Proof equivalence in MLL is PSPACE-complete

TL;DR

This paper proves that deciding proof equivalence in multiplicative linear logic (MLL) is a PSPACE-complete problem, highlighting the computational complexity and implications for proof net representations.

Contribution

It establishes the PSPACE-completeness of MLL proof equivalence using a reduction from Nondeterministic Constraint Logic, impacting the understanding of proof net canonical forms.

Findings

MLL proof equivalence is PSPACE-complete.

Proof equivalence problem is reducible from Nondeterministic Constraint Logic.

Implication that canonical proof nets for MLL with units are unlikely to exist.

Abstract

MLL proof equivalence is the problem of deciding whether two proofs in multiplicative linear logic are related by a series of inference permutations. It is also known as the word problem for star-autonomous categories. Previous work has shown the problem to be equivalent to a rewiring problem on proof nets, which are not canonical for full MLL due to the presence of the two units. Drawing from recent work on reconfiguration problems, in this paper it is shown that MLL proof equivalence is PSPACE-complete, using a reduction from Nondeterministic Constraint Logic. An important consequence of the result is that the existence of a satisfactory notion of proof nets for MLL with units is ruled out (under current complexity assumptions). The PSPACE-hardness result extends to equivalence of normal forms in MELL without units, where the weakening rule for the exponentials induces a similar rewiring problem.