em></span>
      </p>
<p>
        Requires <span class="emphasis"><em>-1 &lt;= k &lt;= 1</em></span> otherwise returns the result
        of <a class="link" href="../error_handling.html#math_toolkit.error_handling.domain_error">domain_error</a>
        (outside this range the result would be complex).
      </p>
<p>
        The final <a class="link" href="../../policy.html" title="Chapter&#160;18.&#160;Policies: Controlling Precision, Error Handling etc">Policy</a> argument is optional and can
        be us