Normal forms for real quadratic forms

We investigate the non-diagonal normal forms of a quadratic form on , in particular for n = 3. For this case it is shown that the set of normal forms is the closure of a 5-dimensional submanifold in the 6-dimensional Grassmannian of 2-dimensional subspaces of . Received: 27 June 2008     

Jacobi forms that characterize paramodular forms

The Fourier Jacobi expansions of paramodular forms are characterized from among all sequences of Jacobi forms by two conditions on the Fourier coefficients of the Jacobi forms: a growth condition and a set of linear relations. Examples, both theoretical and computational, indicate that the growth condition may be superfluous.     

Secord-order cusp forms and mixed mock modular forms

In this paper, we consider the space of second order cusp forms. We determine that this space is precisely the same as a certain subspace of mixed mock modular forms. Based upon Poincaré series of Diamantis and O’Sullivan (Trans. Am. Math. Soc. 360:5629–5666, 2008) which span the space of second order cusp forms, we construct Poincaré series which span a natural (more general) subspace of mixed mock modular forms.     

Cusp forms

LetG andH ⊂G be two real semisimple groups defined overQ. Assume thatH is the group of points fixed by an involution ofG. Letπ ⊂L ²(H\G) be an irreducible representation ofG and letf επ be aK-finite function. Let Γ be an arithmetic subgroup ofG. The Poincaré seriesP _f(g)=Σ_H∩ΓΓ f(γ{}itg) is an automorphic form on Γ\G. We show thatP _f is cuspidal in some cases, whenH ∩Γ\H is compact. Partially supported by NSF Grant # DMS 9103608.     

Modular forms

In this survey there are included results of recent years, concerning the theory of modular forms and representations connected with them of adele groups and Galois groups. There is discussed the hypothetical principle of functoriality of automorphic forms and other conjectures of Langlands concerning automorphic forms and the L-functions connected with them.     

Virtual forms

Quadratic forms over fields of characteristic different from two are generalised to virtual forms over arbitrary fields. These objects are related to Milnor’s K-theory for fields. The connection is established by a sequence of maps corresponding to Delzant’s Stiefel–Whitney classes.     

Construction of Jacobi forms associated to indefinite quadratic forms

Vignéras constructs non-holomorphic theta functions according to indefinite quadratic forms with arbitrary signature. We use Vignéras’ theta functions to create examples of non-holomorphic Jacobi forms associated to indefinite theta series by two different methods.     

Whitney constants and approximation of -quasi-linear forms by -linear forms

We discuss some relations between Whitney constants w_m(B_X,Y) for bounded functions from, the unit ball of a real normed space X into another real normed space Y. In particular, we generalize a result of Tsar’kov that

to any n-dimensional X (here denotes linearized Whitney constant).     

On Siegel modular forms of half-integral weights and Jacobi forms

We will establish a bijective correspondence between the space of the cuspidal Jacobi forms and the space of the half-integral weight Siegel cusp forms which is compatible with the action of the Hecke operators. This correspondence is based on a bijective correspondence between the irreducible unitary representations of a two-fold covering group of a symplectic group and a Jacobi group (that is, a semidirect product of a symplectic group and a Heisenberg group). The classical results due to Eichler-Zagier and Ibukiyama will be reconsidered from our representation theoretic point of view.

     

Differential operators for Hermitian Jacobi forms and Hermitian modular forms

Kim (Arch Math (Basel) 79(3):208–215, 2002) constructs multilinear differential operators for Hermitian Jacobi forms and Hermitian modular forms. However, her work relies on incorrect actions of differential operators on spaces of Hermitian Jacobi forms and Hermitian modular forms. In particular, her results are incorrect if the underlying field is the Gaussian number field. We consider more general spaces of Hermitian Jacobi forms and Hermitian modular forms over \(\mathbb {Q}(i)\), which allow us to correct the corresponding results in Kim (2002).     

Randers space forms

This note chronicles various roles played by the Yasuda-Shimada theorem in some recent developments of Riemann--Finsler geometry. We shall demonstrate that the said theorem is, at various stages of its life, an enigma, an inspiration, a flawed icon, and a powerful catalyst.     

Vector-valued Jacobi-like forms

We introduce vector-valued Jacobi-like forms associated to a representation r: G? GL(n,\Bbb C)\rho: \Gamma \rightarrow GL(n,{\Bbb C}) of a discrete subgroup G ì SL(2,\Bbb C)\Gamma \subset SL(2,{\Bbb C}) in \Bbb Cⁿ{\Bbb C}^n and establish a correspondence between such vector-valued Jacobi-like forms and sequences of vector-valued modular forms of different weights with respect to ρ. We determine a lifting of vector-valued modular forms to vector-valued Jacobi-like forms as well as a lifting of scalar-valued Jacobi-like forms to vector-valued Jacobi-like forms. We also construct Rankin-Cohen brackets for vector-valued modular forms.