docs: private void updateRemoveR() {
          for( int i = 1; i < n+1; i++ ) {
              for( int j = 0; j < n; j++ ) {
                  double sum = 0;
                  for( int k = i-1; k <= j; k++ ) {
                      sum += U_tran.data[i*m+k] * R.data[k*n+j];
                  }
                  R.data[(i-1)*n+j] = sum;
              }
          }
      }