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Recursive properties of Cohen--Macaulay flag simplicial complexes and Lefschetz decompositions from $f$-vectors

Soohyun Park

Abstract

Most applications of the hard Lefschetz theorem related to combinatorial properties of simplicial complexes involve their $h$-vectors. In the context of positivity properties involving $h$-vectors of flag spheres, $f$-vectors with a Lefschetz-type ``Boolean'' decomposition have been studied. In this note, we explore families of flag simplicial complexes where we can see this Boolean decomposition explicitly in terms of transformations connecting different simplicial complexes in this family. Note that we will take complexes in a given dimension to be PL homeomorphic to each other. In particular, the existence of a Boolean decomposition patched from local parts can be phrased in terms of a certain map formally satisfying an analogue of the hard Lefschetz theorem. The map is given by the composition of a double suspension with a ``net single edge subdivision''. Here, the former contributes to the Boolean part and the latter contributes to the disjoint non-Boolean part. The fact that the simplicial complex with the given $f$-vector can be taken to be balanced suggests algebraic versions of maps connected to these decompositions.

Recursive properties of Cohen--Macaulay flag simplicial complexes and Lefschetz decompositions from $f$-vectors

Abstract

Most applications of the hard Lefschetz theorem related to combinatorial properties of simplicial complexes involve their -vectors. In the context of positivity properties involving -vectors of flag spheres, -vectors with a Lefschetz-type ``Boolean'' decomposition have been studied. In this note, we explore families of flag simplicial complexes where we can see this Boolean decomposition explicitly in terms of transformations connecting different simplicial complexes in this family. Note that we will take complexes in a given dimension to be PL homeomorphic to each other. In particular, the existence of a Boolean decomposition patched from local parts can be phrased in terms of a certain map formally satisfying an analogue of the hard Lefschetz theorem. The map is given by the composition of a double suspension with a ``net single edge subdivision''. Here, the former contributes to the Boolean part and the latter contributes to the disjoint non-Boolean part. The fact that the simplicial complex with the given -vector can be taken to be balanced suggests algebraic versions of maps connected to these decompositions.

Paper Structure

This paper contains 1 section, 6 theorems, 4 equations.

Table of Contents

  1. Boolean decompositions, transformations, and local-global properties via Lefschetz maps

Key Result

Theorem 2

Let $\Delta$ be a flag Cohen--Macaulay simplicial complex and $\Gamma$ be a simplicial complex such that $h(\Delta) = f(\Gamma)$. Define $\Gamma_e$ analogously for $\mathop{\mathrm{lk}}\nolimits_\Delta(e)$.

Theorems & Definitions (15)

  • Definition 1
  • Theorem 2
  • Definition 1.1
  • Theorem 1.2
  • Definition 1.3
  • Proposition 1.4
  • proof
  • Proposition 1.5
  • proof
  • Theorem 1.6
  • ...and 5 more