>>197
>"Seminar on complex multiplication"の
>Serreの論説ではいきなりj(τ)が出てくる

なるほど (google検索)
"Seminar on complex multiplication" Serre
で 下記ヒット
これかな？
さすが、プロ数学者は詳しいですね

(参考)
https://en.wikipedia.org/wiki/Complex_multiplication
Complex multiplication
In mathematics, complex multiplication (CM) is the theory of elliptic curves E that have an endomorphism ring larger than the integers.[1] Put another way, it contains the theory of elliptic functions with extra symmetries, such as are visible when the period lattice is the Gaussian integer lattice or Eisenstein integer lattice.

It has an aspect belonging to the theory of special functions, because such elliptic functions, or abelian functions of several complex variables, are then 'very special' functions satisfying extra identities and taking explicitly calculable special values at particular points. It has also turned out to be a central theme in algebraic number theory, allowing some features of the theory of cyclotomic fields to be carried over to wider areas of application. David Hilbert is said to have remarked that the theory of complex multiplication of elliptic curves was not only the most beautiful part of mathematics but of all science.[2]

Citations
4 Serre 1967, p. 295.
9 Serre 1967, p. 293.
References
・Serre, J.-P. (1967). "XIII. Complex multiplication". In Cassels, J.W.S.; Fröhlich, Albrecht (eds.). Algebraic Number Theory. Academic Press. pp. 292–296.