Let f,g, and h be some functions and x and y be scalar variables. The functions are related in the following way:
f(x) + g(y) = h(x-y)
The question is: can functions f, g, and h be anything besides linear functions (f(x)=a*x+b, g(y)=-a*y+c, h(x)=a*x+b+c)? In other words, is there another solution to the functional equation besides the linear one?