Notation for indexing an indexed sequence. I have a notation question. Say I have the following:
$S = \{a_{1}, \ldots, a_{n}\}$ a finite list of integers (order matters).
What I really want is to express that now I pick only $K$ elements, in any order.
So, I came up with this idea $S'  =  \{a_{i_1}, \ldots, a_{i_k}\}$ where $i$ ranges from 1 to $n$. Is it somehow correct?
Thanks in advance!

$$S=\{a_1,...,a_n\}, n\in \mathbb N$$
$$S'=\{b_1,...b_k\}, k\in \mathbb N$$
$$S'\subseteq S$$
This is my approach, don't know for sure this is the best way. $S'\subseteq S$ means, $S'$ is a subset from $S$.
