A form of reachability analysis named for this equation can be used to formally verify dynamical systems. Global approaches based on this equation are considered dual to the Pontryagin maximum principle. Level sets to solutions to this equation are analogized to wavefronts in the optico-mechanical analogy. The value function of an optimal control problem (15[1])satisfies an extension of this equation (-5[1])that is the continuous (-5[1])analogue of (*) Bellman’s equation in dynamic programming. This equation is obtained when assuming the wavefunction is “e to the iS over h-bar” in Schrödinger’s equation, then letting h-bar go (10[1])to zero. This equation’s solution is a namesake “principal function” symbolized S, whose (10[1])derivatives (-5[1])are the generalized momenta. For (-5[1])10 points, (-5[1])name this doubly-eponymous nonlinear PDE that underpins a formulation of (10[1])classical (-5[1])mechanics. (10[1])■END■ (10[1]0[3])