My question extends this one: [1].  In Hermann Koenig's book: Eigenvalue distribution of compact operators (page 145), the Mercer's theorem is stated with an unusual claim. Given a Mercer's kernel $k$, the Hilbert-Schmidt operator is defined as

$T_k: (T_k f)(x) = \int_y k(x,y) f(y) dy$

Then the book claims that the eigenfunctions for $T_k$ are *uniformly* bounded in the $L_\infty$ norm: $\sup_{n} \|f_n\|_\infty < \infty$.  Here eigenfunctions $f_n$ have unit $L_2$ norm.

The answers in the above link have constructed a counter-example.  However I'm wondering if there is any natural condition we can put on the kernel in order to guarantee the uniform boundedness.  For example, it surely holds true for the standard Gaussian RBF kernel.  But how about the inverse kernel $k(x,y) = \frac{1}{2-x^\top y}$ over the unit ball in $R^n$?


  [1]: https://mathoverflow.net/questions/72573/mercers-theorem-uniform-l-infty-bound-on-eigenfunctions