Abstract:
By means of a resonance theorem we will establish the existence of functions in $L_p(\Omega)$ (where $\Omega$ is an $N$-dimensional region) whose expansion in eigenfunctions of the Laplacian is not Riesz-summable of order $a<N\bigl(\frac1p-\frac12\bigr)-\frac12$ if $1\leqslant p<\frac{2N}{N+1}$.