P. Albano et al., Regularity results for the minimum time function of a class of semilinear evolution equations of parabolic type, SIAM J CON, 38(3), 2000, pp. 916-946
Under suitable controllability and smoothness assumptions, the minimum time
function T(x) of a semilinear control system is proved to be locally Lipsc
hitz continuous and semiconcave on the controllable set. These properties a
re then applied to derive optimality conditions relating optimal trajectori
es to the superdifferential of T.