A First Course in Modular Forms
《A First Course in Modular Forms》 · 第 367 页 · Chapter 9「Galois Representations」
The Legendre symbol from elementary number theory, $(a/p)$ where $a\in\mathbb Z$ and $p$ is an odd prime, extends to incorporate the Kronecker symbol, defined only for $a\equiv0,1\pmod4$.