The **Fourier expansion** (at infinity) of a Maass form of a non-cocompact subgroup of PSL$(2,\R)$ reads
$$f(z)=\sum_{n\in\Z}a(n)\sqrt{y}K_{ir}(2\pi|n|y)e^{2\pi inx},$$
where $K_{ir}(x)$ is a $K$-Bessel function and the expansion coefficients $a(n)$ are related to Hecke eigenvalues.

If the Maass form is even, then the expansion simplifies to $$f(z)=\sum_{n=1}^{\infty}c(n)\sqrt{y}K_{ir}(2\pi ny)\cos(2\pi nx),$$ and if the form is odd $$f(z)=\sum_{n=1}^{\infty}c(n)\sqrt{y}K_{ir}(2\pi ny)\sin(2\pi nx).$$ For a prime $p$ the coefficient $c(p)$ equals the corresponding Hecke eigenvalue.

**Knowl status:**

- Review status: reviewed
- Last edited by Nathan Ryan on 2019-05-01 11:23:59

**Referred to by:**

**History:**(expand/hide all)

- 2019-05-01 11:23:59 by Nathan Ryan (Reviewed)
- 2019-04-29 23:16:52 by Nathan Ryan
- 2018-12-26 09:41:55 by David Farmer

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