\documentclass{article}
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\begin{document}

\section{Integration and Summation}
\[ \sum_{a=0}^\infty f_a(c^a) \]
\[ \sum_{a=0}^\infty \sum_{b=-a}^a f_a(c^a) g_b(c^b) \]
\[ \sum_{a=0}^\infty \sum_{b=-a}^a \lim_{c\rightarrow 0} f_a(c^a) g_b(c^b) \]

\section{Operators}
\[  D a \]
\[  D f \]
\[  D f(a) \]
\[  D G H f \]
\[  D G H f (a) \]
\[  D (a) (b) \]

\end{document}
