Generalized Hamiltonian gradient flow of contact type II: singularities, cut locus, and controllability
We study singularities, cut loci and controllability for time-dependent contact Hamilton--Jacobi equations. A contact version of Arnaud's theorem identifies the superdifferential graph of an evolved profile with the contact Hamiltonian image of the differential graph of its positive Lax--Oleinik regularization. This representation defines intrinsic singular characteristics and determines their initial velocity through a minimum-energy selection on the space-time superdifferential. The curves are continuous and locally $1/2$-Hölder, but they need not be locally Lipschitz. The exponent $1/2$ is optimal even for a quadratic Hamiltonian. The intrinsic characteristic issued from a cut point is singular at every later time in the short-time interval. We also establish the contact Lax--Oleinik commutator identities, which identify the superlevel and sublevel sets of the cut time with the corresponding terminal momentum graphs through bi-Lipschitz maps. For reachable terminal data we characterize the associated calibrated sets, and we describe all continuous initial data that reach a prescribed semiconcave profile. The proofs use Herglotz' variational principle and short-time estimates for contact fundamental solutions.