I have a short question concerning the spectral theory of automorphic forms. What is the main property of the unipotent group $N$, which consist of matrices in the form \begin{pmatrix} 1 & t \\ 0 &1 \end{pmatrix} in $GL(2)$, which provides that the cuspidal spectrum decomposes discretely?


The background: Consider $G = GL\_2(\mathbb{A})$, $\Gamma =GL\_2(\mathbb{Q})$ and $Z$ the centrum of $G$, then we decompose
$$Ind\_{ \Gamma Z}^G 1 = \pi \oplus \pi^\bot,$$
where $\pi = Ind\_{N\Gamma Z}^G$. The projection onto $\pi$ is given in terms of the integral
$$ P : \phi(g) \mapsto \int\limits\_{N(\mathbb{A})} \phi(ng) d g.$$
Now given a bounded function $f$ on $\Gamma \backslash G / \Gamma$, with suitable decay properties, we can define the operator


$$Tf : \phi \mapsto f \* \phi.$$


Why is $(1-P)Tf(1-P)$ a Hilbert Schmidt operator?


My guess is that an answer should include the Iwasawa decomposition and an according decomposition of the integral operator. 



Why is the cuspidal spectrum discrete?
This result is due to Gelfand, Graev, Piatetski-Shapiro and has a short proof. I suggest you read Bump: Automorphic forms and representations, Prop. 3.2.3, pp. 285-289. 


Let me switch to $G=\mathrm{PGL}\_2(\mathbb{R})$ and $\Gamma=\mathrm{PGL}\_2(\mathbb{Z})$ for simplicity.
The idea is to consider right convolutions $\rho(\phi)$ by smooth and compactly supported functions $\phi:G\to\mathbb{C}$, which act on the space of automorphic functions $L^2(\Gamma\backslash G)$. This family of operators is sufficiently rich to distinguish automorphic functions: if $f\neq g$ then there is $\phi$ such that $\rho(\phi)f\neq\rho(\phi)g$. Thinking of $\rho(\phi)$ as an operator on $(\Gamma\cap N)\backslash G$, its kernel is given by


$$K(g,h)=\sum\_{\gamma\in\Gamma\cap N}\phi(g^{-1}\gamma h).$$


As $N\cong\mathbb{R}$ and $\Gamma\cap N\cong\mathbb{Z}$, the sum can be analyzed by Poisson summation, using the characters of $N$. It turns out that by subtracting the term corresponding to the trivial character, i.e.


$$K\_0(g,h)=\int\_{N}\phi(g^{-1} n h) \ dn,$$


the rest of the kernel decays rapidly at infinity, hence defines a compact operator. However, on the cuspidal space the operator with kernel $K\_0(g,h)$ acts by zero, hence the operator $\rho(\phi)$ is compact on the cuspidal space. In particular, $\rho(\phi)$ has an eigenvalue with finite multiplicity on any right $G$-invariant subspace of cuspidal functions, and from here the result follows by a standard linear algebra argument.


EDIT: I fixed some inaccuracies.


