A Decomposition Theorem for Topological Semi-small Maps

Abstract

A continuous surjection $f: X \rightarrow Y$ is an almost covering map if there exists a nowhere dense closed set $R \subset Y$ with a finite decomposition $R = \bigsqcup_β T_β$ such that the restriction $f\vert_{f^{-1}(Y \setminus R)}$ is a covering map and, for each $β$, the restriction $f\vert_{f^{-1}(T_β)}$ is a fiber bundle. We refer to an almost covering map as a topological semi-small map if, for each $β$, the fiber of $f\vert_{f^{-1}(T_β)}$ and the space $T_β$ satisfy the same dimension condition as for algebraic semi-small maps. We first study the topological properties of almost coverings and develop the necessary tools. In the next step, we restrict our attention to topological semi-small maps and prove that, under the assumption that for each $β$ and each trivializing neighborhood $U \subset T_β$ the connecting homomorphisms in the Gysin sequence of the normal bundle of $f^{-1}(U) \subset X$ vanish in suitable degrees, the derived direct image of the constant sheaf on the domain admits a decomposition.