No proof nets for MLL with units Proof equivalence in MLL is PSPACE-complete

[Analysis of algorithms and problem complexity]: Nonnumerical algorithms and problems—Complexity of proof procedures Keywords linear logic, proof equivalence, proof nets, constraint logic, PSPACE-completeness 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 ∗-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). 1