Flag positroid pipe dreams

Abstract: We introduce flag positroid pipe dreams (FPPs), whose role in the study of complete flag positroids is analogous to the role of Le-diagrams in the study of positroids. We develop the combinatorics of these diagrams and highlight some of their properties. FPPs are in bijection with intervals in the Bruhat order of the symmetric group, and the number of elbows in an FPP is the dimension of the corresponding Richardson cell in the decomposition of the nonnegative flag variety. We show how complete flag positroids can be built rank by rank via FPPs, and how the Le-diagrams of the positroid constituents of the flag can be obtained from the FPP via a simple standardization operation. Using partial FPPs, we give an alternative proof of a conjecture of Benedetti, Chavez, and Tamayo on the problem of characterizing elementary positroid quotients via cyclic shifts of decorated permutations, in the nonnegatively representable case. Our proof partially addresses a problem of Chen et al. regarding an explicit characterization of the cyclic shift operators purely in terms of decorated permutations. We show that the poset of nonnegatively representable elementary positroid quotients is self-dual. The maximal chains of this poset are in bijection with FPPs.