Essential spectrum and L(2)-solutions of one-dimensional Schrodinger operators

In 1949, Hartman and Wintner showed that if the eigenvalue equations of a one-dimensional Schrodinger operator possess square integrable solutions, then the essential spectrum is' nowhere dense. Furthermore, they conjectured that this statement could be improved and that under this condition the essential spectrum might always be void. This is shown to be false. It is proved that, on the contrary, every closed, nowhere dense set does occur as the essential spectrum of Schrodinger operators which satisfy the condition of existence of L(2)-solutions. The proof of this theorem is based on inverse spectral theory.