OFFSET
1,2
COMMENTS
Note that any such sequence has at least 3 balls and at most n+2.
LINKS
Jeremy Dover, Table of n, a(n) for n = 1..100
FORMULA
a(n) = n! * Sum_{k=3..n+2} (binomial(k,3)+3*binomial(k,4))/(n+2-k)!.
a(n)/n! ~ e*n^4/8. - Vaclav Kotesovec, Feb 03 2017
MATHEMATICA
Table[n! * Sum[(Binomial[k, 3]+3*Binomial[k, 4])/(n+2-k)!, {k, 3, n+2}], {n, 1, 20}] (* Vaclav Kotesovec, Feb 03 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Jeremy Dover, Feb 02 2017
STATUS
approved