Index and Sectional Category

Abstract

Let $G$ be a finite group with order $|G|=\ell$ and $2\leq q\leq \ell$. For a free $G$-space $X$, we introduce a notion of $q$-th index of $(X,G)$, denoted by $\text{ind}_q(X,G)$. Our concept is relevant in the Borsuk-Ulam theory. We draw general estimates for the $q$-th index in terms of the sectional category of the quotient map $X\to X/G$, denoted by $\text{secat}(X\to X/G)$. This property connects a standard problem in Borsuk-Ulam theory to current research trends in sectional category. Under certain hypothesis we observed that $\text{secat}(X\to X/G)=\text{ind}_2(X,G)+1$. As an application of our results, we present new results in Borsuk-Ulam theory and sectional category.