OFFSET
3,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 3..100
Eric Weisstein's World of Mathematics, Chromatic Invariant
Eric Weisstein's World of Mathematics, Crown Graph
FORMULA
a(n) = Sum_{k=2..2*n} Sum_{j=0..n} Sum_{i=0..k-j} (-1)^k*(k-2)!*binomial(n, j)*Stirling2(n-j, i)*Stirling2(n-j, k-j-i). - Andrew Howroyd, Apr 22 2018
MATHEMATICA
Table[Sum[(-1)^k (k - 2)! Binomial[n, j] StirlingS2[n - j, i] StirlingS2[n - j, k - j - i], {k, 2, 2 n}, {j, 0, n}, {i, 0, k - j}], {n, 3, 20}] (* Eric W. Weisstein, Apr 23 2018 *)
PROG
(PARI) a(n)={sum(k=2, 2*n, (-1)^k*(k-2)!*sum(j=0, min(n, k), binomial(n, j)*sum(i=0, k-j, stirling(n-j, i, 2)*stirling(n-j, k-j-i, 2))))} \\ Andrew Howroyd, Apr 22 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Eric W. Weisstein, Nov 16 2017
EXTENSIONS
Terms a(10) and beyond from Andrew Howroyd, Apr 22 2018
STATUS
approved