SEQUENCES OF REAL NUMBERS. II.1. Definition 1. A sequence is a real-valued function f whose domain is the set positive integers. (N). The numbers f(1), f(2), · · · are called the terms of the sequence. f(1) ≡ s1, f(2) ≡ s2, · · · , f(n) ≡ sn, · · · . In discussing sequences the subscript notation is much more common than functional notation.