Numerical analysis of nonlinear eigenvalue problems

We provide a priori error estimates for variational approximations of the ground state eigenvalue and eigenvector of nonlinear elliptic eigenvalue problems of the form $-{div} (A\nabla u) + Vu + f(u^2) u = λu$, $\|u\|_{L^2}=1$. We focus in particular on the Fourier spectral approximation (for periodic problems) and on the $¶_1$ and $¶_2$ finite-element discretizations. Denoting by $(u_δ,λ_δ)$ a variational approximation of the ground state eigenpair $(u,λ)$, we are interested in the convergence rates of $\|u_δ-u\|_{H^1}$, $\|u_δ-u\|_{L^2}$ and $|λ_δ-λ|$, when the discretization parameter $δ$ goes to zero. We prove that if $A$, $V$ and $f$ satisfy certain conditions, $|λ_δ-λ|$ goes to zero as $\|u_δ-u\|_{H^1}^2+\|u_δ-u\|_{L^2}$. We also show that under more restrictive assumptions on $A$, $V$ and $f$, $|λ_δ-λ|$ converges to zero as $\|u_δ-u\|_{H^1}^2$, thus recovering a standard result for {\em linear} elliptic eigenvalue problems. For the latter analysis, we make use of estimates of the error $u_δ-u$ in negative Sobolev norms.