OFFSET
-1,1
COMMENTS
If Y is a 3-subset of an 2n-set X then, for n>=3, a(n-3) is the number of (2 n-2)-subsets of X having at least two elements in common with Y. - Milan Janjic, Dec 16 2007
LINKS
G. C. Greubel, Table of n, a(n) for n = -1..5000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = A100345(n+4, n-1) for n>0.
a(n) = (n+4)*(2*n+3). - Reinhard Zumkeller, Nov 18 2004
From G. C. Greubel, Jul 16 2017: (Start)
a(n) = 3*a(n-1) - 3*a(n-1) + a(n-2).
G.f.: (2*x^2 - 3*x -3)/(x*(x-1)^3).
E.g.f.: (2*x^2 + 13*x + 12)*exp(x). (End)
MAPLE
MATHEMATICA
Table[2 n^2 + 11 n + 12, {n, -1, 50}] (* G. C. Greubel, Jul 16 2017 *)
PROG
(PARI) a(n)=2*n^2+11*n+12 \\ Charles R Greathouse IV, Sep 24 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Jun 12 2002
STATUS
approved