Say $f(x,y)$ follows some PDE such that no nontrivial solutions exist of the form:
$$f(x,y) =X(x)Y(y) $$
and
$$f(x,y) = X(x)+ Y(y)$$
The first one refers to separable and non-separable PDEs; but what about the second one? Is there a name for the combination of these?
