0,3

a(n) = Sum[n!^2 / k!(n-k)!^2 {k=1...n}]

a(n) = Sum[P(n, k)C(n, k) {k=1...n}]

a(2) = 6 because A002720(2) = 7 and 7 - 1 = 6.

Sequence in context: A012718 A006630 A093964 this_sequence A012522 A012326 A112903

Adjacent sequences: A097659 A097660 A097661 this_sequence A097663 A097664 A097665

nonn

Ross La Haye (rlahaye(AT)new.rr.com), Sep 20 2004