Poincaré series (modular form)

In number theory, a Poincaré series is a mathematical series generalizing the classical theta series that is associated to any discrete group of symmetries of a complex domain, possibly of several complex variables. In particular, they generalize classical Eisenstein series. They are named after Henri Poincaré.

If Γ is a finite group acting on a domain D and H(z) is any meromorphic function on D, then one obtains an automorphic function by averaging over Γ:

∑γ∈ΓH(γ(z)).

However, if Γ is a discrete group, then additional factors must be introduced in order to assure convergence of such a series. To this end, a Poincaré series is a series of the form

θk(z)=∑γ∈Γ*(Jγ(z))kH(γ(z))

where Jγ is the Jacobian determinant of the group element γ,[1] and the asterisk denotes that the summation takes place only over coset representatives yielding distinct terms in the series.

The classical Poincaré series of weight 2k of a Fuchsian group Γ is defined by the series

θk(z)=∑γ∈Γ*(cz+d)−2kH(az+bcz+d)

the summation extending over congruence classes of fractional linear transformations

γ=(abcd)

belonging to Γ. Choosing H to be a character of the cyclic group of order n, one obtains the so-called Poincaré series of order n:

θk,n(z)=∑γ∈Γ*(cz+d)−2kexp⁡(2πinaz+bcz+d)

The latter Poincaré series converges absolutely and uniformly on compact sets (in the upper halfplane), and is a modular form of weight 2k for Γ. Note that, when Γ is the full modular group and n = 0, one obtains the Eisenstein series of weight 2k. In general, the Poincaré series is, for n ≥ 1, a cusp form.

## Notes

1. ↑ Or a more general factor of automorphy as discussed in Kollár 1995, §5.2.

## References

* Kollár, János (1995), Shafarevich maps and automorphic forms, M. B. Porter Lectures, Princeton University Press, ISBN 978-0-691-04381-4 .
* Hazewinkel, Michiel, ed. (2001), "Theta-series", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, https://www.encyclopediaofmath.org/index.php?title=T/t092610 .

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