Eichler-Shimura theory for general congruence subgroups (2410.20385v1)

Abstract: This article aims to extend the Eichler-Shimura isomorphism theorem for cusp forms on a congruence subgroup of $\text{SL}_2(\mathbb{Z})$ to the whole space of modular forms. To regularize the Eichler-Shimura integral at infinity we reconstruct the standard formalism of the Eichler-Shimura theory starting from an abstract differential definition of the integral. The article also provides new proof of Hida's twisted Eichler-Shimura isomorphism for the modular forms with nebentypus using the isomorphism theorem mentioned above. We compute the Eichler-Shimura homomorphism for a family of Eisenstein series on $\Gamma(N)$ and prove algebraicity results for the cohomology classes attached to a basis of the space of Eisenstein series on a general congruence subgroup.