Solutions To Mathematics Textbooks/Proofs and Fundamentals/Chapter 4

Exercise 4.1.1

(1), (3) and (6) are functions.

Exercise 4.1.2

The first one and the fourth one are functions (here I've assumed that "mother" and "father" were meant biologically, and that not everyone has precisely one person they'd call a "best friend").

Exercise 4.1.3

(i) and (iv) represent functions.

Exercise 4.1.4

(2) and (3) properly describe functions.