Huygens' principle and Hadamard's conjecture
The present article explains the meaning of Huygens' principle for linear partial differential equations of normal hyperbolic type. The different behaviour of the solutions of such an equation with an odd or an even number of independent variables is pointed out and the famous criterion of J. Hadamard is reproduced. Hadamard also posed the problem to find all differential equations of normal hyperbolic type for which Huygens' principle is valid. The author describes the actual state of the efforts to solve this problem.
scientific article; zbMATH DE number 4204950
scientific article