Let be a complex number with strictly positive imaginary part. Define the holomorphic Eisenstein series of weight, where is an integer, by the following series: This series absolutely converges to a holomorphic function of in the upper half-plane and its Fourier expansion given below shows that it extends to a holomorphic function at. It is a remarkable fact that the Eisenstein series is a modular form. Indeed, the key property is its -invariance. Explicitly if and then and is therefore a modular form of weight. Note that it is important to assume that, otherwise it would be illegitimate to change the order of summation, and the -invariance would not hold. In fact, there are no nontrivial modular forms of weight 2. Nevertheless, an analogue of the holomorphic Eisenstein series can be defined even for, although it would only be a quasimodular form.
The modular invariants and of an elliptic curve are given by the first two Eisenstein series: The article on modular invariants provides expressions for these two functions in terms of theta functions.
Recurrence relation
Any holomorphic modular form for the modular group can be written as a polynomial in and. Specifically, the higher order can be written in terms of and through a recurrence relation. Let, so for example, and. Then the satisfy the relation for all. Here, is the binomial coefficient. The occur in the series expansion for the Weierstrass's elliptic functions:
Define. Then the Fourier series of the Eisenstein series is where the coefficients are given by Here, are the Bernoulli numbers, is Riemann's zeta function and is the divisor sum function, the sum of the th powers of the divisors of. In particular, one has The summation over can be resummed as a Lambert series; that is, one has for arbitrary complex and. When working with the -expansion of the Eisenstein series, this alternate notation is frequently introduced:
Identities involving Eisenstein series
As theta functions
Given, let and define where and are alternative notations for the Jacobi theta functions. Then, thus, an expression related to the modular discriminant, Also, since and, this implies
Products of Eisenstein series
Eisenstein series form the most explicit examples of modular forms for the full modular group. Since the space of modular forms of weight has dimension 1 for, different products of Eisenstein series having those weights have to be equal up to a scalar multiple. In fact, we obtain the identities: Using the -expansions of the Eisenstein series given above, they may be restated as identities involving the sums of powers of divisors: hence and similarly for the others. The theta function of an eight-dimensional even unimodular lattice is a modular form of weight 4 for the full modular group, which gives the following identities: for the number of vectors of the squared length in the root lattice of the type. Similar techniques involving holomorphic Eisenstein series twisted by a Dirichlet character produce formulas for the number of representations of a positive integer ' as a sum of two, four, or eight squares in terms of the divisors of. Using the above recurrence relation, all higher can be expressed as polynomials in and. For example: Many relationships between products of Eisenstein series can be written in an elegant way using Hankel determinants, e.g. Garvan's identity where is the modular discriminant.
Ramanujan identities
gave several interesting identities between the first few Eisenstein series involving differentiation. Let then These identities, like the identities between the series, yield arithmetical convolution identities involving the sum-of-divisor function. Following Ramanujan, to put these identities in the simplest form it is necessary to extend the domain of to include zero, by setting Then, for example Other identities of this type, but not directly related to the preceding relations between, and functions, have been proved by Ramanujan and Giuseppe Melfi, as for example
