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 A028230 Bisection of A001353. Indices of square numbers which are also octagonal. 22
 1, 15, 209, 2911, 40545, 564719, 7865521, 109552575, 1525870529, 21252634831, 296011017105, 4122901604639, 57424611447841, 799821658665135, 11140078609864049, 155161278879431551, 2161117825702177665 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Chebyshev S-sequence with Diophantine property. 4*b(n)^2 - 3*a(n)^2 = 1 with b(n)=A001570(n), n>=0. y satisfying the Pellian x^2 - 3*y^2=1, for even x given by A094347(n). - Lekraj Beedassy, Jun 03 2004 a(n) = L(n,-14)*(-1)^n, where L is defined as in A108299; see also A001570 for L(n,+14). - Reinhard Zumkeller, Jun 01 2005 Product x*y, where the pair (x, y) solves for x^2 - 3y^2 = -2, i.e., a(n)=A001834(n)*A001835(n). - Lekraj Beedassy, Jul 13 2006 Numbers n such that RootMeanSquare(1,3,...,2*A001570(k)-1) = n. - Ctibor O. Zizka, Sep 04 2008 As n increases, this sequence is approximately geometric with common ratio r = lim(n -> Infinity, a(n)/a(n-1)) = (2 + sqrt(3))^2 = 7 + 4 * sqrt(3). - Ant King, Nov 15 2011 REFERENCES R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics. Addison-Wesley, Reading, MA, 1990, p. 329. J. D. E. Konhauser et al., Which Way Did the Bicycle Go?, MAA 1996, p. 104. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..890 Andersen, K., Carbone, L. and Penta, D., Kac-Moody Fibonacci sequences, hyperbolic golden ratios, and real quadratic fields, Journal of Number Theory and Combinatorics, Vol 2, No. 3 pp 245-278, 2011. See Section 9. Alex Fink, Richard K. Guy, and Mark Krusemeyer, Partitions with parts occurring at most thrice, Contributions to Discrete Mathematics, Vol 3, No 2 (2008), pp. 76-114. See Section 13. T. N. E. Greville, Table for third-degree spline interpolations with equally spaced arguments, Math. Comp., 24 (1970), 179-183. W. D. Hoskins, Table for third-degree spline interpolation using equi-spaced knots, Math. Comp., 25 (1971), 797-801. Tanya Khovanova, Recursive Sequences E. Kilic, Y. T. Ulutas, N. Omur, A Formula for the Generating Functions of Powers of Horadam's Sequence with Two Additional Parameters, J. Int. Seq. 14 (2011) #11.5.6, table 4, k=1, t=2. Dino Lorenzini, Z. Xiang, Integral points on variable separated curves, Preprint 2016. F. V. Waugh and M. W. Maxfield, Side-and-diagonal numbers, Math. Mag., 40 (1967), 74-83. Eric Weisstein's World of Mathematics, Octagonal Square Number. Index entries for linear recurrences with constant coefficients, signature (14,-1). FORMULA a(n) = 2*A001921(n)+1. a(n) = 14*a(n-1) - a(n-2) for n>1. a(n) = S(n, 14) + S(n-1, 14) = S(2*n, 4) with S(n, x) := U(n, x/2) Chebyshev's polynomials of the second kind. See A049310. S(-1, x)=0, S(n, 14)=A007655(n+1) and S(n, 4)=A001353(n+1). G.f.: x*(1+x)/(1-14*x+x^2). a(n) = (ap^(2*n+1) - am^(2*n+1))/(ap - am) with ap := 2+sqrt(3) and am := 2-sqrt(3). a(n+1) = Sum_{k=0..n} (-1)^k*binomial(2*n-k, k)*16^(n-k), n>=0. a(n) = sqrt((4*A001570(n-1)^2 - 1)/3). a(n) ~ 1/6*sqrt(3)*(2 + sqrt(3))^(2*n-1). - Joe Keane (jgk(AT)jgk.org), May 15 2002 4*a(n+1) = (A001834(n))^2 + 4*(A001835(n+1))^2 - (A001835(n))^2. E.g. 4*a(3) = 4*209 = 19^2 + 4*11^2 - 3^2 = (A001834(2))^2 + 4*(A001835(3))^2 - A001835(2))^2. Generating floretion: 'i + 2'j + 3'k + i' + 2j' + 3k' + 4'ii' + 3'jj' + 4'kk' + 3'ij' + 3'ji' + 'jk' + 'kj' + 4e. - Creighton Dement, Dec 04 2004 Define f[x,s] = s x + Sqrt[(s^2-1)x^2+1]; f[0,s]=0. a(n) = f[a(n-1),7] + f[a(n-2),7]. - Marcos Carreira, Dec 27 2006 From Ant King, Nov 15 2011: (Start) a(n) = 1/6 * sqrt(3) * ( (tan(5*Pi/12)) ^ (2n-1) - (tan(Pi/12)) ^ (2n-1) ). a(n) = floor (1/6 * sqrt(3) * (tan(5*Pi/12)) ^ (2n-1)). (End) a(n) = A001353(n)^2-A001353(n-1)^2. - Antonio Alberto Olivares, Apr 06 2020 MAPLE seq(coeff(series((1+x)/(1-14*x+x^2), x, n+1), x, n), n = 0..30); # G. C. Greubel, Dec 06 2019 MATHEMATICA LinearRecurrence[{14, - 1}, {1, 15}, 17] (* Ant King, Nov 15 2011 *) CoefficientList[Series[(1+x)/(1-14x+x^2), {x, 0, 30}], x] (* Vincenzo Librandi, Jun 17 2014 *) PROG (Sage) [(lucas_number2(n, 14, 1)-lucas_number2(n-1, 14, 1))/12 for n in range(1, 18)] # Zerinvary Lajos, Nov 10 2009 (PARI) Vec((1+x)/(1-14*x+x^2)+O(x^99)) \\ Charles R Greathouse IV, Jun 16 2014 (PARI) isok(n) = ispolygonal(n^2, 8); \\ Michel Marcus, Jul 09 2017 (MAGMA) I:=[1, 15]; [n le 2 select I[n] else 14*Self(n-1) - Self(n-2): n in [1..30]]; // G. C. Greubel, Dec 06 2019 (GAP) a:=[1, 15];; for n in [3..30] do a[n]:=14*a[n-1]-a[n-2]; od; a; # G. C. Greubel, Dec 06 2019 CROSSREFS Cf. A036428, A046184. Cf. A077416 with companion A077417. Sequence in context: A280160 A239991 A274563 * A122572 A067560 A019553 Adjacent sequences:  A028227 A028228 A028229 * A028231 A028232 A028233 KEYWORD nonn,easy AUTHOR EXTENSIONS Additional comments from Wolfdieter Lang, Nov 29 2002 Incorrect recurrence relation deleted by Ant King, Nov 15 2011 Minor edits by Vaclav Kotesovec, Jan 28 2015 STATUS approved

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Last modified October 25 12:36 EDT 2020. Contains 338012 sequences. (Running on oeis4.)