\[\frac{{\partial}^{2}}{{\partial s}^{2}}\Phi^{(\mathrm{U})}\left(s,t;\mathbf{x}%
\right)\frac{{\partial}^{2}}{{\partial t}^{2}}\Phi^{(\mathrm{U})}\left(s,t;%
\mathbf{x}\right)-\left(\frac{\,{\partial}^{2}}{\,\partial s\,\partial t}\Phi^%
{(\mathrm{U})}\left(s,t;\mathbf{x}\right)\right)^{2}=0.\]