docs: public double[][] getD() {
      double[][] D = new double[n][n];
      for(int i = 0; i < n; i++) {
        final double e_i = e[i];
        final double[] D_i = D[i];
        D_i[i] = d[i];
        if(e_i > 0) {
          D_i[i + 1] = e_i;
        }
        else if(e_i < 0) {
          D_i[i - 1] = e_i;
        } // else: e_i = 0
      }
      return D;
    }