Partial differential equations of the form $$k{\partial \over {\partial t}}u(r,t) = \nabla ^2 u(r,t)$$ (diffusion equation) and $${{\partial ^2 } \over {\partial t^2 }}u(r,t) = c^2 \nabla ^2 u(r,t)$$ (wave equation) are amenable to the use of the Laplace transform.1 Indeed, on taking the Laplace transform of the former, we get. $$ \left( {kpI - \nabla ^2 } \right)U\left( {r,p} \right) = ku\left( {r,0} \right). $$