(4 points) Bonus question: Let Σ be an alphabet, and
f(A,B) = {w in Σ* | there exists x, y, z, t in Σ* such that w = xy = zt and x in A and z in B}.
Show that regular languages are closed under f.
4. (4 points) Bonus question: Let Z be an alphabet, and
f(A,B) = {w ∈ Z* | ∃x, y, z, t ∈ Z*. w = xy = zt ∧ x ∈ A ∧ z ∈ B}
Show that regular languages are closed under f.