Linear homology in a nutshell

Jonathan Fine

Published 2019-07-31Version 1

In 1985 Bayer and Billera defined a flag vector $f(X)$ for every convex polytope $X$, and proved some fundamental properties. The flag vectors $f(X)$ span a graded ring $\mathcal{R}=\bigoplus_{d\geq0}\mathcal{R}_d$. Here $\mathcal{R}_d$ is the span of the $f(X)$ with $\dim X=d$. It has dimension the Fibonacci number $F_{d+1}$. This paper introduces and explores the conjecture, that $\mathcal{R}$ has a counting basis $\{e_i\}$. If true then the equation $f(X) = \sum g_i(X)e_i$ conjecturally provides a formula for the Betti numbers $g_i(X)$ of a new homology theory. As the $g_i(X)$ are linear functions of $f(X)$, we call the new theory linear homology. Further, assuming the conjecture each $g_i$ will have a rank $r\geq0$. The rank zero part of linear homology will be (middle perversity) intersection homology. The higher rank $g_i$ measure successively more complicated singularities. In dimension $d$ we will have $\dim\mathcal{R}_d$ linearly independent Betti numbers. This paper produces a basis $\{e_i\}$ for $\mathcal{R}$, that is conjecturally a counting basis.