How to insert the "X is a subset of Y, but is not equal to Y" symbol? [duplicate]

Question

How can I insert the "X is a subset of Y, but is not equal to Y" symbol? I have tried using $\subsetneq$, as in $X \subsetneq Y$, but this yields the error message:

Undefined control sequence.

There is no problem using $\subseteq$.

Code:

\title{Subset-Not-Equal-To}
\author{}
\date{}

\documentclass[8pt]{article}

\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amsthm}

\begin{document}
\maketitle

$X \subsetneq Y$
\end{document}

Answer 1

You have to load the amssymb package. Here is a possible solution:

\documentclass{article}

\usepackage{amssymb}

\begin{document}

Type ``\verb|$A \subsetneq  B$|'' to get ``$A \subsetneq B$''.

Type ``\verb|$A \subsetneqq B$|'' to get ``$A \subsetneqq B$''.

\end{document}