Select each relation R ⊆ R × R which equals a function f such that y = f(x) for all x in its domain.
1) R = {(x, y) | 2x - y = 1}
2) R = {(x, y) | x² + y² = 13}
3) R = {(4, 3), (3, 2), (2, 3)}
4) R = {(1, 2), (2, 3), (2, 4)}