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A092236 a(n) = (3^n + 2*3^(n/2)*cos(n*Pi/6))/3. 0
1, 2, 4, 9, 24, 72, 225, 702, 2160, 6561, 19764, 59292, 177633, 532170, 1595052, 4782969, 14346720, 43040160, 129127041, 387400806, 1162241784, 3486784401, 10460412252, 31381236756, 94143533121, 282430067922, 847289140884, 2541865828329, 7625595890664 (list; graph; refs; listen; history; text; internal format)
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

Table of n, a(n) for n=0..28.

The Art of Problem Solving, IMO Longlists 1992 (From Neil Fernandez, Jul 29 2014)

FORMULA

G.f.: (1-4x+4x^2) / [(1-3x)(1-3x+3x^2)].

CROSSREFS

Sequence in context: A148084 A156804 A081913 * A336023 A009283 A236756

Adjacent sequences:  A092233 A092234 A092235 * A092237 A092238 A092239

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Feb 18 2004

STATUS

approved

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Last modified October 22 18:45 EDT 2021. Contains 348175 sequences. (Running on oeis4.)