Eta invariant

In

eigenvalues minus the number of negative eigenvalues. In practice both numbers are often infinite so are defined using zeta function regularization. It was introduced by Atiyah, Patodi, and Singer (1973, 1975) who used it to extend the Hirzebruch signature theorem to manifolds with boundary. The name comes from the fact that it is a generalization of the Dirichlet eta function

They also later used the eta invariant of a self-adjoint operator to define the eta invariant of a compact odd-dimensional smooth manifold.

Michael Francis Atiyah, H. Donnelly, and I. M. Singer (1983) defined the

Hilbert modular surface can be expressed in terms of the value at s=0 or 1 of a Shimizu L-function

Definition

The eta invariant of self-adjoint operator A is given by η_A(0), where η is the analytic continuation of

\eta (s)=\sum _{\lambda \neq 0}{\frac {\operatorname {sign} (\lambda )}{|\lambda |^{s}}}

and the sum is over the nonzero eigenvalues λ of A.