%I #15 Apr 23 2018 10:31:34
%S 2,1,11,362,21234,1965624,264398280,48773612880,11824686110160,
%T 3646938237505920,1394586705296776320,647624841502298284800,
%U 359025601255648673068800,234214938700483636606233600,177617896085186117264114611200,154944426571409730022474894387200
%N Chromatic invariant of the n-cocktail party graph.
%H Andrew Howroyd, <a href="/A295166/b295166.txt">Table of n, a(n) for n = 1..100</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ChromaticInvariant.html">Chromatic Invariant</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/CocktailPartyGraph.html">Cocktail Party Graph</a>
%F a(n) = Sum_{k=0..n} binomial(n,k)*(-1)^(n+k)*(n+k-2)! for n > 1. - _Andrew Howroyd_, Apr 22 2018
%t 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 *)
%t seq = Join[{2}, Table[-Gamma[n - 1] HypergeometricU[n - 1, 2 n, -1], {n, 2, 20}]] (* _Eric W. Weisstein_, Apr 23 2018 *)
%o (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
%K nonn
%O 1,1
%A _Eric W. Weisstein_, Nov 16 2017
%E a(16) from _Andrew Howroyd_, Apr 22 2018