J. P. Kormes "The solution of the differential equation $\left( {{{a^2 \partial ^2 } \mathord{\left/ {\vphantom {{a^2 \partial ^2 } {\partial t^2 - \Delta }}} \right. \kern-\nulldelimiterspace} {\partial t^2 - \Delta }}} \right)\left( {{{\partial ^2 } \mathord{\left/ {\vphantom {{\partial ^2 } {\partial t^2 - \Delta }}} \right. \kern-\nulldelimiterspace} {\partial t^2 - \Delta }}} \right)u = f\left( {x,y,z,t} \right)$ by Hadamard's method," Bulletin of the American Mathematical Society, Bull. Amer. Math. Soc. 50(12), 842-855, (December 1944)