$$\Psi ^{(\mathrm {U} )}\left(\mathbf {x} \right)=\int _{-\infty }^{\infty }\int _{-\infty }^{\infty }\exp \left(i\Phi ^{(\mathrm {U} )}\left(s,t;\mathbf {x} \right)\right)\mathrm {d} s\mathrm {d} t$$