Let $X$ be a list of real numbers. The length of $X$ is finite, i.e., $|X|=m<\infty$.
The largest element of $X$ can be denoted by $\max X$.
My question is
Is there any mathematical notation that represents $n\leq m$ largest elements of $X$? or is there any mathematical way to represent the sublist composed of the $n$ largest elements of $X$?
Yes. If you use a subscript in parentheses, that indicates order. For example, if $X_1 = 5, X_2 = 7, X_3 = 1, X_4 = 6$, then $X_{(1)} = 1, X_{(2)} = 5, X_{(3)} = 6,$ and $X_{(4)} = 7$. The notation is used for order statistics in probability theory. So the largest k items in a list of N times would be $X_{(n-k+1)}, X_{(n-k+2)}, ..., X_{(n)}.$