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Trace formulas for Hecke operators, Gaussian hypergeometric functions, and the modularity of a threefold. (English) Zbl 1277.11037

Summary: We present simple trace formulas for Hecke operators \(T_k(p)\) for all \(p>3\) on \(S_k(\varGamma _{0}(3))\) and \(S_k(\varGamma _{0}(9))\), the spaces of cusp forms of weight \(k\) and levels 3 and 9. These formulas can be expressed in terms of special values of Gaussian hypergeometric series and lend themselves to recursive expressions in terms of traces of Hecke operators on spaces of lower weight. Along the way, we show how to express the traces of Frobenius of a family of elliptic curves equipped with a 3-torsion point as special values of a Gaussian hypergeometric series over \(\mathbb F_q\), when \(q \equiv 1\). As an application, we use these formulas to provide a simple expression for the Fourier coefficients of \(\eta (3z)^8\), the unique normalized cusp form of weight 4 and level 9, and then show that the number of points on a certain threefold is expressible in terms of these coefficients.

MSC:

11F25 Hecke-Petersson operators, differential operators (one variable)
11G20 Curves over finite and local fields
11G40 \(L\)-functions of varieties over global fields; Birch-Swinnerton-Dyer conjecture
11T24 Other character sums and Gauss sums
33E50 Special functions in characteristic \(p\) (gamma functions, etc.)

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