Bumpless pipe dreams (BPDs) are combinatorial objects used to study Schubert and Grothendieck polynomials. In 2025, Weigandt introduced a co-BPD object corresponding to each BPD and used them to prove change of bases formulas between these polynomials. She posed the open problem of characterizing which permutations have only reduced co-BPDs associated to their BPDs. In this talk, we present a pattern-avoidance characterization of these permutations. This is joint work with Josh Arroyo.

arXiv: https://arxiv.org/abs/2512.24361