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 A077422 Chebyshev sequence T(n,11) with Diophantine property. 9
 1, 11, 241, 5291, 116161, 2550251, 55989361, 1229215691, 26986755841, 592479412811, 13007560326001, 285573847759211, 6269617090376641, 137646002140526891, 3021942430001214961, 66345087457886202251, 1456569981643495234561, 31978194508699008958091 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Numbers n such that 30*(n^2-1) is square. - Vincenzo Librandi, Aug 08 2010 Except for the first term, positive values of x (or y) satisfying x^2 - 22xy + y^2 + 120 = 0. - Colin Barker, Feb 19 2014 LINKS Colin Barker, Table of n, a(n) for n = 0..745 Tanya Khovanova, Recursive Sequences Index entries for linear recurrences with constant coefficients, signature (22,-1). FORMULA a(n+1)^2 - 30*(2*b(n))^2 = 1, n>=0, with the companion sequence b(n)=A077421(n). a(n) = 22*a(n-1) - a(n-2), a(-1) := 11, a(0)=1. a(n) = T(n, 11) = (S(n, 22)-S(n-2, 22))/2 = S(n, 22)-11*S(n-1, 22) with T(n, x), resp. S(n, x), Chebyshev's polynomials of the first, resp. second, kind. See A053120 and A049310. S(n, 22)=A077421(n). a(n) =  (ap^n + am^n)/2 with ap := 11+2*sqrt(30) and am := 11-2*sqrt(30). a(n) = sum(((-1)^k)*(n/(2*(n-k)))*binomial(n-k, k)*(2*11)^(n-2*k), k=0..floor(n/2)), n>=1. a(n+1) = sqrt(1 + 30*(2*A077421(n))^2), n>=0. a(n) = Cosh[2n*ArcSinh[Sqrt[5]]] - Herbert Kociemba, Apr 24 2008 G.f.: (1-11*x)/(1-22*x+x^2). - Philippe Deléham, Nov 17 2008 MATHEMATICA Table[Cos[n*ArcCos[11]] // Round, {n, 0, 15}]  (* Jean-François Alcover, Dec 19 2013 *) PROG (Sage) [lucas_number2(n, 22, 1)/2 for n in range(0, 20)] # Zerinvary Lajos, Jun 26 2008 (MAGMA) [n: n in [1..10000000] |IsSquare(30*(n^2-1))] // Vincenzo Librandi, Aug 08 2010 (PARI) Vec((1-11*x)/(1-22*x+x^2) + O(x^100)) \\ Colin Barker, Jun 15 2015 CROSSREFS Cf. A090730. Sequence in context: A264465 A090921 A089328 * A267642 A267665 A196258 Adjacent sequences:  A077419 A077420 A077421 * A077423 A077424 A077425 KEYWORD nonn,easy AUTHOR Wolfdieter Lang, Nov 29 2002 STATUS approved

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Last modified September 21 13:00 EDT 2020. Contains 337272 sequences. (Running on oeis4.)