Algebraic Topology
《Algebraic Topology》 · 第 437 页 · Section 4.D
For a graded free \(\mathbb Z\)-module \(A=\bigoplus_iA_i\), its Poincaré series is the formal power series \(p_A(t)=\sum_i a_it^i\), where \(a_i=\operatorname{rank}A_i\).