2012/02/29 by Vicenţiu Paşol, Vicentiu Pasol, Alexandru A. Popa · 5 citations
Mathematics · #Advanced Algebra and Geometry #Advanced Mathematical Identities #Algebraic Geometry and Number Theory #math.NT #msc:11F11 #msc:11F27 #msc:11F67
paper · pdf · doi:10.1112/plms/pdt003
32 pages. Final version, with a minor error corrected compared with the published version: in Sec. 5.1, p.10, an extraneous characterization of the double coset $Δ_n$ in item (1) has been removed; Proc. London Math. Soc., online first February 14, 2013
arxiv created 2013/07/16 · arxiv updated 2013/07/17
We study the space of period polynomials associated with modular forms of integral weight for finite index subgroups of the modular group. For the modular group, this space is endowed with a pairing, corresponding to the Petersson inner product on modular forms via a formula of Haberland, and with an action of Hecke operators, defined algebraically by Zagier. We generalize Haberland's formula to (not necessarily cuspidal) modular forms for finite index subgroups, and we show that it conceals two stronger formulas. We extend the action of Hecke operators to period polynomials of modular forms, we show that the pairing on period polynomials appearing in Haberland's formula is nondegenerate, and we determine the adjoints of Hecke operators with respect to it. We give a few applications for Γ1(N): an extension of the Eichler-Shimura isomorphism to the entire space of modular forms; the determination of the relations satisfied by the even and odd parts of period polynomials associated with cusp forms, which are independent of the period relations; and an explicit formula for Fourier coefficients of Hecke eigenforms in terms of their period polynomials, generalizing the Coefficients Theorem of Manin.