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1. Functions A function f from X to Y (in symbols f : X ? Y) is a relation from X to Y such that Dom(f) = X and if two pairs (x,y) and (x,y) ? f, then y = y Example: Dom(f) = X = {a, b, c, d}, Rng(f) = {1, 3, 5} f(a) = f(b) = 3, f(c) = 5, f(d) = 1.