Created the Abstract Algebra theorems and definitions cheat sheet

part-2/chapters/chapter-9/normal-subgroups.tex

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+\section{Normal Subgroups}
+
+\begin{definition}[Normal Subgroup]
+	A subgroup $H$ of a group $G$ is called a \textit{normal} subgroup of $G$ if $aH = Ha$ for all $a$ in $G$. We denote this by $H \triangleleft G$.
+\end{definition}
+
+\begin{theorem}[Normal Subgroup Test]
+	A subgroup $H$ of $G$ is normal in $G$ if and only if $xHx^{-1} \subseteq H$ for all $x$ in $G$.
+\end{theorem}