LECTURE NOTES IN ANALYSIS (2011)
《LECTURE NOTES IN ANALYSIS (2011)》 · 第 161 页 · Section 3.5「Equivalence and Reduction of Binary Quadratic Forms」，Definition 3.9
If $d$ is not a perfect square then the number of equivalence classes of binary quadratic forms of discriminant $d$ is called the class number of $d$, denoted $H(d)$.