 class="math inline">\(c\)</span>.</p></li>
<li><p>If <code>loss='tukey'</code> then <span class="math inline">\(f\)</span> is Tukey’s byweight function given by
<span class="math display">\[
l_{c}(r) = \left\{ \begin{array}{l}
r^2
&amp; \text{for} \,\, r \leq c
\\
c^2
&amp; \text{otherwise}
\end{array} \right\}
\]</span> which is quadratic/constant in the residuals below/above the
threshold determined by <span class="math inline">\(c\)</span>.</p></li>
</ol>
<p>In practice a smooth approximation to both 