In an n-particle system, the number of distinct unordered pairs is the binomial coefficient C(n,2) = n·(n-1)/2 — the leading interaction-channel count for pairwise-interaction Hamiltonians. Small cases: n=2 → 1 pair, n=3 → 3, n=4 → 6, n=5…
In an n-particle system, the number of distinct unordered pairs is the binomial coefficient C(n,2) = n·(n-1)/2 — the leading interaction-channel count for pairwise-interaction Hamiltonians. Small cases: n=2 → 1 pair, n=3 → 3, n=4 → 6, n=5…