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A Numerical Survey of the Reduction of Modular Curve Genus by Fricke’s Involutions

In: Number Theory

Author

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  • Harvey Cohn

    (City College (City University of New York), Mathematics Department)

Abstract

The modular curve of order N, relating j(z) and j(z/N), can be simplified by a system of involutions first due to Fricke. A computation is presented for finding the genus of the simplified curves by using the method of isometric circles due to Poincaré. This is an internal construction (in memory) for the fundamental domain. A survey of the results for N ≤ 310 confirms the 64 values of N (largest 119) for which the genus reduces to 0 and 65 values of N (largest 288) for which it reduces to 1. It also reveals 55 values of N (largest 390) for which the genus reduces to 2. While other surveys have been made, (e.g., by A. Ogg and by A.O.L. Atkin), the present one has the advantage of using minimal computational tools and theoretical techniques. The elliptic cases are interesting because in some instances e. g., N = 40,48,63,65,75, the involution process does not produce a genus reduction for the torus, and Poincaré’s method shows how this works geometrically. Also, in many cases the elliptic curves can be parametrised analytically using discriminant functions in a manner originally introduced by Fricke.

Suggested Citation

  • Harvey Cohn, 1991. "A Numerical Survey of the Reduction of Modular Curve Genus by Fricke’s Involutions," Springer Books, in: David V. Chudnovsky & Gregory V. Chudnovsky & Harvey Cohn & Melvyn B. Nathanson (ed.), Number Theory, chapter 4, pages 85-104, Springer.
    • Handle: RePEc:spr:sprchp:978-1-4757-4158-2_4
    DOI: 10.1007/978-1-4757-4158-2_4
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