This work shows that, in one dimension, due to its topology, a closed-loop product of short-time propagators is always positive, despite the fact that each anti-symmetric free fermion propagator can be of either sign.
@article{arxiv.2209.06785,
title = {A topological proof that there is no sign problem in one dimensional Path Integral Monte Carlo simulation of fermions},
author = {Siu A. Chin},
journal= {arXiv preprint arXiv:2209.06785},
year = {2022}
}