Mathematical notation of substring

Question

Let $\{a_1,a_2,\dots,a_n\}$ be an ordered set.

Is there any established mathematical notation for the first $k$ (resp. last $k$ ) elements of this ordered set?

This corresponds to the computer science equivalent of "substring".

Thanks.

Answer 1

You could say the following. Let $a_1 < a_2 < \ldots < a_n$ be a chain. For $1 \leqslant k \leqslant n$, the chain $a_1 < a_2 < \ldots < a_k$ is called the prefix of length $k$ of the original chain. Its suffix of length $k$ is the chain $a_{n-k + 1} < \ldots < a_n$.