Algebraic Topology
《Algebraic Topology》 · 第 417 页 · Section 4.3
The map \(W\cup CA\to K(\pi_nX,n+1)\) associated to an extension problem determines an obstruction class \(\omega_n\in H^{n+1}(W,A;\pi_nX)\); the required lift exists if and only if \(\omega_n=0\).