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diff --git a/vorlesungen/punktgruppen/script.tex b/vorlesungen/punktgruppen/script.tex new file mode 100644 index 0000000..2a6d95c --- /dev/null +++ b/vorlesungen/punktgruppen/script.tex @@ -0,0 +1,63 @@ +\documentclass[a4paper]{article} + +\usepackage[cm]{manuscript} +\usepackage{xcolor} + +\newcommand{\scene}[1]{\noindent[ #1 ]\par} +\newenvironment{totranslate}{\color{blue!70!black}}{} + +\begin{document} + +\section{Intro} + +\section{Geometrie} +\begin{totranslate} +We'll start with geometric symmetries as they are the simplest to grasp. + +\scene{Intro} + To mathematically formulate the concept, we will think of symmetries as + actions to perform on an object, like this square. The simplest action, is to + take this square, do nothing and put it back down. Another action could be to + flip it along an axis, or to rotate it around its center by 90 degrees. + +\scene{Cyclic Groups} + Let's focus our attention on the simplest class of symmetries: those + generated by a single rotation. We will gather the symmetries in a group + \(G\), and denote that it is generated by a rotation \(r\) with these angle + brackets. + + Take this pentagon as an example. By applying the rotation \emph{action} 5 + times, it is the same as if we had not done anything, furthermore, if we + \emph{act} a sixth time with \(r\), it will be the same as if we had just + acted with \(r\) once. Thus the group only contain the identity and the + powers of \(r\) up to 4. + + In general, groups with this structure are known as the ``Cyclic Groups'' of + order \(n\), where the action \(r\) can be applied \(n-1\) times before + wrapping around. + + % You can think of them as the rotational symmetries of an \(n\)-gon. + +\scene{Dihedral Groups} + Okay that was not difficult, now let's spice this up a bit. Consider this + group for a square, generated by two actions: a rotation \(r\) and a + reflection \(\sigma\). Because we have two actions we have to write in the + generator how they relate to each other. + + Let's analyze this expression. Two reflections are the same as the identity. + Four rotations are the same as the identity, and a rotation followed by a + reflection, twice, is the same as the identity. + + This forms a group with 8 possible unique actions. This too can be generalized + to an \(n\)-gon, and is known as the ``Dihedral Group'' of order \(n\). +\end{totranslate} + +\scene{Symmetrische Gruppe} +\scene{Alternierende Gruppe} + +\section{Algebra} + +\section{Krystalle} + +\end{document} +% vim:et ts=2 sw=2: |