Mercer's theorem
= Mercer's theorem
{c}
{wiki}
Mercer's theorem says that a continuous symmetric positive-semidefinite kernel on a compact domain has an orthonormal eigenfunction expansion
$$
K(s,t)=\sum_{k\geq1}\lambda_k\phi_k(s)\phi_k(t),
$$
with nonnegative eigenvalues and absolute uniform convergence under its standard hypotheses.