Looking for an “S” symbol

Question

I can’t figure out what this

symbol is. It is used in the context of “column space of a matrix X”. It is not one of the Greek letters and my guess is that it is just an ordinary “S” with some sort of text style. I have looked through all common ones but could not find an appropriate one.

Answer 1

I can't decide whether this is a hit or a miss (yours is a little thinner). It looks like the item is span (or initial of some other word).

\documentclass[border=3mm]{standalone}
\usepackage[mathscr]{euscript}
\begin{document}
$\mathscr{S}$
\end{document}

Personal rant : Math with text decorations sucks big time. Notation is for the reader not for the author's Gaussian fantasies (patent pending).