Evaluation problems for the Thompson group and the Brin-Thompson group, and their relation to the word problem

TL;DR

This paper investigates the evaluation problems for the Thompson group V and the Brin-Thompson group 2V, showing they are equivalent to their respective word problems and analyzing their computational complexity.

Contribution

It establishes the equivalence of evaluation problems to word problems for V and 2V, and characterizes their complexity classes.

Findings

Evaluation problems reduce to word problems for V and 2V.

Evaluation problems are equivalent to word problems in general.

Complexity varies: context-free for V, P-complete for 2V.

Abstract

The Thompson group $V$, as well as the Brin-Thompson group $2V$, is finitely generated and can be defined as a monoid acting on bitstrings, respectively pairs of bitstrings. Therefore evaluation problems can be defined for $V$ and $2V$. We show that these evaluation problems reduce to the corresponding word problems, and that in general, these evaluation problems are actually equivalent to the word problems. The long-input version of the evaluation problem is deterministic context-free and reverse deterministic context-free for $V,$ and P-complete for $2V$.