Ordered, countable sets of numbers, x₁, x₂, x₃,…, not necessarily all different. In general such sets are called sequences, whereas the term progression is usually confined to the special types: the arithmetic, in which the difference x_k − x_k−1 between successive terms is constant; the geometric, in which the ratio x_k/x_k−1 is constant; and the harmonic, in which the reciprocals of the terms are in arithmetic progression.