(**) \Longleftrightarrow \widehat{\Phi}^{-1}\left(x,\, y\right)\Phi\left(x,\, y\right)=C \Longleftrightarrow \widehat{\Phi}\left(y,\, x\right)\Phi\left(x,\, y\right)=C \mid \frac{\partial}{\partial x}