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arXiv · 1602.07426

The Hauptmodul at elliptic points of certain arithmetic groups

Jay Jorgenson·Lejla Smajlović·Holger Then
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Abstract

Let $N$ be a square-free integer such that the arithmetic group $\Gamma_0(N)^+$ has genus zero; there are $44$ such groups. Let $j_N$ denote the associated Hauptmodul normalized to have residue equal to one and constant term equal to zero in its $q$-expansion. In this article we prove that the Hauptmodul at any elliptic point of the surface associated to $\Gamma_0(N)^+$ is an algebraic integer. Moreover, for each such $N$ and elliptic point $e$, we show how to explicitly evaluate $j_{N}(e)$ and provide the list of generating polynomials (with small coefficients) of the class fields or their subfields corresponding to the orders over the imaginary quadratic extension of rationals stemming from the elliptic points under consideration.

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Jay Jorgenson, Lejla Smajlović, Holger Then. 2016-02-24. The Hauptmodul at elliptic points of certain arithmetic groups. https://arxiv.org/abs/1602.07426

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