LogCFL, improving previous bounds.

Abstract

The Graph Isomorphism problem restricted to graphs of bounded treewidth or bounded tree distance width are known to be solvable in polynomial time [Bod90],[YBFT99]. We give restricted space algorithms for these problems proving the following results: - Isomorphism for bounded tree distance width graphs is in L and thus complete for the class. We also show that for this kind of graphs a canon can be computed within logspace. - For bounded treewidth graphs, when both input graphs are given together with a tree decomposition, the problem of whether there is an isomorphism which respects the decompositions (i.e. considering only isomorphisms mapping bags in one decomposition blockwise onto bags in the other decomposition) is in L. - For bounded treewidth graphs, when one of the input graphs is given with a tree decomposition the isomorphism problem is in LogCFL. - As a corollary the isomorphism problem for bounded treewidth graphs is in LogCFL. This improves the known TC1 upper bound for the problem given by Grohe and Verbitsky [GroVer06].