Characteristic Polynomials of Graph- and Digraph-Deleted Catalan Arrangements

Abstract

We develop a finite-field stratification for characteristic polynomials of deletion subarrangements of the full $m$-Catalan arrangement. It reduces the count to cyclic placements of rigid blocks and yields falling-factorial expansions for deletions indexed by graphs, digraphs, and gain-labeled digraphs. The coefficients are graphical Stirling numbers for zero-layer deletions, directed matching numbers when the deleted layer $\ell$ satisfies $1\le \ell \le \lfloor m/2\rfloor$, directed path-cover numbers when $\lfloor m/2\rfloor <\ell \le m$, and admissible gain-labeled arc sets for multilayer deletions. For $\ell =m$, a complementary path-cover expansion yields factorization consequences. The method also gives formulas for directed Ish-type arrangements in terms of path-cycle covers and outdegrees.

25 pages