Some formulas for the coefficients of Drinfeld modular forms

We obtain some formulas for t-expansion coefficients of meromorphic Drinfeld modular forms for GL₂(F_q[T]). Let j(z) be the Drinfeld modular invariant. As an application we show that the values of j(z) at points in the divisor of Drinfeld modular forms for GL₂(F_q[T]) are algebraic over F_q(T).     

Compactification of Drinfeld modular varieties and Drinfeld modular forms of arbitrary rank

We give an abstract characterization of the Satake compactification of a general Drinfeld modular variety. We prove that it exists and is unique up to unique isomorphism, though we do not give an explicit stratification by Drinfeld modular varieties of smaller rank which is also expected. We construct a natural ample invertible sheaf on it, such that the global sections of its k-th power form the space of (algebraic) Drinfeld modular forms of weight k. We show how the Satake compactification and modular forms behave under all natural morphisms between Drinfeld modular varieties; in particular we define Hecke operators. We give explicit results in some special cases.     

Coefficients of Drinfeld modular forms and Hecke operators

Consider the space of Drinfeld modular forms of fixed weight and type for Γ₀(n)⊂GL₂(Fq[T]). It has a linear form bn, given by the coefficient of tm+n(q−1) in the power series expansion of a type m modular form at the cusp infinity, with respect to the uniformizer t. It also has an action of a Hecke algebra. Our aim is to study the Hecke module spanned by b₁. We give elements in the Hecke annihilator of b₁. Some of them are expected to be nontrivial and such a phenomenon does not occur for classical modular forms. Moreover, we show that the Hecke module considered is spanned by coefficients bn, where n runs through an infinite set of integers. As a consequence, for any Drinfeld Hecke eigenform, we can compute explicitly certain coefficients in terms of the eigenvalues. We give an application to coefficients of the Drinfeld Hecke eigenform h.     

Congruence properties of the coefficients of the Drinfeld modular invariant

 Let be the expansion at infinity of the Drinfeld modular invariant. We know that the coefficients c _n 's are in the polynomial ring 𝔽_q[T]. In this text, we prove for these coefficients congruence properties modulo powers of p, where p  𝔽 _q [T] is a polynomial of degree 1. Received: 19 April 2002 Mathematics Subject Classification (2000): 11F52, 11F33, 11G09     

Special values of Drinfeld modular forms and algebraic independence

Let A be a polynomial ring in one variable over a finite field and k be its fraction field. Let f be a Drinfeld modular form of nonzero weight for a congruence subgroup of GL₂(A) so that the coefficients of the q _∞-expansion of f are algebraic over k. We consider n CM points α ₁, . . . , α _n on the Drinfeld upper half plane for which the quadratic fields k(α ₁), . . . , k(α _n ) are pairwise distinct. Suppose that f is non-vanishing at these n points. Then we prove that f(α ₁), . . . , f(α _n ) are algebraically independent over k.     

Ramanujan type congruences for modular forms of several variables

We give congruences between the Eisenstein series and a cusp form in the cases of Siegel modular forms and Hermitian modular forms. We should emphasize that there is a relation between the existence of a prime dividing the (k?1)th generalized Bernoulli number and the existence of non-trivial Hermitian cusp forms of weight k. We will conclude by giving numerical examples for each case.     

Iwasawa invariants for the False-Tate extension and congruences between modular forms

For an ordinary prime p?3, we consider the Hida family associated to modular forms of a fixed tame level, and their Selmer groups defined over certain Galois extensions of Q(μp) whose Galois group is G≅Zp?Zp. For Selmer groups defined over the cyclotomic Zp-extension of Q(μp), we show that if the μ-invariant of one member of the Hida family is zero, then so are the μ-invariants of the other members, while the λ-invariants remain the same only in a branch of the Hida family. We use these results to study the behavior of some invariants from non-commutative Iwasawa theory in the Hida family.     

Hecke operators for weakly holomorphic modular forms and supersingular congruences

We consider the action of Hecke operators on weakly holomorphic modular forms and a Hecke-equivariant duality between the spaces of holomorphic and weakly holomorphic cusp forms. As an application, we obtain congruences modulo supersingular primes, which connect Hecke eigenvalues and certain singular moduli.

     

Linear recursion congruences with periodic coefficients

The paper deals with the problem of the primitive period of a sequence satisfying a linear recursion congruence, modulo a power of a prime, with periodic coefficients.Translated from Matematicheskie Zametki, Vol. 3, No. 6, pp. 625–632, June, 1968.     

The zeros of certain weakly holomorphic Drinfeld modular forms

Duke and Jenkins (Pure Appl Math Q 4(4):1327–1340, 2008) constructed a canonical basis for the space of weakly holomorphic modular forms for \({{\rm SL}_2(\mathbb{Z})}\) and investigated the zeros of the basis elements. In this paper we give an analogy in the Drinfeld setting of the result given by Duke and Jenkins (Pure Appl Math Q 4(4):1327–1340, 2008).     

On Ramanujan congruences for modular forms of integral and half-integral weights

In 1916 Ramanujan observed a remarkable congruence: . The modern point of view is to interpret the Ramanujan congruence as a congruence between the Fourier coefficients of the unique normalized cusp form of weight and the Eisenstein series of the same weight modulo the numerator of the Bernoulli number . In this paper we give a simple proof of the Ramanujan congruence and its generalizations to forms of higher integral and half-integral weights.

     

Action of Hecke operators on two distinguished Drinfeld modular forms

We study the action of Hecke operators on certain non-standard Fourier expansions for the Drinfeld-Eisenstein series E _q-1 and the Drinfeld discriminant function Δ, and we find an equation which “explains” an old result of D. Goss: these two distinct modular forms have the same eigenvalues.     

Congruences for the coefficients of weakly holomorphic modular forms

Recent works have used the theory of modular forms to establishlinear congruences for the partition function and for tracesof singular moduli. We show that this type of phenomenon iscompletely general, by finding similar congruences for the coefficientsof any weakly holomorphic modular form on any congruence subgroup     

Ramanujan and coefficients of meromorphic modular forms

The study of Fourier coefficients of meromorphic modular forms dates back to Ramanujan, who, together with Hardy, studied the reciprocal of the weight 6 Eisenstein series. Ramanujan conjectured a number of further identities for other meromorphic modular forms and quasi-modular forms which were subsequently established by Berndt, Bialek, and Yee. In this paper, we place these identities into the context of a larger family by making use of Poincaré series introduced by Petersson and a new family of Poincaré series which we construct here and which are of independent interest. In addition we establish a number of new explicit identities. In particular, we give the first examples of Fourier expansions for meromorphic modular form with third-order poles and quasi-meromorphic modular forms with second-order poles.