Given a sequence $x$, what is the name/notation for the unary operator that operates on $x$ to express the number of elements in $x$?
- e.g. If $x = (x_1, x_2, x_3)$, then the unary operator operating on $x$ would yield $3$.
- e.g. If $x = (x_1, x_2, x_3, ... \infty)$, then the unary operator operating on $x$ would yield $\infty$.
What I am not asking:
- the length of a vector $||v||$
- the dimension of a vector space $V$
Analogy for what I am asking:
The unary operator in question operates similar to how the cardinality operator operates on a set variable (e.g. if $\ X = \{4, 9, 3\}$, then $(|X| = 3)\equiv true$.)
Edit: 08 July 2023
Updated the title and body of this question to request the notation/name of the operator.