Interpretations of the $\textbf{cd}$-index

Abstract

We extend the recent work of Caster--Guyer--Samper (2026+) to establish a combinatorial interpretation of the $\textbf{cd}$-index for all $S$-partitionable posets, which includes all polytopes and $S$-shellable spheres. This interpretation is inductive on rank, but does not require passing to the order complex of the poset. We also provide topological interpretations of each $\textbf{cd}$-monomial. Furthermore, we establish a recurrence on the $\textbf{cd}$-indices of shelling components of the $d$-simplex that does not require the use of André permutations, a unimodality result for $\textbf{cd}$-words ending in $\textbf{d}$ for shelling components of the $d$-simplex and the $d$-cube, and provide two families of subdivisions of the boundary of the $d$-simplex whose sequence of $\textbf{cd}$-indices is unbounded on all but the monomial $\textbf{c}^{d+1}$ and all but the two monomials $\textbf{c}^{d+1}$ and $\textbf{d}\textbf{c}^{d-1}$.