A note on the reduction from LTLf to LTL

Abstract

LTLf, a finite word variant of LTL, can be reduced to LTL by introducing a new atomic proposition indicating the prefix of the infinite words that correspond to the finite words that the original LTLf formula was considering. Such a reduction was originally proposed by De Giacomo and Vardi (IJCAI'13). However, while any LTL formula reduced from LTLf describes an obligation property in the hierarchy of Manna and Pnueli (PODC'90), the aforementioned reduction does not provide an LTL formula that belongs to the syntactic obligation fragment of LTL. This note shows how the reduction was fixed in Spot in order to ensure that the resulting LTL formula is always a syntactic obligation. Doing so allows algorithms specialized to syntactic obligation to be used on LTLf formulas. For instance, in previous work (CAV'26) we described a specialized translation from syntactic obligations to minimal, weak, deterministic Büchi automata that would not be usable with the original reduction.