pan class="math notranslate nohighlight">\(\beta\)</span> function is greater, and in the vicinity of the focus, the influence of the space charge can be neglected.</p>
</div>
<div class="section" id="Bush’s-Theorem">
<h2>Bush’s Theorem<a class="headerlink" href="#Bush’s-Theorem" title="Permalink to this headline">¶</a></h2>
<p>As an addition to the derivation of the equation of the envelope of the beam, we prove an important auxiliary theorem called the Bush theorem. It connects the angular velocity