A sphere-product counterexample to the additivity of homology nilpotency

Abstract

Homology nilpotency need not increase under a product with a sphere. We construct a simply connected, closed smooth manifold $M$ of dimension $53$, rationally elliptic with $\dim_{\mathbb Q}\bigl(π_*(M)\otimes\mathbb Q\bigr)=7$, such that $cat_0(M)=10$ and $Hnil(M\times S^2)=Hnil(M)=11$. The inequality $Hnil(X\times Y)\leq Hnil(X)+Hnil(Y)$ can therefore be strict even for a product of a closed manifold with $S^2$ as the second factor.