There are rich connections between classical analysis and number theory. For instance, analytic number theory contains many examples of asymptotic expressions derived from estimates for analytic functions, such as in the proof of the Prime Number Theorem. In combinatorial number theory, exact formulas for number-theoretic quantities are derived from relations between analytic functions. Elliptic functions, especially theta functions, are an important class of such functions in this context, which had been made clear already in Jacobi's Fundamenta Nova. Theta functions are also classically connected with Riemann surfaces and with the modular group Gamma, which provide another path for insights into number theory. Hershel Farkas and Irwin Kra uncover combinatorial identities by means of the function theory on Riemann surfaces related to the principal congruence subgroups Gamma.