# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a295166 Showing 1-1 of 1 %I A295166 #15 Apr 23 2018 10:31:34 %S A295166 2,1,11,362,21234,1965624,264398280,48773612880,11824686110160, %T A295166 3646938237505920,1394586705296776320,647624841502298284800, %U A295166 359025601255648673068800,234214938700483636606233600,177617896085186117264114611200,154944426571409730022474894387200 %N A295166 Chromatic invariant of the n-cocktail party graph. %H A295166 Andrew Howroyd, Table of n, a(n) for n = 1..100 %H A295166 Eric Weisstein's World of Mathematics, Chromatic Invariant %H A295166 Eric Weisstein's World of Mathematics, Cocktail Party Graph %F A295166 a(n) = Sum_{k=0..n} binomial(n,k)*(-1)^(n+k)*(n+k-2)! for n > 1. - _Andrew Howroyd_, Apr 22 2018 %t A295166 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 A295166 seq = Join[{2}, Table[-Gamma[n - 1] HypergeometricU[n - 1, 2 n, -1], {n, 2, 20}]] (* _Eric W. Weisstein_, Apr 23 2018 *) %o A295166 (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 A295166 nonn %O A295166 1,1 %A A295166 _Eric W. Weisstein_, Nov 16 2017 %E A295166 a(16) from _Andrew Howroyd_, Apr 22 2018 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE