I have a very long expression which can be reduced down quite a lot if I define the following functions
F[x_] := Sqrt[π] x Exp[x^2] Erfc[x] G[x_] := Sqrt[π] x Exp[x^2] Erf[x]
where the arguments $x$ that will appear are different (let's say $x1$, $x2$, $x3$...). The result is not a polynomial so I can't get
Collect to work. I know other people have asked about this but I tried what was suggested in the posts and I couldn't get it to work.
I thought that defining a rule like
Erfc[x] -> F[x]/(Sqrt[π] x Exp[x]) Erf[x] -> G[x]/(Sqrt[π] x Exp[x^2])
could help, but I didn't get it to work.