2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/230390We give a functorial construction of k-invariants for ring spectra and use these to classify extensions in the Postnikov tower of a ring spectrum.This is the version published by Algebraic & Geometric Topology on 1 November 2006Algebraic Topology55P43, 55S45text