A First Course in Modular Forms
《A First Course in Modular Forms》 · 第 5 页 · Section 1.1「First definitions and examples」
For even $k>2$, define the Eisenstein series of weight $k$ by $G_k(\tau)=\sum'_{(c,d)\in\mathbb Z^2}(c\tau+d)^{-k}$.