: <math> f(x,y) = f_1(x) \cdot f_2(y) {\;\circ\!\!-\!\!\bullet\;} \overset{\sim}{f_1} (k_x) \dot \overset{\sim}{f_2} (k_y)</math> | : <math> f(x,y) = f_1(x) \cdot f_2(y) {\;\circ\!\!-\!\!\bullet\;} \overset{\sim}{f_1} (k_x) \dot \overset{\sim}{f_2} (k_y)</math> |