Unitary One-Parameter Groups with Finite speed of Propagation

E. C. Svendsen
1982 Proceedings of the American Mathematical Society  
Suppose that £ is a Hermitian vector bundle over a Riemannian manifold and that U is a one-parameter group of linear operators on the set of smooth sections of £ with compact support. We prove that if U satisfies a smoothness condition, is unitary, and propagates initial data with finite speed, then it can be constructed from the solutions of a first-order symmetric hyperbolic system of partial differential equations.
doi:10.2307/2043559 fatcat:qxatahaakjgmxo5bx7bgwijffi