rix)</code> Cartan matrix from Coxeter matrix</p><p>The  argument should be the Coxeter matrix  <code>M</code> for a Coxeter group <code>W</code> and the   result  is  the  Cartan  Matrix   <code>C</code>  for  the  standard  reflection representation  of <code>W</code>. We have <code>C[s,t]=-2cos(œÄ/M[s,t])</code>, where <code>M[s,s]==1</code> and  by  convention  <code>œÄ/M[s,t]==0</code>  if  <code>M[s,t]==‚àû</code>,  which we represent by <code>M[s,t]==0</code>.  Since  <code>M</code>  is  symm