docs: private void hqr2BackTransformation(int nn, int low, int high) {
      for(int j = nn - 1; j >= low; j--) {
        final int last = j < high ? j : high;
        for(int i = low; i <= high; i++) {
          final double[] Vi = V[i];
          double sum = 0.;
          for(int k = low; k <= last; k++) {
            sum += Vi[k] * H[k][j];
          }
          Vi[j] = sum;
        }
      }
    }