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This question already has an answer here:

I need this symbols used by Boole and Peirce, any ideas?

enter image description here

marked as duplicate by egreg symbols Apr 2 at 21:42

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

5
\documentclass{article}
\newcommand\inclusion{\ooalign{$-$\cr$<$}}
\begin{document}
$\inclusion$
\end{document}

enter image description here

Depending on how it is used, you can wrap it in a \mathrel for example.

If you need it across math styles:

\documentclass{article}
\usepackage{scalerel}
\newcommand\inclusion{\mathrel{\ThisStyle{\ooalign{$\SavedStyle-$\cr$\SavedStyle<$}}}}
\begin{document}
$\inclusion_{\inclusion_{\inclusion}}$

$ X\inclusion B$
\end{document}

enter image description here

Look, Mom! I didn't use stackengine!!

4

The ⪪ symbol is U+2AAA in Unicode, and \smt in the unicode-math package. It’s not present in the default Latin Modern Math font, but it is in several others.

\documentclass[varwidth]{standalone}
\usepackage{unicode-math}

\defaultfontfeatures{Scale=MatchLowercase}
\setmathfont{STIX Two Math}

\begin{document}
\( S \smt T \)
\end{document}

S ⪪ T

In legacy PDFTeX documents, it’s available from the stix or stix2 package.

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