Mock modular form

Abstract: The recent development of the theory of modular forms and associated zeta functions, together with all its arithmetic significance, is quite pleasing, and our knowledge in this field is evergrowing, but the forms of half integral weight have attracted only casual attention, in spite of their importance and ancientness. Indeed, the connection of such forms with zeta functions was never clarified. When Hecke developed his theory of Euler product forthe forms of integral weight, he pointed out the impossibility of a similar theory for the forms of half integral weight, and that only partial information could be obtained for the Fourier coefficients of such forms (Werke, p. 639). He explained this in more detail in his last paper [3], which might have given a rather negative and somewhat misleading impression that one would not be able to do much except in some special cases. A treatment of a more general type of modular form was given by Wohlfahrt [12]. In fact, he defined Hecke operators whose degree is the square of a prime, and showed a certain multiplicative relation, as predicted by Hecke, for the Fourier coefficients, but discussed neither Euler product, nor connection with zeta functions. In the present paper, we try to reveal a more affirmative aspect of the subject. To be specific, put, for each positive integer N,

Abstract: We construct some families of automorphic forms on Grassmannians which have singularities along smaller sub Grassmannians, using Harvey and Moore's extension of the Howe (or theta) correspondence to modular forms with poles at cusps. Some of the applications are as follows. We construct families of holomorphic automorphic forms which can be written as infinite products, which give many new examples of generalized Kac-Moody superalgebras. We extend the Shimura and Maass-Gritsenko correspondences to modular forms with singularities. We prove some congruences satisfied by the theta functions of positive definite lattices, and find a sufficient condition for a Lorentzian lattice to have a reflection group with a finite volume fundamental domain. We give some examples suggesting that these automorphic forms with singularities are related to Donaldson polynomials and to mirror symmetry for K3 surfaces.

Abstract: The mock theta functions were invented by the Indian mathematician Srinivasa Ramanujan, who lived from 1887 until 1920. He discovered them shortly before his death. In this dissertation, I consider several of the examples that Ramanujan gave of mock theta functions, and relate them to real-analytic modular forms of weight 1/2. In Chapter 1, I consider a sum, which was also studied by Lerch. This Lerch sum transforms almost as a Jacobi form under substitutions in (upsilon, nu, tau ). I show that the transformation behaviour becomes that of a Jacobi form if we add a (relatively simple) correction term. This correction term is real-analytic in (upsilon, nu, tau) but not holomorphic. For special values of (upsilon, nu), we could call the Lerch sum (considered as a function of tau ) a mock theta function, although these examples were not considered by Ramanujan. In Chapter 2, I consider theta functions for indefinite quadratic forms. These indefinite theta functions are modified versions of theta functions introduced by Gottsche and Zagier. I find elliptic and modular transformation properties of these functions, similar to the properties of theta functions associated to positive definite quadratic forms. In the case of positive definite quadratic forms, the theta functions are holomorphic. The theta functions in the chapter are not holomorphic. By taking special values of certain parameters, we get most of the examples of mock theta functions given by Ramanujan. Andrews gave most of the fifth order mock theta functions as Fourier coefficients of meromorphic Jacobi forms, namely certain quotients of ordinary Jacobi theta-series. This is the motivation for the study of the modularity of Fourier coefficients of meromorphic Jacobi forms, in Chapter 3. We find that modularity follows on adding a real-analytic correction term to the Fourier coefficients. In Chapter 4, I use the results from Chapter 2 to get the modular transformation properties of the seventh-order mock v-functions and most of the fifth-order functions. The final result is that we can write each of these mock theta-functions as the sum of two functions ? and G, where: - ? is a real-analytic modular form of weight 1/2 and is an eigenfunction of the appropriate Casimir operator with eigenvalue 3/16 (this is also the eigenvalue of holomorphic modular forms of this weight); and - G is a theta series associated to a negative definite unary quadratic form. Moreover G is bounded as ? tends vertically to any rational limit. Many of the results of Chapter 4 could be deduced using the methods from Chapter 1 or Chapter 3 instead of Chapter 2. This means that I have actually given three approaches to proving modularity properties of the mock theta -functions.