this code
$X = <X_1 ...,X_n>$
gives
how to have angle brackets render like this
Another option it is to use bracket package. Here I'm putting a MWE with the fonts newtxtext (Times New Roman) for the text and newtxmath for mathematical symbols of your image.
\documentclass[a4paper,12pt]{article}
\usepackage{newtxtext,newtxmath}
\usepackage{braket}
\usepackage{blindtext}
\begin{document}
\blindtext
\[X=\braket{X_1,\ldots,X_n}\]
\blindtext
\end{document}
<and>in math mode are regular relational symbols. You must use\langleand\rangle, respectively. – ivankokan Oct 24 '19 at 13:28\ldotsinstead of...Regards. – Sebastiano Oct 24 '19 at 14:07\dotsc(AMS) for "dots after comma". – ivankokan Oct 24 '19 at 14:53\dotscan be used, instead of\dotsc; the result is the same in this case. See $x_1, \dotsc, x_n$ vs. $x_1$, $\dots$, $x_n$. Also for\dotscthe packageamsmathis needed. – dexteritas Oct 24 '19 at 15:40