(j * (j + 1)) / 2 + j0 < ((i + 1) * (i + 2)) / 2, 0 ≤ j0, 0 ≤ i, j0 ≤ j, (i * (i + 1)) / 2 ≤ (j * (j + 1)) / 2 + j0 ⊦ i = j