Let $\Omega\subset \mathbb R^2$ be open bounded with smooth boundary. It is well known that the laplace equation $$ -\Delta u=0 $$ has a set of eigenvalues $0<\lambda_1<\lambda_2\leq\lambda_3<...$, and the corresponding eigenfunction forms a ONB in $L^2$.
My question: for any $\epsilon>0$ fixed, do we have a similar set of eigenfunctions for the following equation? $$ -\operatorname{div}\left(\frac{\nabla u}{\sqrt{|\nabla u|^2+\epsilon}}\right)=0. $$ If there do exists a countable set of eigenfunctions, what about the regularity? Could they form an ONB as the laplace equation?
I think, without $\epsilon$ term the above equation is ill-posted. But how about adding $\epsilon$ term? would it be helpful?
Thank you!
PS: by eigenfunction I am looking for solution like $$ -\operatorname{div}\left(\frac{\nabla u}{\sqrt{|\nabla u|^2+\epsilon}}\right)=\lambda u, $$ and also I am interested in regularity as well.
