# Minakshisundaram–Pleijel zeta function

The **Minakshisundaram–Pleijel zeta function** is a zeta function encoding the eigenvalues of the Laplacian of a compact Riemannian manifold. It was introduced by Subbaramiah Minakshisundaram and Åke Pleijel (1949). The case of a compact region of the plane was treated earlier by Torsten Carleman (1935).

## DefinitionEdit

For a compact Riemannian manifold *M* of dimension *N* with eigenvalues
of the Laplace–Beltrami operator , the zeta function is given for sufficiently large by

(where if an eigenvalue is zero it is omitted in the sum). The manifold may have a boundary, in which case one has to prescribe suitable boundary conditions, such as Dirichlet or Neumann boundary conditions.

More generally one can define

for *P* and *Q* on the manifold, where the are normalized eigenfunctions. This can be analytically continued to a meromorphic function of *s* for all complex *s*, and is holomorphic for .

The only possible poles are simple poles at the points for *N* odd, and at the points for *N* even. If *N* is odd then vanishes at . If *N* is even, the residues at the poles can be explicitly found in terms of the metric, and by the Wiener–Ikehara theorem we find as a corollary the relation

- ,

where the symbol indicates that the quotient of both the sides tend to 1 when T tends to .^{[1]}

The function can be recovered from by integrating over the whole manifold *M*:

- .

## Heat kernelEdit

The analytic continuation of the zeta function can be found by expressing it in terms of the heat kernel

as the Mellin transform

In particular, we have

where

is the trace of the heat kernel.

The poles of the zeta function can be found from the asymptotic behavior of the heat kernel as *t*→0.

## ExampleEdit

If the manifold is a circle of dimension *N*=1, then the eigenvalues of the Laplacian are *n*^{2} for integers *n*. The zeta function

where ζ is the Riemann zeta function.

## ApplicationsEdit

Apply the method of heat kernel to asymptotic expansion for Riemannian manifold (M,g) we obtain the two following theorems. Both are the resolutions of the inverse problem in which we get the geometric properties or quantities from spectra of the operators.

1) Minakshisundaram–Pleijel Asymptotic Expansion

Let (M,g) be an *n*-dimensional Riemannian manifold. Then, as *t*→0+, the trace of the heat kernel has an asymptotic expansion of the form:

In dim=2, this means that the integral of scalar curvature tells us the Euler characteristic of M, by the Gauss–Bonnet theorem.

In particular,

where S(x) is scalar curvature, the trace of the Ricci curvature, on M.

2) Weyl Asymptotic Formula Let M be a compact Riemannian manifold, with eigenvalues with each distinct eigenvalue repeated with its multiplicity. Define N(λ) to be the number of eigenvalues less than or equal to , and let denote the volume of the unit disk in . Then

as . Additionally, as ,

This is also called Weyl's law, refined from the Minakshisundaram–Pleijel asymptotic expansion.

## ReferencesEdit

**^**Minakshisundaram, Subbaramiah; Pleijel, Åke (1949). "Some properties of the eigenfunctions of the Laplace-operator on Riemannian manifolds".*Canadian Journal of Mathematics*.**1**: 242–256. doi:10.4153/CJM-1949-021-5. ISSN 0008-414X. MR 0031145. Archived from the original on 2012-03-20. Retrieved 2011-02-12.

- Berger, Marcel; Gauduchon, Paul; Mazet, Edmond (1971),
*Le spectre d'une variété riemannienne*, Lecture Notes in Mathematics,**194**, Berlin, New York: Springer-Verlag, doi:10.1007/BFb0064643, MR 0282313 - Carleman, Torsten (1935), "Propriétés asymptotiques des fonctions fondamentales des membranes vibrantes.",
*8. Skand. Mat.-Kongr.*(in French): 34–44, Zbl 0012.07001