“…-Along this line, the quatitative semiconcavity and convexity estimate of the associated fundamental solutions have been obtained in [7] recently, which is useful for the intrinsic study of the global propagation of singularities of the viscosity solutions of (5) and (6) ( [8,4,5,6]); -When the Lagrangian has the form L(x, v) − λ u, by solving the associated Carathéodory equation (9) directly, one gets the representation formula for the associated viscosity solutions immediately ( [19,40,43,13]). The representation formula bridges the PDE aspects of the problem with the dynamical ones; -Consider a family of Lagrangians in the form {L(x, v) + ∑ k i=1 a i j u i }, a problem of Herglotz' variational principle in the vector form is closely connected to certain stochastic model of weakly coupled Hamilton-Jacobi equations (see, for instance, [20,23,38]).…”