Arithmetic progression

In mathematics, an arithmetic progression, or arithmetic sequence, is a sequence. The difference between each two terms which follow one another is always the same. This is different from a geometric progression, where the ratio of change between two terms is constant.
If the initial term of an arithmetic progression is <math>a_1</math> and the common difference is <math>d</math>, then the <math>n</math>-th term of the sequence (<math>a_n</math>) is given by:

<math>a_n = a_1 + (n - 1)d</math>

A finite portion of an arithmetic progression is called a finite arithmetic progression and sometimes just called an arithmetic progression. The sum of a finite arithmetic progression is called an arithmetic series.