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Further properties of a function of Ogg and Ligozat. (English) Zbl 1238.11068

Summary: Certain identities of Ramanujan may be succinctly expressed in terms of the rational function \(\breve{g}_{\chi} = \breve{f}_{\chi}-\frac{1}{\breve {f}_{\chi}}\) on the modular curve \(X_{0}(N)\), where \(\breve{f}_{\chi} = w_{N}f_{\chi}\) and \(f _{\chi }\) is a certain modular unit on the Nebentypus cover \(X_{\chi}(N)\) introduced by Ogg and Ligozat [in: B. Mazur, Publ. Math., Inst. Hautes Étud. Sci. 47, 33–186 (1977), p. 107–108].for prime \(N \equiv 1 \pmod 4\) and \(w_{N}\) is the Fricke involution. These correspond to levels \(N = 5,13\), where the genus \(g_{N}\) of \(X_{0}(N)\) is zero. In this paper we study slightly more general kind of relations for each \(\breve{g}_{\chi}\) such that \(X_{0}(N)\) has genus \(g_{N}=1,2,\) and also for each \(h_{\chi} = g_{\chi}+\breve{g}_{\chi}\) such that the Atkin-Lehner quotient \(X_{0}^{+}(N)\) has genus \(g_{N}^{+} = 1,2\). Finally we study the normal closure of the field of definition of the zeros of the latter.

MSC:

11G16 Elliptic and modular units
20H05 Unimodular groups, congruence subgroups (group-theoretic aspects)
11G18 Arithmetic aspects of modular and Shimura varieties

Software:

PARI/GP; Macaulay2; Magma

References:

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