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+\documentclass{article}
+\begin{document}
+The union of two sets $\mathcal{A}$ and $\mathcal{B}$ is the set of elements
+that are in at least one of the two sets,  and is designated as
+$\mathcal{A\cup B}$. This operation is commutative
+$\mathcal{A\cup B = B\cup A}$ and associative $\mathcal{(A\cup B)\cup C =
+A\cup(B\cup C)}$.  If $\mathcal{A\subseteq B}$, then
+$\mathcal{A\cup B = B}$. It then follows that $\mathcal{A\cup A = A}$,
+$\mathcal{A\cup\{\emptyset\} = A}$ and $\mathcal{J\cup A = J}$. 
+
+\end{document}