Is there notation for subset containing the n smallest (or largest)? Suppose I have a discrete set $A=\{a_1,\ldots,a_k\}$ and I wish to write an expression for  the minimum of this set. I would write:
$$
\min \{a_1,\ldots,a_k\}
$$
But what if rather than just an element, I want the subset of $A$ containing the $n$ smallest elements of $A$? How would I write that?

I don't think there is a standard notation. It is likely to be most clear if you define your own notation in words before using it. Clear notation that springs to mind for me would be $\min_n (A)$ or $\min^{(n)} (A)$, but it needs explanation.
You can define the concept using only existing notation, but it is perhaps less clear than using words:
$$a_1=\min(A) \\ a_{i+1}=\min(A\setminus\{a_1,\dots,a_i\})\ \text{for}\ i\ge 1\\ \text{min}_n(A)=\{a_1,\dots,a_n\}$$
David Sheard's comment suggests another definition.
