\documentclass{article}

\input{preamble}

\newcommand{\link}[1]{\lean{../../..}{Bookshelf/Real/Sequence/Arithmetic}
  {Real.Arithmetic.#1}}

\begin{document}

\section*{Sum of Arithmetic Series}%
\label{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}