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A078343
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a(0) = -1, a(1) = 2; a(n) = 2*a(n-1) + a(n-2).
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19
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-1, 2, 3, 8, 19, 46, 111, 268, 647, 1562, 3771, 9104, 21979, 53062, 128103, 309268, 746639, 1802546, 4351731, 10506008, 25363747, 61233502, 147830751, 356895004, 861620759, 2080136522, 5021893803, 12123924128, 29269742059, 70663408246, 170596558551
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OFFSET
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0,2
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COMMENTS
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Inverse binomial transform of -1, 1, 6, 22, 76, 260,.. (see A111566). Binomial transform of -1, 3, -2, 6, -4, 12, -8, 24, -16,... (see A162255). - R. J. Mathar, Oct 02 2012
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REFERENCES
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H. S. M. Coxeter, 1998, Numerical distances among the circles in a loxodromic sequence, Nieuw Arch. Wisk, 16, pp. 1-9.
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LINKS
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FORMULA
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For the unsigned version: a(1)=1; a(2)=2; a(n) = Sum_{k=2..n-1} (a(k) + a(k-1)).
a(n) is asymptotic to (1/4)*(-2+3*sqrt(2))*(1+sqrt(2))^n.
a(n) = 2*A000129(n) - A000129(n-1) if n>0; abs(a(n)) = sum_{k=0..floor(n/2)} (C(n-k-1, k) - C(n-k-1, k-1))2^(n-2k). - Paul Barry, Dec 23 2004
O.g.f.: (1-4*x)/(-1 + 2*x + x^2). - R. J. Mathar, Feb 15 2008
a(n) = -(-1)^n * A048654(-n) = ( (-2+3*sqrt(2))*(1+sqrt(2))^n + (-2-3*sqrt(2))*(1-sqrt(2))^n )/4 for all n in Z. - Michael Somos, Jun 30 2022
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EXAMPLE
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G.f. = -1 + 2*x + 3*x^2 + 8*x^3 + 19*x^4 + 46*x^5 + 111*x^6 + ... - Michael Somos, Jun 30 2022
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MAPLE
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f:=proc(n) option remember; if n=0 then RETURN(-1); fi; if n=1 then RETURN(2); fi; 2*f(n-1)+f(n-2); end;
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MATHEMATICA
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LinearRecurrence[{2, 1}, {-1, 2}, 40] (* Harvey P. Dale, Apr 15 2019 *)
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PROG
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(Haskell)
a078343 n = a078343_list !! n
a078343_list = -1 : 2 : zipWith (+)
(map (* 2) $ tail a078343_list) a078343_list
(Magma) m:=30; R<x>:=PowerSeriesRing(Integers(), m); Coefficients(R!((1-4*x)/(-1+2*x+x^2))); // G. C. Greubel, Jul 26 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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EXTENSIONS
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STATUS
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approved
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