e can use the <a href="http://functions.wolfram.com/ElementaryFunctions/ArcTanh/06/01/03/01/" target="_top">approximation</a>:
      </p>
<p>
        <span class="inlinemediaobject"><img src="../../../equations/atanh2.svg"></span>
      </p>
<p>
        Otherwise the <a href="http://functions.wolfram.com/ElementaryFunctions/ArcTanh/02/" target="_top">primary
        definition</a>:
      </p>
<p>
        <span class="inlinemediaobject"><img src="../../../equations/atanh1.svg"></span>
      </p>
<p>
        