I have the following statement to prove but not sure how to get started :
Let f: A → B and g: B → A be arbitrary functions. Show that there are subsets A1, A2 ⊆ A and B1, B2 ⊆ B such that : A1 ∪ A2 = A, A1 ∩ A2 = ∅ and B1 ∪ B2 = B, B1 ∩ B2 = ∅ and f(A1) = B1, g(B2) = A2.
I believe we need to use the notion of partial order here.
Thanks for your help.