Explicit formulas for Hecke operators on cusp forms, Dedekind symbols and period polynomials

Shinji Fukuhara

Published 2005-06-19, updated 2006-03-28Version 2

Let S_{w+2} be the vector space of cusp forms of weight w+2 on the full modular group, and let S_{w+2}^* denote its dual space. Periods of cusp forms can be regarded as elements of S_{w+2}^*. The Eichler-Shimura isomorphism theorem asserts that odd (or even) periods span S_{w+2}^*. However, periods are not linearly independent; in fact, they satisfy the Eichler-Shimura relations. This leads to a natural question: which periods would form a basis of S_{w+2}^*. First we give an answer to this question. Passing to the dual space S_{w+2}, we will determine a new basis for S_{w+2}. The even period polynomials of this basis elements are expressed explicitly by means of Bernoulli polynomials. Next we consider three spaces--S_{w+2}, the space of even Dedekind symbols of weight w with polynomial reciprocity laws, and the space of even period polynomials of degree w. There are natural correspondences among these three spaces. All these spaces are equipped with compatible action of Hecke operators. We will find explicit form of period polynomials and the actions of Hecke operators on the period polynomials. Finally we will obtain explicit formulas for Hecke operators on S_{w+2} in terms of Bernoulli numbers B_k and divisor functions sigma_k(n), which are quite different from the Eichler-Selberg trace formula.