Higher dimensionality

It is natural to think of a recursive three-dimensional low-pass dip filter as the functional form  

$${\alpha \over \alpha \ +\ { k_x^2 \ +\ k_y^2 \over -\,i\,\omega} }$$ (25)

This, however, leads to an infeasible Crank-Nicolson situation. Multidimensional low-pass filtering is possible with  

$$\left( {\alpha_x \over \alpha_x \ +\ { k_x^2 \over -\,i\,\om... ..._y \over \alpha_y \ +\ { k_y^2 \over -\,i\,\omega} } \ \right)$$ (26)