I am documenting a piece of software for which the user uses an equation to determine an input value where one of the variables, $x$ may be a natural number in the interval (or set?) $[16, 31]$.
What is the best way to denote this?
I could use $x \in \Bbb{N}\,?\,[16,\,31]$ but should there be something where the $?$ is?
Or I might use $x \in [16,\,31]$ but then is this open to non-natural numbers? Another option that suffers a similar problem is using $16\leq{x}\leq31$.
I can't decide between them and feel that this means there should be a better way. Is there?
If I recall correctly, "$[16 . . 31]$" is reasonably common notation for this. See the end of this section of the wiki page; alternatives include "$\{16 .. 31\}$" and "$16..31$".
Alternatively, you can always write "$\mathbb{Z}\cap [16, 31]$".