## What is the relation of the Kuznetsov-Bruggeman trace formula and the Selberg trace formula?

I have read that there is an elementary way to show that the above mentioned trace fromulas are equivalent in the sense, that each of them can be derived directly from the other. There should exist a short elegant method by Zagier. Where?

In short, I know how to deduce the Selberg trace formula from Arthur's trace formula at least in principle, how should one proceed to deduce the Kuznetsov formula from the arthur trace formula. What is the utility of the Kuznetsov formula? For which applications is this trace formula more suitable than the Selberg trace formula?

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Sorry to give a reference to my own paper, but perhaps what you are looking for is contained in section 2 of this paper; see also Theorem 1.3. The basic idea is that the Selberg and Kuznetsov trace formulae both involve spectral sums but with different weights. To get Selberg weights from Kuznetsov one needs to multiply by $L(1, \text{sym}^2 u_j)$ where $u_j$ is a Hecke-Maass form associated to the spectral parameter $t_j$. Thus one obtains a sum of Kloosterman sums of the form $\sum_{c} \sum_{n} \frac{S(n^2, 1;c)}{ c n}$ with some weight function. An application of Poisson summation in $n$ leads to values of quadratic Dirichlet $L$-functions at $1$ which by the class number formula can be expressed in terms of class numbers. These are then connected to the geometric side of the Selberg trace formula using Sarnak's thesis. In the paper referenced above this is carried out in the reverse direction.