%I M3599 #133 Jul 25 2024 14:05:11
%S 0,4,24,140,816,4756,27720,161564,941664,5488420,31988856,186444716,
%T 1086679440,6333631924,36915112104,215157040700,1254027132096,
%U 7309005751876,42600007379160,248291038523084,1447146223759344,8434586304032980
%N a(n) = 6*a(n-1) - a(n-2).
%C Solutions y of the equation 2x^2-y^2=2; the corresponding x values are given by A001541. - _N-E. Fahssi_, Feb 25 2008
%C The lower intermediate convergents to 2^(1/2) beginning with 4/3, 24/17, 140/99, 816/577, form a strictly increasing sequence; essentially, numerators=A005319 and denominators=A001541. - _Clark Kimberling_, Aug 26 2008
%C Numbers n such that (ceiling(sqrt(n*n/2)))^2 = 1 + n*n/2. - _Ctibor O. Zizka_, Nov 09 2009
%C All nonnegative solutions of the indefinite binary quadratic form X^2 + 4*X*Y -4*Y^2 of discriminant 32, representing -4 are (X(n), Y(n)) = (a(n), A001653(n+1)), for n >= 0. - _Wolfdieter Lang_, Jun 13 2018
%C Also the number of edge covers in the n-triangular snake graph. - _Eric W. Weisstein_, Jun 08 2019
%C All of the positive integer solutions of a*b+1=x^2, a*c+1=y^2, b*c+1=z^2, x+z=2*y, 0<a<b<c are given by a=A001542(n), b=A005319(n), c=A001542(n+1), x=A001541(n), y=A001653(n+1), z=A002315(n) with 0<n. - _Michael Somos_, Jun 26 2022
%C a(n) is the sum of 4*n consecutive powers of the silver ratio 1+sqrt(2), starting at (1+sqrt(2))^(-2*n) and ending at (1+sqrt(2))^(2*n-1). - _Greg Dresden_ and Ruxin Sheng, Jul 25 2024
%D P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 160, middle display.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Marius A. Burtea, <a href="/A005319/b005319.txt">Table of n, a(n) for n = 0..100</a>
%H K. Andersen, L. Carbone, and D. Penta, <a href="https://pdfs.semanticscholar.org/8f0c/c3e68d388185129a56ed73b5d21224659300.pdf">Kac-Moody Fibonacci sequences, hyperbolic golden ratios, and real quadratic fields</a>, Journal of Number Theory and Combinatorics, Vol 2, No. 3 pp 245-278, 2011. See Section 9.
%H Hacène Belbachir, Soumeya Merwa Tebtoub, and László Németh, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL23/Nemeth/nemeth7.html">Ellipse Chains and Associated Sequences</a>, J. Int. Seq., Vol. 23 (2020), Article 20.8.5.
%H John M. Campbell, <a href="http://arxiv.org/abs/1105.3399">An Integral Representation of Kekulé Numbers, and Double Integrals Related to Smarandache Sequences</a>, arXiv preprint arXiv:1105.3399 [math.GM], 2011.
%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>
%H Simon Plouffe, <a href="https://arxiv.org/abs/0911.4975">Approximations de séries génératrices et quelques conjectures</a>, Dissertation, Université du Québec à Montréal, 1992; arXiv:0911.4975 [math.NT], 2009.
%H Simon Plouffe, <a href="/A000051/a000051_2.pdf">1031 Generating Functions</a>, Appendix to Thesis, Montreal, 1992
%H Soumeya M. Tebtoub, Hacène Belbachir, and László Németh, <a href="https://hal.archives-ouvertes.fr/hal-02918958/document#page=18">Integer sequences and ellipse chains inside a hyperbola</a>, Proceedings of the 1st International Conference on Algebras, Graphs and Ordered Sets (ALGOS 2020), hal-02918958 [math.cs], 17-18.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/EdgeCover.html">Edge Cover</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/TriangularSnakeGraph.html">Triangular Snake Graph</a>
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (6,-1).
%F G.f.: 4*x / ( 1-6*x+x^2 ). - _Simon Plouffe_ in his 1992 dissertation.
%F G.f. for signed version beginning with 1: (1+2*x+x^2)/(1+6*x+x^2).
%F For any term n of the sequence, 2*n^2 + 4 is a perfect square. Limit_{n->infinity} a(n)/a(n-1) = 3 + 2*sqrt(2). - _Gregory V. Richardson_, Oct 06 2002
%F a(n) = ((3+2*sqrt(2))^n - (3-2*sqrt(2))^n) / sqrt(2). - _Gregory V. Richardson_, Oct 06 2002
%F (-1)^(n+1) = A090390(n+1) + A001542(n+1) + A046729(n) - a(n) (conjectured). - _Creighton Dement_, Nov 17 2004
%F For n > 0, a(n) = A000129(n+1)^2 - A000129(n-1)^2; a(n) = A046090(n-1) + A001652(n); e.g., 816 = 120 + 696; a(n) = A001653(n) - A001653(n-1); e.g., 816 = 985 - 169. - _Charlie Marion_ Jul 22 2005
%F a(n) = 4*A001109(n). - _M. F. Hasler_, Mar 2009
%F For n > 1, a(n) is the denominator of continued fraction [1,4,1,4,...,1,4] with (n-1) repetitions of 1,4. For the numerators, see A001653. - _Greg Dresden_, Sep 10 2019
%F 1/a(n) - 1/a(n+1) = 1/(Pell(2*n+1) - 1/Pell(2*n+1)) for n >= 1, where Pell(n) = A000129(n). - _Peter Bala_, Aug 21 2022
%F E.g.f.: sqrt(2)*exp(3*x)*sinh(2*sqrt(2)*x). - _Stefano Spezia_, Nov 25 2022
%F a(n) = 2*A000129(2*n). - _Tanya Khovanova_ and MIT PRIMES STEP senior group, Apr 17 2024
%e G.f. = 4*x + 24*x^2 + 140*x^3 + 816*x^4 + 4756*x^5 + ... - _Michael Somos_, Jun 26 2022
%t LinearRecurrence[{6, -1}, {0, 4}, 22] (* _Jean-François Alcover_, Sep 26 2017 *)
%t Table[((3 + 2 Sqrt[2])^n - (3 - 2 Sqrt[2])^n)/Sqrt[2], {n, 20}] // Expand (* _Eric W. Weisstein_, Jun 08 2019 *)
%t CoefficientList[Series[(4 x)/(1 - 6 x + x^2), {x, 0, 20}], x] (* _Eric W. Weisstein_, Jun 08 2019 *)
%t a[ n_] := 4*ChebyShevU[n-1, 3]; (* _Michael Somos_, Jun 26 2022 *)
%o (Magma) a:=[0,4]; [n le 2 select a[n] else 6*Self(n-1) - Self(n-2):n in [1..22]]; // _Marius A. Burtea_, Sep 19 2019
%o (PARI) {a(n) = 4*polchebyshev(n-1, 2, 3)}; /* _Michael Somos_, Jun 26 2022 */
%Y Cf. A000129, A001109, A001541, A001542, A001652, A001653, A046090, A046729, A090390.
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_