Poincaré series (modular form)

{{distinguish|text=the Hilbert–Poincaré series}}

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:

:\sum_{\gamma\in\Gamma} H(\gamma(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

:\theta_k(z) = \sum_{\gamma\in\Gamma^*} (J_\gamma(z))^k H(\gamma(z))

where Jγ is the Jacobian determinant of the group element γ,Or a more general factor of automorphy as discussed in {{harvnb|Kollár|1995|loc=§5.2}}. 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

:\theta_k(z) = \sum_{\gamma\in\Gamma^*} (cz+d)^{-2k}H\left(\frac{az+b}{cz+d}\right)

the summation extending over congruence classes of fractional linear transformations

:\gamma=\begin{pmatrix}a&b\\c&d\end{pmatrix}

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:

:\theta_{k,n}(z) = \sum_{\gamma\in\Gamma^*} (cz+d)^{-2k}\exp\left(2\pi i n\frac{az+b}{cz+d}\right)

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

References

  • {{Citation | last1=Kollár | first1=János | title=Shafarevich maps and automorphic forms | publisher=Princeton University Press | series=M. B. Porter Lectures | isbn=978-0-691-04381-4 |mr=1341589 | year=1995|authorlink=János Kollár}}.
  • {{Springer|title=Theta-series|id=T/t092610|first=E.D.|last=Solomentsev}}.

{{DEFAULTSORT:Poincare series (modular form)}}

Category:Automorphic forms

Category:Modular forms

Category:Series (mathematics)