How to distinguish a local semigroup from a global semigroup
  For a given autonomous time-dependent system that generates either a global, in time, semigroup or else only a local, in time, semigroup, a test involving a linear eigenvalue problem is given which determines which of 'global' or 'local' holds. Numerical examples are given. A linear transformation A is defined so that one has 'global' or 'local' depending on whether A does not or does have a positive eigenvalue. There is a possible application to Navier-Stokes problems. 
John W. Neuberger
math.AP