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 A081071 a(n) = Lucas(4*n+2)-2, or Lucas(2*n+1)^2. 6
 1, 16, 121, 841, 5776, 39601, 271441, 1860496, 12752041, 87403801, 599074576, 4106118241, 28143753121, 192900153616, 1322157322201, 9062201101801, 62113250390416, 425730551631121, 2918000611027441, 20000273725560976 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Conjecture: a(n) = Fibonacci(4*n+3) + Sum_{k=2..2*n} Fibonacci(2*k). - Alex Ratushnyak, May 06 2012 REFERENCES Hugh C. Williams, Edouard Lucas and Primality Testing, John Wiley and Sons, 1998, p. 75. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (8,-8,1). FORMULA a(n) = 8*a(n-1) - 8*a(n-2) + a(n-3). a(n) = -2 + (3/2) * ((7/2 - (3/2)*sqrt(5))^n + (7/2 + (3/2)*sqrt(5))^n) + (1/2)*sqrt(5)*((7/2 + (3/2)*sqrt(5))^n - (7/2 - (3/2)*sqrt(5))^n), with n >= 0. - Paolo P. Lava, Dec 01 2008 G.f.: -(1+8*x+x^2)/((x-1)*(x^2-7*x+1)). - Colin Barker, Jun 26 2012 From Peter Bala, Nov 19 2019: (Start) Sum_{n >= 1} 1/(a(n) + 5) = (3*sqrt(5) - 5)/30. Sum_{n >= 1} 1/(a(n) - 5) = (15 - 4*sqrt(5) )/60. Sum_{n >= 1} (-1)^(n+1)/(a(n) - 5) = 1/12. Sum_{n >= 1} (-1)^(n+1)/(a(n) - 25/a(n)) = (5 + 2*sqrt(5))/120. (End) Sum_{n>=0} 1/a(n) = (1/sqrt(5)) * Sum_{n>=1} n/F(2*n), where F(n) is the n-th Fibonacci number (A000045). - Amiram Eldar, Oct 05 2020 MAPLE luc := proc(n) option remember: if n=0 then RETURN(2) fi: if n=1 then RETURN(1) fi: luc(n-1)+luc(n-2): end: for n from 0 to 40 do printf(`%d, `, luc(4*n+2)-2) od: # James A. Sellers, Mar 05 2003 MATHEMATICA CoefficientList[Series[-(1+8*x+x^2)/((x-1)*(x^2-7*x+1)), {x, 0, 40}], x] (* or *) LinearRecurrence[{8, -8, 1}, {1, 16, 121}, 50] (* Vincenzo Librandi, Jun 26 2012 *) LucasL[4*Range[0, 20]+2]-2 (* Harvey P. Dale, Nov 25 2012 *) PROG (Magma) I:=[1, 16, 121]; [n le 3 select I[n] else 8*Self(n-1)-8*Self(n-2)+Self(n-3): n in [1..30]]; // Vincenzo Librandi, Jun 26 2012 (PARI) x='x+O('x^30); Vec((1+8*x+x^2)/((1-x)*(x^2-7*x+1))) \\ G. C. Greubel, Dec 21 2017 CROSSREFS Cf. A000032 (Lucas numbers), A000045, A002878 is Lucas(2n+1), A081069. Sequence in context: A191902 A294140 A014765 * A217022 A069658 A069667 Adjacent sequences: A081068 A081069 A081070 * A081072 A081073 A081074 KEYWORD nonn,easy AUTHOR R. K. Guy, Mar 04 2003 EXTENSIONS More terms from James A. Sellers, Mar 05 2003 STATUS approved

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Last modified December 7 19:19 EST 2022. Contains 358669 sequences. (Running on oeis4.)