se estimates are plugged into <span class="arithmatex">\(\eqref{eq:18-6a}\)</span>, which requires the inversion of a <span class="arithmatex">\(p\times p\)</span> singular matrix <span class="arithmatex">\(\hat\Sigma\)</span> when <span class="arithmatex">\(p\gg N\)</span>. Regularized overcomes the issue by replacing <span class="arithmatex">\(\hat\Sigma\)</span> with </p>
<div class="arithmatex">\[\begin{equation}
    \hat\Sigma(\gamma) = \gamma\hat\Sigma + (1-\gamma)\text{diag}(\hat\Sigma),  \non
\end{e