\documentclass{article}
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\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[Sum of Arithmetic Series]

Let $(a_i)_{i \geq 0}$ be an arithmetic sequence with common difference $d$.
Then for some $n \in \mathbb{N}$,
$$\sum_{i=0}^n a_i = \frac{(n + 1)(a_0 + a_n)}{2}.$$

\end{theorem}

\begin{proof}

Common.Sequence.Arithmetic.sum\_recursive\_closed

\end{proof}

\end{document}
