Eisenstein Series - Eisenstein Series For The Modular Group

Eisenstein Series For The Modular Group

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:

G_{2k}(\tau) = \sum_{ (m,n)\in\mathbb{Z}^2\backslash(0,0)} \frac{1}{(m+n\tau )^{2k}}.

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

G_{2k} \left( \frac{ a\tau +b}{ c\tau + d} \right) = (c\tau +d)^{2k} G_{2k}(\tau)

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.

Read more about this topic:  Eisenstein Series

Famous quotes containing the words series and/or group:

    “The professional celebrity, male and female, is the crowning result of the star system of a society that makes a fetish of competition. In America, this system is carried to the point where a man who can knock a small white ball into a series of holes in the ground with more efficiency than anyone else thereby gains social access to the President of the United States.”
    —C. Wright Mills (1916–1962)

    “If the Russians have gone too far in subjecting the child and his peer group to conformity to a single set of values imposed by the adult society, perhaps we have reached the point of diminishing returns in allowing excessive autonomy and in failing to utilize the constructive potential of the peer group in developing social responsibility and consideration for others.”
    —Urie Bronfenbrenner (b. 1917)