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A068010
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Number of subsets of {1,2,3,...,n} that sum to 0 mod 3.
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2
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1, 1, 2, 4, 6, 12, 24, 44, 88, 176, 344, 688, 1376, 2736, 5472, 10944, 21856, 43712, 87424, 174784, 349568, 699136, 1398144, 2796288, 5592576, 11184896, 22369792, 44739584, 89478656, 178957312, 357914624, 715828224, 1431656448, 2863312896
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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LINKS
| Sophie LeBlanc, Jan 20 2002, sci.math posting
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FORMULA
| a(0)=1, a(1)=1, a(n) = 2*a(n-1) if 3 does not divide n-1 and a(n) = 2*a(n-1)-(2^((n-1)/3)) if 3 divides n-1.
a(n) = (2^n + 2^((n + 1 + (4/sqrt(3))*cos(((4*n)+1)*Pi/6))/3))/3. [Fred Galvin]
Empirical G.f.: (1-x-2*x^3)/(1-2*x-2*x^3+4*x^4). [Colin Barker, Feb 03 2012]
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MAPLE
| A068010 := n -> (2^n + 2^((n + 1 + (4/sqrt(3))*cos(((4*n)+1)*Pi/6))/3))/3;
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CROSSREFS
| 3rd row of A068009.
Sequence in context: A050293 A048115 A047151 * A095848 A136339 A019505
Adjacent sequences: A068007 A068008 A068009 * A068011 A068012 A068013
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KEYWORD
| nonn,changed
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AUTHOR
| Antti Karttunen, Feb 11 2002
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