On the Möbius Function and Topology of General Pattern Posets

We introduce a formal definition of a pattern poset which encompasses several previously studied posets in the literature. Using this definition we present some general results on the Möbius function and topology of such pattern posets. We prove our results using a poset fibration based on the embeddings of the poset, where embeddings are representations of occurrences. We show that the Möbius function of these posets is intrinsically linked to the number of embeddings, and in particular to so called normal embeddings. We present results on when topological properties such as Cohen-Macaulayness and shellability are preserved by this fibration. Furthermore, we apply these results to some pattern posets and derive alternative proofs of existing results, such as Björner’s results on subword order. Moreover, we conjecture that shellability is preserved by poset fibrations satisfying certain conditions, which would generalise a result of Quillen’s showing Cohen-Macaulayness is preserved under similar conditions, and we prove a restricted form of this conjecture.