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A011920
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a(n) = b(n)*(b(n)+1) = b(n) + ... + c(n), where b(n) = A011916(n), c(n) = A011918(n).
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2
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12, 1980, 378840, 73419192, 14241916260, 2762844014580, 535977297450672, 103976830083273840, 20170969020163148220, 3913064012542622257452, 759114247456742016195720, 147264250942490855924510760
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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REFERENCES
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Mario Velucchi "Seeing couples" in Recreational and Educational Computing, to appear 1997.
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LINKS
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FORMULA
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a(n) = +209*a(n-1) -2926*a(n-2) +2926*a(n-3) -209*a(n-4) +a(n-5).
G.f.: -12*x*(1-44*x+11*x^2)/ ((x-1) * (x^2-14*x+1) * (x^2-194*x+1)). (End)
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MAPLE
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A011922 := proc(n) (2+sqrt(1+((((2+sqrt(3))^(2*n)-(2-sqrt(3))^(2*n))^2)/4)))/3 ; expand(%) ; simplify(%) ; end proc:
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MATHEMATICA
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LinearRecurrence[{209, -2926, 2926, -209, 1}, {12, 1980, 378840, 73419192, 14241916260}, 20] (* Harvey P. Dale, Jan 01 2021 *)
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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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Mario Velucchi (mathchess(AT)velucchi.it)
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EXTENSIONS
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STATUS
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approved
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