OFFSET
3,1
LINKS
Alois P. Heinz, Table of n, a(n) for n = 3..1000
Vaclav Kotesovec, Recurrence (of order 9)
FORMULA
a(n) ~ 3^n/6 * (1 + 3*sqrt(3/(Pi*n))/2+sqrt(3)*(1+2*cos(2*Pi*n/3))/(Pi*n)). - Vaclav Kotesovec, Aug 29 2014
EXAMPLE
a(4) = 12: aabc, aacb, abac, abca, acab, acba, baac, baca, bcaa, caab, caba, cbaa.
MATHEMATICA
Table[Sum[n!/Product[IntegerPartitions[n, {3}][[k, j]]!, {j, 1, 3}], {k, 1, Length[IntegerPartitions[n, {3}]]}], {n, 3, 30}] (* Vaclav Kotesovec, Aug 29 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jun 21 2013
STATUS
approved