The principal fibration sequence and the second cohomotopy set

Abstract

Let $p:E -> B$ be a principal fibration with classifying map $w:B -> C$. It is well-known that the group $[X,ΩC]$ acts on $[X,E]$ with orbit space the image of $p_#$, where $p_#: [X,E] -> [X,B]$. The isotropy subgroup of the map of $X$ to the base point of $E$ is also well-known to be the image of $[X, ΩB]$. The isotropy subgroups for other maps $e:X -> E$ can definitely change as $e$ does. The set of homotopy classes of lifts of $f$ to the free loop space on $B$ is a group. If $f$ has a lift to $E$, the set $p_#^{-1}(f)$ is identified with the cokernel of a natural homomorphism from this group of lifts to $[X, ΩC]$. As an example, $[X,S^2]$ is enumerated for $X$ a 4-complex.