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 $a_{1}$ and the common difference is $d$, then the $n$-th term of the sequence ($a_{n}$) is given by:

$a_{n}=a_{1}+(n-1)d$

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.