Definition:Arithmetic Sequence

An arithmetic progression is a sequence $$\left \langle{a_n}\right \rangle$$ in $$\R$$ defined as:
 * $$a_n = a_1 + \left({n-1}\right) d$$ for $$n = 1, 2, 3, \ldots$$

where:
 * $$a_1 \in \R$$ is called the initial term;
 * $$d \in \R: d \ne 0$$ is called the common difference.

Thus its general form is:
 * $$a_1, a_1 + d, a_1 + 2 d, a_1 + 3 d, \ldots$$