OFFSET
0,1
COMMENTS
If Y is a 3-subset of an n-set X then, for n>=9, a(n-9) is the number of 7-subsets of X having at most one element in common with Y. - Milan Janjic, Nov 23 2007
FORMULA
G.f.: (3-2*x)/(1-x)^8.
a(n)= binomial(n+6, 6)*(n+21)/7 = 3*b(n)-2*b(n-1), with b(n):=binomial(n+7, 7); cf. A000580.
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Jun 11 2004
STATUS
approved