OFFSET
0,2
COMMENTS
From an Internat. Math. Olympiad problem.
Comment from Neil Fernandez, Apr 29 2014: The question was proposed at the IMO in 1992 and got on to the long list, as question 19, but not the shortlist.
REFERENCES
E. Lozansky and C. Rousseau, Winning Solutions, Springer, 1996; see p. 154.
LINKS
The Art of Problem Solving, IMO Longlists 1992 (From Neil Fernandez, Jul 29 2014)
Index entries for linear recurrences with constant coefficients, signature (6,-12,9).
FORMULA
G.f.: (1 - 4*x + 4*x^2)/((1 - 3*x)*(1 - 3*x + 3*x^2)).
a(n) = Sum_{k=0..n} binomial(n, k) * Sum_{j=0..floor(k/3)} binomial(k, 3*j). - Joseph M. Shunia, Jul 25 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Feb 18 2004
STATUS
approved