\displaystyle \tilde x '' + \frac {1} {\beta ^ 2 \gamma} \gamma '\tilde x' + \frac {1} {2 \beta ^ 2 \gamma} \gamma '' \tilde x + k_s \tilde x = 0,
   \\
   \displaystyle \tilde y '' + \frac {1} {\beta ^ 2 \gamma} \gamma '\tilde y' + \frac {1} {2 \beta ^ 2 \gamma} \gamma '' \tilde y + k_s \tilde y = 0,
 \end{cases}
\end{equation*}\end{split}\]</div>
</div>
<div class="section" id="Literature">
<h2>Literature<a class="headerlink" href="#Literature" title="Permalink to this headline">¶</a></h2>
<ol cla