A100646 Denominator of Cotesian number C(n,2).

6, 8, 15, 144, 280, 640, 14175, 2240, 199584, 87091200, 875875, 22353408000, 5003856000, 229605376, 10854718875, 941525544960000, 1013940928000, 3064383995904000, 82324272054024, 2996771880960000, 255484332230400000, 809280523999877529600000, 5699209469078125

Offset: 2

Author

N. J. A. Sloane, Dec 05 2004

Keywords

• nonn
• frac

Examples

			1/6, 3/8, 2/15, 25/144, 9/280, 49/640, -464/14175, 27/2240, -16175/199584, -3237113/87091200, -105387/875875, -1737125143/22353408000, -770720657/5003856000, -25881785/229605376, ... = A100645/A100646 = A002179/A002176 (the latter not being in lowest terms)
		

References

• Charles Jordan, Calculus of Finite Differences, Chelsea 1965, p. 513.

Links

• Vincenzo Librandi, Table of n, a(n) for n = 2..200

Crossrefs

Cf. A100645.

See A002176 for further references. A diagonal of A100640/A100641.

Programs

Mathematica

cn[n_, 0] := Sum[n^j*StirlingS1[n, j]/(j+1), {j, 1, n+1}]/n!; cn[n_, n_] := cn[n, 0]; cn[n_, k_] := 1/n!*Binomial[n, k]*Sum[n^(j+m)*StirlingS1[k, j]* StirlingS1[n-k, m]/((m+1)*Binomial[j+m+1, m+1]), {m, 1, n}, {j, 1, k+1}]; a[n_] := Denominator[cn[n, 2]]; Table[a[n], {n, 2, 24}]  (* Jean-François Alcover, Oct 08 2013 *)