Exercise 2.28 (Dual and Self-Dual Functions). Let be given the following functions:

f1 = (11100111);

f2 = (01110001);

f3 = (11001101);

f4 = x1x2 ∨ x2(x3 ⊕ x4).

Find for each function its dual function! Is one of these functions a self-dual function?