betac</a>(v2
                    / 2, v1 / 2, v2 / (v2 + v1 * x))
                  </p>
                  <p>
                    The first is used for v1 * x &gt; v2, otherwise the second is
                    used.
                  </p>
                  <p>
                    The aim is to keep the <span class="emphasis"><em>x</em></span> argument to <a class="link" href="../../sf_beta/ibeta_function.html" title="Incomplete Beta Functions">ibeta</a> well
                    away from 1 to avoid round