Therefore, the range set can have at the most two elements of the
co-domain {1, 2, 3}, showing that f  is not onto, a contradiction Hence, f must be one-one Example 14 Show that a one-one function f : {1, 2, 3} → {1, 2, 3} must be onto Solution Since f is one-one, three elements of {1, 2, 3} must be taken to 3 different
elements of the co-domain {1, 2, 3} under f