OFFSET
6,1
COMMENTS
If a 3-set Y and a 3-set Z, having one element in common, are subsets of an n-set X then a(n+2) is the number of 3-subsets of X intersecting both Y and Z. - Milan Janjic, Oct 03 2007
LINKS
Colin Barker, Table of n, a(n) for n = 6..1000
Milan Janjic, Two Enumerative Functions.
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = n + a(n-1) (with a(6)=3). - Vincenzo Librandi, Aug 06 2010
G.f.: x^6*(3*x^2-x-3) / (x-1)^3. - Colin Barker, Mar 18 2015
Sum_{n>=6} 1/a(n) = 4423/6120 + 2*Pi*tan(sqrt(145)*Pi/2)/sqrt(145). - Amiram Eldar, Dec 13 2022
MATHEMATICA
Drop[Accumulate[Range[60]]-18, 5] (* Harvey P. Dale, Dec 08 2017 *)
PROG
(PARI) Vec(x^6*(3*x^2-x-3)/(x-1)^3 + O(x^100)) \\ Colin Barker, Mar 18 2015
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 21 1999
STATUS
approved