Arithmetic sequence

An arithmetic sequence (or arithmetic progression) is a (finite or infinite) sequence of (real or complex) numbers such that the difference of consecutive elements is the same for each pair.

Examples for arithmetic sequences are
 * 2, 5, 8, 11, 14, 17 (finite, length 6: 6 elements, difference 3)
 * 5, 1, &minus;3, &minus;7 (finite, length 4: 4 elements, difference &minus;4)
 * 1, 3, 5, 7, 9, ... (2n &minus; 1), ... (infinite, difference 2)

Mathematical notation
A finite sequence
 * $$ a_1,a_2,\dots,a_n = \{ a_i \mid i=1,\dots,n \}         = \{ a_i \}_{i=1,\dots,n} $$

or an infinite sequence
 * $$ a_0,a_1,a_2,\dots = \{ a_i \mid i\in\mathbb N \}         = \{ a_i \}_{i\in\mathbb N} $$

is called arithmetic sequence if
 * $$ a_{i+1}-a_i = d $$

for all indices i. (The index set need not start with 0 or 1.)

General form
Thus, the elements of an arithmetic sequence can be written as
 * $$ a_i = a_1 + (i-1)d $$

Sum
The sum (of the elements) of a finite arithmetic sequence is
 * $$ a_1 + a_2 +\cdots+ a_n = \sum_{i=1}^n a_i = (a_1 + a_n){n \over 2} = na_1 + d {n(n-1) \over 2}$$