Editing Sequence (section)

===Subsequences===
A '''[[subsequence]]''' of a given sequence is a sequence formed from the given sequence by deleting some of the elements without disturbing the relative positions of the remaining elements. For instance, the sequence of positive even integers (2, 4, 6, ...) is a subsequence of the positive integers (1, 2, 3, ...). The positions of some elements change when other elements are deleted. However, the relative positions are preserved.

Formally, a subsequence of the sequence <math>(a_n)_{n\in\mathbb N}</math> is any sequence of the form <math display=inline>(a_{n_k})_{k\in\mathbb N}</math>, where <math>(n_k)_{k\in\mathbb N}</math> is a strictly increasing sequence of positive integers.