Cohomological Dimension, Connectivity, and Lusternik--Schnirelmann category

Abstract

Dranishnikov~\cite{D2} proved that \[{\rm cat} X\leq {\rm cd}(\pi_1(X))+\Bigl\lceil\frac{{\rm hd} (X)-1}{2}\Bigr\rceil.\] where ${\rm cd}(\pi)$ denotes the cohomological dimension of a group $\pi$ and ${\rm hd}(X)$ denotes the homotopy dimension of $X$. Furthermore, there is a well-known inequality of Grossman,~\cite{G}: \[ {\rm cat} X\leq \Bigl\lceil\frac{{\rm hd} (X)}{k+1}\Bigr\rceil \text{ if } \pi_i(X)=0 \text{ for } i\leq k. \] We make a synthesis and generalization of both of these results, by demonstrating the main result: \[ {\rm cat}\leq {\rm cd}(\pi_1(X))+\Bigl\lceil\frac{{\rm hd} (X)-1}{k+1}\Bigr\rceil \text { if }\pi_i(X)=0 \text{ for } i=2, \ldots, k. \] The proof of the main theorem uses the Oprea--Strom inequality ${\rm cat} X\leq {\rm hd} (B\pi_1(X))+{\rm cat}^1X$, \cite{OS} where ${\rm cat}^1$ is the Clapp-Puppe ${\rm cat} \mathcal{A}$ with $\mathcal{A}$ the class of 1-dimensional CW complexes. The inequality clarified the Dranishnikov inequality.