Polyhedral and tropical geometry of flag positroids

Abstract

A flag positroid of ranks $r:=(r_1<\cdots <r_k)$ on $[n]$ is a flag matroid that can be realized by a real $r_k\times n$ matrix $A$ such that the $r_i\times r_i$ minors of $A$ involving rows $1,2,\dots <!--FUNCTION\; APPLICATION-->,r_i$ are nonnegative for all $1\le i\le k$. In this paper we explore the polyhedral and tropical geometry of flag positroids, particularly when $r:=(a,a+1,\dots <!--FUNCTION\; APPLICATION-->,b)$ is a sequence of consecutive numbers. In this case we show that the nonnegative tropical flag variety ${\mathrm{TrFl}<!--FUNCTION\; APPLICATION-->}_{r,n}^{\ge 0}$ equals the nonnegative flag Dressian ${\mathrm{FlDr}<!--FUNCTION\; APPLICATION-->}_{r,n}^{\ge 0}$, and that the points $\mu =({\mu }_a,\dots <!--FUNCTION\; APPLICATION-->,{\mu }_b)$ of ${\mathrm{TrFl}<!--FUNCTION\; APPLICATION-->}_{r,n}^{\ge 0}={\mathrm{FlDr}<!--FUNCTION\; APPLICATION-->}_{r,n}^{\ge 0}$ give rise to coherent subdivisions of the flag positroid polytope $P(\underset{¯}{\mu })$ into flag positroid polytopes. Our results have applications to Bruhat interval polytopes: for example, we show that a complete flag matroid polytope is a Bruhat interval polytope if and only if its $(\le 2)$-dimensional faces are Bruhat interval polytopes. Our results also have applications to realizability questions. We define a positively oriented flag matroid to be a sequence of positively oriented matroids $({\chi }_1,\dots <!--FUNCTION\; APPLICATION-->,{\chi }_k)$ which is also an oriented flag matroid. We then prove that every positively oriented flag matroid of ranks $r=(a,a+1,\dots <!--FUNCTION\; APPLICATION-->,b)$ is realizable.