Period polynomials and explicit formulas for Hecke operators on Gamma₀(2)

Let S_{w+2}(\Gamma_0(N)) be the vector space of cusp forms of weight w+2 on the congruence subgroup \Gamma_0(N). We first determine explicit formulas for period polynomials of elements in S_{w+2}(\Gamma_0(N)) by means of Bernoulli polynomials. When N=2, from these explicit formulas we obtain new bases for S_{w+2}(\Gamma_0(2)), and extend the Eichler-Shimura-Manin isomorphism theorem to \Gamma_0(2). This implies that there are natural correspondences between the spaces of cusp forms on \Gamma_0(2) and the spaces of period polynomials. Based on these results, we will find explicit form of Hecke operators on S_{w+2}(\Gamma_0(2)). As an application of our main theorems, we will also give an affirmative answer to a speculation of Imamo\=glu and Kohnen on a basis of S_{w+2}(\Gamma_0(2)).