Recurrence of singularities for second order isotropic pseudodifferential operators

Let H be a self‐adjoint isotropic elliptic pseudodifferential operator of order 2. Denote by the solution of the Schrödinger equation with initial data . If u₀ is compactly supported the solution is smooth for small , but not for all t. We determine the wavefront set of in terms of the wavefront set of u₀ and the principal and subprincipal symbol of H.