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A247560
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a(n) = 3*a(n-1) - 4*a(n-2) with a(0) = a(1) = 1.
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5
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1, 1, -1, -7, -17, -23, -1, 89, 271, 457, 287, -967, -4049, -8279, -8641, 7193, 56143, 139657, 194399, 24569, -703889, -2209943, -3814273, -2603047, 7447951, 32756041, 68476319, 74404793, -50690897, -449691863, -1146312001, -1640168551, -335257649, 5554901257
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OFFSET
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0,4
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LINKS
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FORMULA
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G.f.: (1 - 2*x) / (1 - 3*x + 4*x^2).
a(n) = 3*a(n-1) - 4*a(n-2) for all n in Z.
a(n) = a(-1-n) * 2^(2*n + 1) for all n in Z.
a(n) = (-1)^n * A087168(n) for all n in Z.
A247565(n) = 2^n + a(n) for all n in Z.
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EXAMPLE
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G.f. = 1 + x - x^2 - 7*x^3 - 17*x^4 - 23*x^5 - x^6 + 89*x^7 + 271*x^8 + ...
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MAPLE
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A247560:=n->simplify((1/14*I)*sqrt(7)*((3/2+(1/2*I)*sqrt(7))^n-(3/2-(1/2*I)*sqrt(7))^n)+1/2*((3/2+(1/2*I)*sqrt(7))^n+(3/2-(1/2*I)*sqrt(7))^n)): seq(A247560(n), n=0..40); # Wesley Ivan Hurt, Oct 02 2014
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MATHEMATICA
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a[ n_] := Re[ (1 - 1/Sqrt[-7]) (3 + Sqrt[-7])^n / 2^n];
LinearRecurrence[{3, -4}, {1, 1}, 40] (* Harvey P. Dale, Jun 13 2017 *)
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PROG
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(PARI) {a(n) = real( (1 + quadgen(-7))^n )};
(Haskell)
a247560 n = a247560_list !! n
a247560_list = 1 : 1 : zipWith (-) (map (* 3) $ tail a247560_list)
(map (* 4) a247560_list)
(Sage)
[((1-1/sqrt(-7))*(3+sqrt(-7))^n/2^n).real() for n in range(34)] # Peter Luschny, Oct 02 2014 (after Somos)
(Magma) I:=[1, 1]; [n le 2 select I[n] else 3*Self(n-1) - 4*Self(n-2): n in [1..30]]; // G. C. Greubel, Aug 04 2018
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CROSSREFS
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KEYWORD
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sign,easy
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AUTHOR
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STATUS
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approved
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