Low frequency asymptotic analysis of a string with rapidly oscillating density

We consider the eigenvalue problem associated to the vibrations of a string with a rapidly oscillating bounded periodic density. It is well known that when the size of the microstructure is small enough with respect to the wa velength of the eigenfunctions 1/root lambda(epsilon), eigenvalues and eige nfunctions can be approximated by those of the limit system where the oscil lating density is replaced by its average. On the other hand, it has been o bserved that when the size of the microstructure is of the order of the wav elength of the eigenfunctions (epsilon similar to 1/root lambda(epsilon)), singular phenomena may occur. In this paper we study the behavior of the eigenvalues and eigenfunctions w hen 1/root lambda(epsilon) approaches the critical size epsilon. To do this we use the WKB approximation which allows us to nd an explicit formula for eigenvalues and eigenfunctions with respect to. In particular, our analysi s provides all order correction formulas for the limit eigenvalues and eige nfunctions below the critical size.