(K\)</span>-vector <span class="math inline">\(a\)</span>,</p>
<p><span class="math display">\[
a^{\top} x a &gt; 0.
\]</span></p>
<p>The number of free parameters required to specify a <span class="math inline">\(K \times K\)</span>
correlation matrix is <span class="math inline">\(\binom{K}{2}\)</span>.</p>
<p>There is more than one way to map from <span class="math inline">\(\binom{K}{2}\)</span> unconstrained
parameters to a <span class="math inline">\(K \times K\)</span> correlation matrix. Stan implem