Torsion exponents in stable homotopy and the Hurewicz homomorphism

Abstract

We give estimates for the torsion in the Postnikov sections $τ_{[1, n]} S^0$ of the sphere spectrum, and show that the $p$-localization is annihilated by $p^{n/(2p-2) + O(1)}$. This leads to explicit bounds on the exponents of the kernel and cokernel of the Hurewicz map $π_*(X) \to H_*(X; \mathbb{Z})$ for a connective spectrum $X$. Such bounds were first considered by Arlettaz, although our estimates are tighter and we prove that they are the best possible up to a constant factor. As applications, we sharpen existing bounds on the orders of $k$-invariants in a connective spectrum, sharpen bounds on the unstable Hurewicz of an infinite loop space, and prove an exponent theorem for the equivariant stable stems.