Exercise 2.31 (Monotone Functions). Which functions of the given set are monotone? Check the increasing as well as the decreasing possibility.

1 f1 = (x → (x → y)) → (y → z);

2 f2 = x1x2 ⊕ x1x3 ⊕ x1x4 ⊕ x2x3 ⊕ x2x4 ⊕ x3x4;

3 f3 = (0000000010111111);

4 f4 = (0001010101010111).