docs: public double[][] getU() {
          double[][] X = new double[n][n];
          double[][] U = X;
          for (int i = 0; i < n; i++) {
              for (int j = 0; j < n; j++) {
                  if (i <= j) {
                      U[i][j] = LU[i][j];
                  } else {
                      U[i][j] = 0.0;
                  }
              }
          }
          return X;
      }