Embedding Bumpless Pipedreams as Bruhat Chains

Schubert polynomials are distinguished representatives of Schubert cycles in the cohomology of the flag variety. In the spirit of Bergeron and Sottile, we use the Bruhat order to give $(n-1)!$ different combinatorial formulas for the Schubert polynomial of a permutation in Sₙ. By the work of Lenart and Sottile, one extreme of the formulas recover the classical pipedream (PD) formula. We prove the other extreme corresponds to bumpless pipedreams (BPDs). We give two applications of this perspective to view BPDs: using the Fomin–Kirillov algebra, we solve the problem of finding a BPD analogue of Fomin and Stanley’s algebraic construction on PDs; we also establish a bijection between PDs and BPDs using Lenart’s growth diagram, which conjecturally agrees with the existing bijection of Gao and Huang.