MALL proof equivalence is Logspace-complete, via binary decision diagrams

TL;DR

This paper proves that proof equivalence in the MALL- fragment of linear logic is LOGSPACE-complete by relating it to binary decision diagram equivalence, contrasting with the higher complexity in MLL.

Contribution

It establishes the LOGSPACE-completeness of proof equivalence in MALL- and links it to binary decision diagrams, providing new insights into proofnets for this logic fragment.

Findings

Proof equivalence in MALL- is LOGSPACE-complete.

Proof equivalence in MALL- is equivalent to BDD equivalence.

Contrasts with PSPACE-completeness in MLL.

Abstract

Proof equivalence in a logic is the problem of deciding whether two proofs are equivalent modulo a set of permutation of rules that reflects the commutative conversions of its cut-elimination procedure. As such, it is related to the question of proofnets: finding canonical representatives of equivalence classes of proofs that have good computational properties. It can also be seen as the word problem for the notion of free category corresponding to the logic. It has been recently shown that proof equivalence in MLL (the multiplicative with units fragment of linear logic) is PSPACE-complete, which rules out any low-complexity notion of proofnet for this particular logic. Since it is another fragment of linear logic for which attempts to define a fully satisfactory low-complexity notion of proofnet have not been successful so far, we study proof equivalence in MALL- (multiplicative-additive without units fragment of linear logic) and discover a situation that is totally different from the MLL case. Indeed, we show that proof equivalence in MALL- corresponds (under AC0 reductions) to equivalence of binary decision diagrams, a data structure widely used to represent and analyze Boolean functions efficiently. We show these two equivalent problems to be LOGSPACE-complete. If this technically leaves open the possibility for a complete solution to the question of proofnets for MALL-, the established relation with binary decision diagrams actually suggests a negative solution to this problem.