The finite multiple harmonic sum over strictly decreasing indices
MultipleHarmonicNumber[n]
MultipleHarmonicNumber[n, {r1, …, rk}]
MultipleHarmonicNumber[n, {r1, …, rk}, {s1, …, sk}]
MultipleHarmonicNumber[3, {2, 1}]
→ 5/12MultipleHarmonicNumber[6, {1}]
→ 49/20