g(f(x,y) ,y) = k*f(g (x,y) ,y) ...



g and f functions on R*R , k a given constant <> 0 .
The idea is building pairs {g,f} such as
g(f(x,y) ,y) = k*f(g (x,y) ,y) is verified.
I've found solutions for the value k = 1

Alain
.