Concept history

Concept created

Let $(a_{n})_{n\in \N}$ be a [[Sequence|sequence]] in a set $X$. A subsequence of $(a_{n})$ is a sequence of the form
$$(a_{n_{k}})_{k\in \N}$$
where $(n_{k})$ is a [[strictly increasing sequence|strictly increasing sequence]] of [[Natural number|natural numbers]].