docs: public Matrix getD()
      {
          Matrix X = new DenseMatrix(n, n);
          for (int i = 0; i < n; i++)
          {
              X.set(i, i, d[i]);
              
              if (e[i] > 0)
                  X.set(i, i+1, e[i]);
              else if (e[i] < 0)
                  X.set(i, i-1, e[i]);
          }
          return X;
      }