Hamilton’s maximum principle
for systems states that given a reaction-diffusion equation (semi-linear heat-type
equation) for sections of a vector bundle over a manifold, if the solution is initially in
a subset invariant under parallel translation and convex in the fibers and if the ODE
associated to the PDE preserves the subset, then the solution remains in the subset
for positive time. We generalize this result to the case where the subsets are
time-dependent and where there is an avoidance set from which the solution is
disjoint. In applications the existence of an avoidance set can sometimes
be used to prove the preservation of a subset of the vector bundle by the
PDE.