1. arithmetic progression refers to a series in which the difference between each term and its previous term is equal to the same constant from the second term, usually expressed by a and p. This constant is called arithmetic progression's tolerance and often expressed by the letter D.

2. Sequence is a function with positive integer set (or its finite set) as its domain, and it is an ordered number. Every number in a series is called an item in this series. The number ranked first is called 1 item of this series (usually also called the first item), the number ranked second is called the second item of this series, and so on, and the number ranked n is called the n item of this series, usually represented by an.