Pattern-avoiding polytopes and Cambrian lattices

In 2017, Davis and Sagan found that a pattern-avoiding
Birkhoff subpolytope and an order polytope have the same normalized
volume. They ask whether the two polytopes are unimodularly
equivalent. We give an affirmative answer to a generalization of this
question.

For each Coxeter element c in the symmetric group, we define a
pattern-avoiding Birkhoff subpolytope, and an order polytope of the
heap poset of the c-sorting word of the longest permutation. We show
the two polytopes are unimodularly equivalent. As a consequence, we
show the normalized volume of the pattern-avoiding Birkhoff
subpolytope is equal to the number of the longest chains in a
corresponding Cambrian lattice. In particular, when c =
s_1s_2…s_{n-1}, this resolves the question by Davis and Sagan. This
talk is based on ongoing joint work with E. Banaian, S. Chepuri and E.
Gunawan.