Algebraic Topology
《Algebraic Topology》 · 第 487 页 · Section 4.L
The operations \(\operatorname{Sq}^i\) generate an algebra \(\mathcal A_2\), called the Steenrod algebra, and \(H^*(X;\mathbb Z_2)\) is an \(\mathcal A_2\)-module for every space \(X\).