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A005189
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Number of n-term 2-sided generalized Fibonacci sequences.
(Formerly M2976)
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2
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1, 1, 1, 3, 14, 85, 626, 5387, 52882, 582149, 7094234, 94730611, 1374650042, 21529197077, 361809517954, 6492232196699, 123852300381986, 2502521367966277, 53379537613065002, 1198434678728086019, 28245547605034208074, 697186985180529270101
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OFFSET
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0,4
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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If n <= 2 then a(n) = 1 otherwise a(n) = 2*(n-1)*a(n-1)-(n-2)^2*a(n-2).
E.g.f.: (e*Ei(1/(x-1)) - e*Ei(-1)-1)/(e^(x/(x-1))*(x-1)), where Ei is the exponential integral function. - Jean-François Alcover, Sep 05 2015, after Fishburn et al.
0 = a(n)*(-24*a(n+2) + 99*a(n+3) - 78*a(n+4) + 17*a(n+5) - a(n+6)) + a(n+1)*(-15*a(n+2) + 84*a(n+3) - 51*a(n+4) + 6*a(n+5)) + a(n+2)*(-6*a(n+2) + 34*a(n+3) - 15*a(n+4)) + a(n+3)*(+10*a(n+3)) for all n in Z. - Michael Somos, Dec 02 2016
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EXAMPLE
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G.f. = 1 + x + x^2 + 3*x^3 + 14*x^4 + 85*x^5 + 626*x^6 + 5387*x^7 + ...
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MAPLE
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if n <= 2 then 1 else 2*(n-1)*procname(n-1)-(n-2)^2*procname(n-2); fi; end;
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MATHEMATICA
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$Assumptions = Element[x, Reals]; F[x_] := (E*ExpIntegralEi[1/(x-1)] - E*ExpIntegralEi[-1]-1)/(E^(x/(x-1))*(x-1)); Join[{1}, CoefficientList[ Normal[Series[F[x], {x, 0, 18}]], x]*Range[0, 18]!] (* Jean-François Alcover, Sep 05 2015 *)
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PROG
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(PARI) {a(n) = if(n<3, n>=0, 2*(n-1)*a(n-1) - (n-2)^2*a(n-2))}; /* Michael Somos, Dec 02 2016 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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