OFFSET
1,1
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..100
Eric Weisstein's World of Mathematics, Chromatic Invariant
Eric Weisstein's World of Mathematics, Cocktail Party Graph
FORMULA
a(n) = Sum_{k=0..n} binomial(n,k)*(-1)^(n+k)*(n+k-2)! for n > 1. - Andrew Howroyd, Apr 22 2018
MATHEMATICA
Join[{2}, Table[Sum[Binomial[n, k] (-1)^(k + n) (n + k - 2)!, {k, 0, n}], {n, 2, 20}]] (* Eric W. Weisstein, Apr 23 2018 *)
seq = Join[{2}, Table[-Gamma[n - 1] HypergeometricU[n - 1, 2 n, -1], {n, 2, 20}]] (* Eric W. Weisstein, Apr 23 2018 *)
PROG
(PARI) a(n)={if(n<2, [2][n], sum(k=0, n, binomial(n, k)*(-1)^(n+k)*(n+k-2)!))} \\ Andrew Howroyd, Apr 22 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Eric W. Weisstein, Nov 16 2017
EXTENSIONS
a(16) from Andrew Howroyd, Apr 22 2018
STATUS
approved