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 A078057 Expansion of (1+x)/(1-2*x-x^2). 63

%I

%S 1,3,7,17,41,99,239,577,1393,3363,8119,19601,47321,114243,275807,

%T 665857,1607521,3880899,9369319,22619537,54608393,131836323,318281039,

%U 768398401,1855077841,4478554083,10812186007,26102926097,63018038201,152139002499,367296043199,886731088897

%N Expansion of (1+x)/(1-2*x-x^2).

%C Let x_n be the sequence 1,3,7,17,41,99,239,... (this sequence or A001333) and let y_n = 1,2,5,12,29,70,169,... (A000129). Then {+- x_n +- y_n*sqrt(2) } are the units in the ring of algebraic integers Z[ sqrt(2) ].

%C Consider a string of n red, blue and green beads (with start and end points distinct and not interchangeable). If one pairing is disallowed, so that a red bead cannot immediately follow a blue bead or vice versa, how many different strings exist of any given length? Answer is a(n). E.g., a(3)=17 because there are 17 strings of length 3: RRR, RRG, RGR, RGG, RGB, GRR, GRG, GGR, GGG, GGB, GBG, GBB, BGR, BGG, BGB, BBG, BBB - _Wayne VanWeerthuizen_, May 02 2004

%C The number of Khalimsky-continuous functions with one fixed endpoint. - Shiva Samieinia (shiva(AT)math.su.se), Oct 08 2007

%C The sequence (-1)^C(n+1,2)*a(n) with g.f. (1-3x-x^2-x^3)/(1+6x^2+x^4) is the Hankel transform of the signed central binomial coefficients (-1)^C(n+1,2)*A001405(n). - _Paul Barry_, Jun 24 2008

%C An elephant sequence, see A175655. For the central square six A[5] vectors, with decimal values between 21 and 336, lead to this sequence. For the corner squares these vectors lead to the companion sequence A000129 (without the leading 0). - _Johannes W. Meijer_, Aug 15 2010

%C Sequence is related to rhombus substitution tilings showing 8-fold rotational symmetry (see A001333). - _L. Edson Jeffery_, Apr 04 2011

%C Number of length-n strings of 3 letters {0,1,2} with no two adjacent nonzero letters identical. The general case (strings of L letters) is the sequence with g.f. (1+x)/(1-(L-1)*x-x^2). - _Joerg Arndt_, Oct 11 2012

%C Row sums of A035607, when seen as a triangle read by rows. - _Reinhard Zumkeller_, Jul 20 2013

%D A. Froehlich and M. J. Taylor, Algebraic Number Theory, Cambridge, 1991 (see p. 3).

%D Thomas Koshy, Pell and Pell-Lucas Numbers with Applications, Springer, New York, 2014.

%H Vincenzo Librandi, <a href="/A078057/b078057.txt">Table of n, a(n) for n = 0..1000</a>

%H C. Bautista-Ramos and C. Guillen-Galvan, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL15/Bautista/bautista4.html">Fibonacci numbers of generalized Zykov sums</a>, J. Integer Seq., 15 (2012), Article 12.7.8

%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>

%H Emanuele Munarini, <a href="http://www.emis.de/journals/INTEGERS/papers/j29/j29.Abstract.html">Combinatorial properties of the antichains of a garland</a>, Integers, 9 (2009), 353-374.

%H Shiva Samieinia, <a href="http://www2.math.su.se/reports/2007/6/">Digital straight line segments and curves</a>, Licentiate Thesis, Stockholm University, Department of Mathematics, Report 2007:6.

%H S. Samieinia, <a href="http://dx.doi.org/10.4171/PM/1858">The number of continuous curves in digital geometry</a>, Port. Math. 67 (1) (2010) 75-89

%H Gy. Tasi and F. Mizukami, <a href="http://dx.doi.org/10.1023/A:1019163812482">Quantum algebraic-combinatoric study of the conformational properties of n-alkanes</a>, J. Math. Chemistry, 25, 1999, 55-64 (see p. 63).

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,1).

%F a(n) = 2*a(n-1) + a(n-2); a(0)=1; a(1)=3. - _Wayne VanWeerthuizen_, May 02 2004

%F a(n) = 2*a(n-1) + a(n-2); a(n+1)/a(n) tends to silver ratio 1+sqrt(2) as n tends to infinity. - Shiva Samieinia (shiva(AT)math.su.se), Oct 08 2007

%F a(n) = Sum_{k, 0<=k<=n}A147720(n,k)*3^k*(-1/3)^(n-k). - _Philippe DelĂ©ham_, Nov 15 2008

%F a(n) = (1/2)*[1+sqrt(2)]^n-(1/2)*sqrt(2)*[1-sqrt(2)]^n+(1/2)*[1-sqrt(2)]^n+(1/2)*[1+sqrt(2)]^n *sqrt(2), with n>=0. - _Paolo P. Lava_, Nov 20 2008

%F a(n) = Pell(n)+Pell(n+1) with Pell(n) = A000129(n). - _Johannes W. Meijer_, Aug 15 2010

%F G.f.: G(0)/(2*x) -1/x, where G(k)= 1 + 1/(1 - x*(2*k-1)/(x*(2*k+1) - 1/G(k+1))); (continued fraction). - _Sergei N. Gladkovskii_, Jun 20 2013

%F a(n) = T(n+1, i) / i^(n+1) , where T(n, x) denotes the Chebyshev polynomial of the first kind. - _Michael Somos_, Jul 28 2018

%e G.f. = 1 + 3*x + 7*x^2 + 17*x^3 + 41*x^4 + 99*x^5 + 239*x^6 + 577*x^7 + ... - _Michael Somos_, Jul 28 2018

%t Expand[Table[((1 + Sqrt[2])^n + (1 - Sqrt[2])^n)/2, {n, 1, 30}]] (* _Artur Jasinski_, Dec 10 2006 *)

%t CoefficientList[Series[(1 + x)/(1 - 2 x - x^2), {x, 0, 30}], x] (* _Vincenzo Librandi_, Jun 16 2014 *)

%t a[ n_] := ChebyshevT[n+1, I] / I^(n+1); (* _Michael Somos_, Jul 28 2018 *)

%o a078057 = sum . a035607_row -- _Reinhard Zumkeller_, Jul 20 2013

%o (PARI) {a(n) = polchebyshev(n+1, 1, I) / I^(n+1)}; /* _Michael Somos_, Jul 28 2018 */

%Y Essentially the same as A001333, which has many more references.

%Y Cf. A131887, A131935, A000129.

%K nonn,easy

%O 0,2

%A _N. J. A. Sloane_, Nov 17 2002

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Last modified January 24 00:58 EST 2019. Contains 319404 sequences. (Running on oeis4.)