TITLE: Mathematical expression for $n$ largest values in a set
QUESTION [13 upvotes]: I would like to know how to express mathematically  $n$ largest values of a given set $A$. 
For example: I have an unsorted set $A$ with $100$ values $\in \mathbb{N}$ and want to get a new set $B$ with the $5$ largest values.
Thank you in advance!

REPLY [19 votes]: $B={\rm argmax}_{A'\subset A, |A'|=n}\sum_{a\in A'} a $