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A025247
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a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ...+ a(n-3)*a(3) for n >= 4.
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4
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2, 0, 1, 2, 4, 9, 22, 56, 146, 388, 1048, 2869, 7942, 22192, 62510, 177308, 506008, 1451866, 4185788, 12119696, 35227748, 102753800, 300672368, 882373261, 2596389190, 7658677856, 22642421206, 67081765932, 199128719896, 592179010350, 1764044315540
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OFFSET
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1,1
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LINKS
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FORMULA
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G.f.: (1+2*x-sqrt(1-4*x+4*x^2-4*x^3))/2. - Michael Somos, Jun 08 2000
Conjecture: n*a(n) +2*(3-2*n)*a(n-1) +4*(n-3)*a(n-2) +2*(9-2*n)*a(n-3)=0. - R. J. Mathar, Aug 14 2012
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MATHEMATICA
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Rest[CoefficientList[Series[(1+2x-Sqrt[1-4x+4x^2-4x^3])/2, {x, 0, 40}], x]] (* Harvey P. Dale, Apr 23 2011 *)
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PROG
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(PARI) a(n)=polcoeff((2*x-sqrt(1-4*x+4*x^2-4*x^3+x*O(x^n)))/2, n)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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