Intersection statistics for antichains in minuscule posets

Abstract

For a finite poset $P$, we study the expected size of the intersection of two independent uniformly random antichains. Equivalently, we evaluate the sum of $|A\cap A'|$ over all ordered pairs of antichains. For general posets this statistic appears to have little structure, but for the classical minuscule posets with uniform combinatorial models it admits closed-form expressions. Though the proofs are elementary and combinatorial, the resulting formulas admit a natural interpretation in terms of weight diagrams of minuscule representations.