Automorphic forms and cohomology theories on Shimura curves of small discriminant

We apply Lurie's theorem to produce spectra associated to 1-dimensional formal group laws on the Shimura curves of discriminants 6, 10, and 14. We compute rings of automorphic forms on these curves and the homotopy of the associated spectra. At p=3, we find that the curve of discriminant 10 recovers much the same as the topological modular forms spectrum, and the curve of discriminant 14 gives rise to a model of a truncated Brown-Peterson spectrum as an E_∞ ring spectrum.     

Shimura lifts of half-integral weight modular forms arising from theta functions

In 1973, Shimura (Ann. Math. (2) 97:440–481, 1973) introduced a family of correspondences between modular forms of half-integral weight and modular forms of even integral weight. Earlier, in unpublished work, Selberg explicitly computed a simple case of this correspondence pertaining to those half-integral weight forms which are products of Jacobi’s theta function and level one Hecke eigenforms. Cipra (J. Number Theory 32(1):58–64, 1989) generalized Selberg’s work to cover the Shimura lifts where the Jacobi theta function may be replaced by theta functions attached to Dirichlet characters of prime power modulus, and where the level one Hecke eigenforms are replaced by more generic newforms. Here we generalize Cipra’s results further to cover theta functions of arbitrary Dirichlet characters multiplied by Hecke eigenforms.      

Isogeny formulas for the Picard modular form and a three terms arithmetic geometric mean

In this paper we study the Picard modular forms and show a new three terms arithmetic geometric mean (AGM) system. This AGM system is expressed via the Appell hypergeometric function of two variables. The Picard modular forms are expressed via the theta constants, and they give the modular function for the family of Picard curves. Our theta constants are “Neben type” modular forms of weight 1 defined on the complex 2-dimensional hyperball with respect to an index finite subgroup of the Picard modular group. We define a simultaneous 3-isogeny for the family of Jacobian varieties of Picard curves. Our main theorem shows the explicit relations between two systems of theta constants which are corresponding to isogenous Jacobian varieties. This relation induces a new three terms AGM which is a generalization of Borweins' cubic AGM.     

Tate classes and L-functions on a product of a quaternionic Shimura surface and a Picard modular surface

We compute the space of Tate classes on a product of a quaternionic Shimura surface and a Picard modular surface in terms of automorphic representations including the exact determination of their field of definition and prove the equality between the dimension of the space of Tate classes and the order of the pole at s=3 of the L-function in some special cases.     

L-Functions of Jacobi forms with Shimura type

For every Jacobi form of Shimura type over H × C, a system of L-functions associated to it is given. These L-functions can be analytically continued to the whole complex plane and satisfy a kind of functional equation. As a consequence, Hecke's inverse theorem on modular forms is extended to the context of Jacobi forms with Shimura type.     

Elliptic points of the Picard modular group

We explicitly compute the elliptic points and isotropy groups for the action of the Picard modular group over the Gaussian integers on 2-dimensional complex hyperbolic space.      

L-Functions of Jacobi forms with Shimura type

For every Jacobi form of Shimura type over H × ℂ, a system of L-functions associated to it is given. These L-functions can be analytically continued to the whole complex plane and satisfy a kind of functional equation. As a consequence, Hecke’s inverse theorem on modular forms is extended to the context of Jacobi forms with Shimura type.     

A note on the discriminant of a space curve

The notion of afree divisor was introduced by K. Saito, who also proved that the discriminant in the semi-universal deformation of an isolated complete intersection is such a free fivisor. In this note we show that the discriminant of the semi-universal deformation of areduced space curve also has this property.     

Integration of modular forms with theta series

Dirichlet series, having holomorphic analytic continuation to the whole complex plane and satisfying a functional equation of standard type, are obtained by considering Rankin type integrals of products of elliptic modular forms for the group SL₂()by theta series of integral quadratic forms of determinant ±1. In a series of cases the constructed Dirichlet series are Mellin transforms of elliptic modular forms of higher weight than the initial forms.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova Akademii Nauk SSSR, Vol. 183, pp. 5–21, 1990.     

A note on the interpretation of closed H-surfaces in physical terms

A variational problem for closed H-surfaces contains conditions which firstly are not evident if compared with related geometrical problems. The formulation turns out to be quite natural, however, if the problem is derived from a mechanical one and hence can be interpreted in physical terms.     

The Picard group of the moduli of G-bundles on a curve

Let G be a complex semi-simple group, and X a compact Riemann surface. The moduli space of principal G-bundles on X, and in particular the holomorphic line bundles on this space and their global sections, play an important role in the recent applications of Conformal Field Theory to algebraic geometry. In this paper we determine the Picard group of this moduli space when G is of classical or G₂ coarse moduli space and the moduli stack).     

Class numbers of definite quaternary forms with square discriminant

Let V be a definite quaternary space over Q having square discriminant. We derive an explicit formula for the number of proper classes of maximal integral lattices in V.     

A modularity criterion for Klein forms, with an application to modular forms of level 13

We find some modularity criterion for a product of Klein forms of the congruence subgroup Γ₁(N) (Theorem 2.6) and, as its application, construct a basis of the space of modular forms for Γ₁(13) of weight 2 (Example 3.4). In the process we face with an interesting property about the coefficients of certain theta function from a quadratic form and prove it conditionally by applying Hecke operators (Proposition 4.3).     

A canonical subspace of modular forms of half-integral weight

We characterise the space of newforms of weight k + 1/2 on Γ₀(4N), N odd and square-free (studied by the second and third authors with Vasudevan) under the Atkin-Lehner W(4) operator. As an application, we show that the (±1)-eigensubspaces of the W(4) operator on the space of modular forms of weight k + 1/2 on Γ₀(4N) is mapped to modular forms of weight 2k on Γ₀(N), under a class of Shimura maps. The existence of such subspaces having this mapping property was conjectured by Zagier in a private communication. One of the special features of the (±1)-eigensubspaces is that the (2k + 1)-th power of the classical theta series of weight 1/2 belongs to the +  eigensubspace and hence this gives interesting congruences for r _2k+1(p ²).