# selberg trace from classical physics

considering the Hamitlonian for the Selberg Operator $y^{2} ( \partial _{x}^{2}+ \partial _{y}^{2})$ given in the Hamiltonian form

$H=g_{ab}p^{a}p^{b}$ with $ds^{2} = \frac{dx^{2}+dy^{2}}{y^{2}}$ being the metric

can we obtain from this H the functional determinant expression for the selberg zeta ??

can we obtain from the integral $d(E)= \iint \delta(E-H(x,y,p)$ the 'smooth' part of the eigenvalue countign fucniton for the Laplacian i mean the factor

$\int_{0}^{E}dptanh(\pi p) p$ i mean if semiclassical physics can be applied to the problem of the Selberg zeta function.

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Your question is very hard to read. Please edit it. I think you should check the first article in "Frontiers in Number Theory, Physics, and Geometry". Its called "Quantum and Arithmetical Chaos" by Eugene Bogomolny, he discusses the Selberg trace formula, the Selberg zeta function and some physics. – Marc Palm Apr 16 '12 at 20:28
I would suggest to cross-post it to theoreticalphysics.stackexchange.com - if do so - let me know - I am also interested to read what physicists will say... – Alexander Chervov Apr 17 '12 at 16:07

This is not really an answer, but somewhat long comment. Selberg's trace formula is, of course, crying to be interpreted by means techniques coming from physics, however, personally I did not see some text which would satisfy me in all respects. Let me suggest some steps towards physical point of view (which are standard):

1) use Feynman-Kac formula: Tr (e^{tH} ) = path integral_{ over all closed paths} EXP( lenght of path)

Here H = is Laplacian

2) Left hand side is obviosuly $sum_{eigenvalues} e^{t \lambda_i} - the "spectral part" of the Selberg's formula. To compare with Wikipedia, take function "h(u)" (in Wikipedia) to be equal to exp(t u). 3) by the Stationary phase approximation we can expect that the principal contribution comes from the minimums of the action - in this case these would be closed geodesics. (This corresponds to limit from quantum mechanics to classical mechanics) Remark: Steps 1-3 are morally true for any Laplacian on any Riemannian manifold 4) By some kind of trick one must show for this particular case of Riemann surfaces with hyperbolic metric, THE ONLY contribution comes from extremums i.e. geodesics. (Such things happens in many other situations - e.g. Duistermaat Heckman formula, supersymmetric quantum mechanics etc...; however for the particular case of the Selberg formula I do not know the reference where this should be exposed in the manner which I would love, but may be I just did not search enough). Concerning yours particular question about the term$ \int_{0}^{E}dptanh(\pi p) p $, unfortunately I cannot derive it, but just some comments. This term corresponds to closed paths of zero length i.e. just points on the manifold (for each point you have trivial path which starts and immediately stops in this point). So contribution of such paths reduces to$\int_{manifold} something$. When you consider the so-called super-symmetric quantum this "something" is exactly Todd class and you get Atiyah-Singer theorem for Dirac operator (or "d-bar" operator is almost the same). But Selberg's formula is more subtle - it is NOT super-symmetric quantum mechanics, but usual quantum mechanics. So my feeling is that you can do something similar and write down some explicit integral over the Riemann surface (i.e. 2d integral) - than you can take explicitly integral in one of the variables and the rest of integral will give you desired term$ \int_{0}^{E}dptanh(\pi p) p $. It might be that "tanh" in this formula has the same origin as "sinh" in Atiyah-Singer theorems - which physically have a clear explanation as a regularized determinant of the harmonic oscillator. I guess that should be written somewhere, but I do not know the reference. - I assume that you ask whether we can compute the continuous part of the spectrum!? 1.There is no continuous spectrum, if the surface is compact. 2.If the surface has finite volume, then, in general, we can not compute the contionuous contribution to the Selberg trace formula. We do not have any non-trivial bounds in full generality, can not even say whether there are any nontrivial discrete eigenvalues at all. 3.For modular surfaces, the continuous spectrum is related to the logarithmic derivative of certain$L$-functions. The computation are very complicated in full generaltiy. For$PSL_2(\mathbb{Z}) \backslash PSL_2(\mathbb{R})\$ it can be related to the Riemann zeta function.

4.For infinite volume surfaces, there is no trace formula, but you can do still some spectral analysis, but I do not know any results here.

If you like more analysis, I can recommend "Iwaniec - Spectral theory of automorphic forms", but if you prefer some group theoretic arguments "Deitmar, Echterhoff - Principle of harmonic analysis" is pretty good, but treats also only the compact case.

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