\documentclass{article}

\input{../../../preamble}

\newcommand{\link}[1]{\lean{../../..}
  {Common/Real/Sequence/Arithmetic} % Location
  {Real.Arithmetic.#1} % Fragment
  {Real.Arithmetic.#1} % Presentation
}

\begin{document}

\section{\proceeding{Sum of Arithmetic Series}}%
\hyperlabel{sec:sum-arithmetic-series}%

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

\begin{proof}

  \link{sum\_recursive\_closed}

\end{proof}

\end{document}