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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
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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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
G.f.: (1-x-2*x^3)/(1-2*x-2*x^3+4*x^4). - Colin Barker, Feb 03 2012
a(0)=1, a(1)=1, a(2)=2, a(n) = 2*a(n-3) + 2^(n - 2), n>=3. - Baris Arslan, Mar 27 2017
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EXAMPLE
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a(4)=6 because we have: {}, {3}, {1,2}, {2,4}, {1,2,3}, {2,3,4}. - Geoffrey Critzer, Jan 18 2014
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MAPLE
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A068010 := n -> (2^n + 2^((n + 1 + (4/sqrt(3))*cos(((4*n)+1)*Pi/6))/3))/3;
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MATHEMATICA
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Table[nn=(n^2+n)/2; Total[Table[Coefficient[Series[Product[1+x^i, {i, 1, n}], {x, 0, nn}], x^(3k)], {k, 1, nn}]]+1, {n, 1, 33}] (* Geoffrey Critzer, Jan 18 2014 *)
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PROG
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(PARI) a(n)=([0, 1, 0, 0; 0, 0, 1, 0; 0, 0, 0, 1; -4, 2, 0, 2]^n*[1; 1; 2; 4])[1, 1] \\ Charles R Greathouse IV, Mar 27 2017
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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