started with formula collection

.gitignore

@@ -45,3 +45,8 @@ build/**
 cliparser_impl.*
 clilexer_impl.*
 stack.hh
+
+# Temporary tex files
+*.log
+*.aux
+*.pdf

doc/formulas/formulas.tex

+%
+% collection of useful formulas
+% @author Tobias Weber <tweber@ill.fr>
+% @date 13-jul-2018
+% @license see 'LICENSE' file
+%
+
+\documentclass{article}
+
+\usepackage[a4paper]{geometry}
+\geometry{tmargin=2.5cm, bmargin=2.5cm, lmargin=2cm, rmargin=2cm}
+
+
+\begin{document}
+Collection of useful formulas, T. Weber, July 13, 2018.
+
+
+\section{Scattering Triangle}
+
+
+\subsection*{Scattering Angle $a_4$}
+
+\begin{equation}
+\left| Q \right> = \left| k_i \right> - \left| k_f \right> \\
+\end{equation}
+\begin{equation}
+\left< Q | Q \right> = \left( \left< k_i \right| - \left< k_f \right| \right) \cdot \left( \left| k_i \right> - \left| k_f \right> \right)
+\end{equation}
+\begin{equation}
+\left< Q | Q \right> = \left< k_i | k_i \right> + \left< k_f | k_f \right> - 2 \left< k_i | k_f \right>
+\end{equation}
+\begin{equation}
+Q^2 = k_i^2 + k_f^2 - 2 k_i k_f \cos a_4
+\end{equation}
+\begin{equation}
+a_4 = \arccos \left( \frac{k_i^2 + k_f^2 - Q^2}{2 k_i k_f} \right)
+\end{equation}
+
+
+
+\subsection*{Rocking Angle $a_3$}
+
+
+\end{document}