On the eigenvalues of Sturm--Liouville operators with potentials from   Sobolev spaces
  We study asymptotic behavior of the eigenvalues of Strum--Liouville operators $Ly= -y'' +q(x)y $ with potentials from Sobolev spaces $W_2^{\theta -1}, \theta \geqslant 0$, including the non-classical case $\theta \in [0,1)$ when the potentials are distributions. The results are obtained in new terms. Define the numbers $$ s_{2k}(q)= \lambda_{k}^{1/2}(q)-k, \quad s_{2k-1}(q)= \mu_{k}^{1/2}(q)-k-1/2, $$ where $\{\lambda_k\}_1^{\infty}$ and $\{\mu_k\}_1^{\infty}$ are the sequences of the eigenvalues of the operator $L$ generated by the Dirichlet and Dirichlet--Neumann boundary conditions, respectivaly. We construct special Hilbert spaces $\hat l_2^{\theta}$ such that the map $F: W^{\theta-1}_2 \to \hat l_2^{\theta}$, defined by formula $F(q)=\{s_n\}_1^{\infty}$, is well-defined for all $\theta\geqslant 0$. The main result is the following: for all fixed $\theta>0$ the map $F$ is weekly nonlinear, i.e. it admits a representation of the form $F(q) =Uq+\Phi(q)$, where $U$ is the isomorphism between the spaces $W^{\theta-1}_2 $ and $\hat l_2^{\theta}$, and $\Phi(q)$ is a compact map. Moreover we prove the estimate $\|\Phi(q)\|_{\tau} \leqslant C\|q\|_{\theta-1}$, where the value of $\tau=\tau(\theta)>\theta$ is given explicitly and the constant $C$ depends only of the radius of the ball $\|q\|_{\theta} \leqslant R$ but does not depend on the function $q$, running through this ball 
A.M. Savchuk, A.A. Shkalikov