frac {1} {a}.\]</div>
<p>We take into account external focusing, assuming a superposition of fields (it is not always true, for example, in nonlinear media this is not true), we obtain:</p>
<div class="math notranslate nohighlight">
\[a '' + k (z) a -\displaystyle\frac {2I} {I_a (\beta\gamma) ^ 3}\frac {1} {a} = 0,\]</div>
<p>which resembles the equation of the envelope:</p>
<div class="math notranslate nohighlight">
\[w '' + kw -\displaystyle\frac {1} {w ^ 3} = 0,\]</div>
<p>where <span class="math notrans