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A070997 a(n) = 8*a(n-1) - a(n-2), a(0)=1, a(-1)=1. 26

%I

%S 1,7,55,433,3409,26839,211303,1663585,13097377,103115431,811826071,

%T 6391493137,50320119025,396169459063,3119035553479,24556114968769,

%U 193329884196673,1522082958604615,11983333784640247,94344587318517361

%N a(n) = 8*a(n-1) - a(n-2), a(0)=1, a(-1)=1.

%C A Pellian sequence.

%C In general, sum{k=0..n, binomial(2n-k,k)j^(n-k)}=(-1)^n*U(2n,I*sqrt(j)/2), I=sqrt(-1); - _Paul Barry_, Mar 13 2005

%C a(n) = L(n,8), where L is defined as in A108299; see also A057080 for L(n,-8). - _Reinhard Zumkeller_, Jun 01 2005

%C Number of 01-avoiding words of length n on alphabet {0,1,2,3,4,5,6,7} which do not end in 0. - _Tanya Khovanova_, Jan 10 2007

%C Hankel transform of A158197. [From _Paul Barry_, Mar 13 2009]

%C For positive n, a(n) equals the permanent of the (2n)X(2n) tridiagonal matrix with sqrt(6)'s along the main diagonal, and 1's along the superdiagonal and the subdiagonal. [From John M. Campbell, Jul 08 2011]

%C Values of x (or y) in the solutions to x^2 - 8xy + y^2 + 6 = 0. - _Colin Barker_, Feb 05 2014

%H Vincenzo Librandi, <a href="/A070997/b070997.txt">Table of n, a(n) for n = 0..1000</a>

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

%H J.-C. Novelli, J.-Y. Thibon, <a href="http://arxiv.org/abs/1403.5962">Hopf Algebras of m-permutations,(m+1)-ary trees, and m-parking functions</a>, arXiv preprint arXiv:1403.5962, 2014

%H <a href="/index/Ch#Cheby">Index entries for sequences related to Chebyshev polynomials.</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (8,-1).

%F For all members x of the sequence, 15*x^2 - 6 is a square. Lim. n-> Inf. a(n)/a(n-1) = 4 + sqrt(15). - _Gregory V. Richardson_, Oct 12 2002

%F a(n) = (5+sqrt(15))/10 * (4+sqrt(15))^n + (5-sqrt(15))/10 * (4-sqrt(15))^n

%F a(n) ~ 1/10*sqrt(10)*(1/2*(sqrt(10)+sqrt(6)))^(2*n+1)

%F a(n) = U(n, 4)-U(n-1, 4) = T(2*n+1, sqrt(5/2))/sqrt(5/2), with Chebyshev's U and T Polynomials and U(-1, x) := 0. U(n, 4)=A001090(n+1), n>=-1.

%F Let q(n, x)=sum(i=0, n, x^(n-i)*binomial(2*n-i, i)); then q(n, 6)=a(n) - _Benoit Cloitre_, Nov 10 2002

%F a(n)a(n+3) = 48 + a(n+1)a(n+2). - _Ralf Stephan_, May 29 2004

%F a(n)=(-1)^n*U(2n, I*sqrt(6)/2), U(n, x) Chebyshev polynomial of second kind, I=sqrt(-1); - _Paul Barry_, Mar 13 2005

%F G.f.: (1-x)/(1-8*x+x^2). a(n)=a(-1-n).

%F a(n) = Jacobi_P(n,-1/2,1/2,4)/Jacobi_P(n,-1/2,1/2,1). - _Paul Barry_, Feb 03 2006

%F [a(n), A001090(n+1)] = [1,6; 1,7]^(n+1) * [1,0]. - _Gary W. Adamson_, Mar 21 2008

%e 1 + 7*x + 55*x^2 + 433*x^3 + 3409*x^4 + 26839*x^5 + ...

%t CoefficientList[Series[(1 - x)/(1 - 8*x + x^2), {x, 0, 30}], x] (* _Vincenzo Librandi_, Jan 26 2013 *)

%t a[c_, n_] := Module[{},

%t p := Length[ContinuedFraction[ Sqrt[ c]][[2]]];

%t d := Denominator[Convergents[Sqrt[c], n p]];

%t t := Table[d[[1 + i]], {i, 0, Length[d] - 1, p}];

%t Return[t];

%t ] (* Complement of A041023 *)

%t a[15, 20] (* _Gerry Martens_, Jun 07 2015 *)

%o (PARI) {a(n) = subst( 9*poltchebi(n) - poltchebi(n-1), x, 4) / 5} /* _Michael Somos_, Jun 07 2005 */

%o (PARI) {a(n) = if( n<0, n=-1-n); polcoeff( (1 - x) / (1 - 8*x + x^2) + x * O(x^n), n)} /* _Michael Somos_, Jun 07 2005 */

%o (Sage) [lucas_number1(n,8,1)-lucas_number1(n-1,8,1) for n in xrange(1, 21)]# [From _Zerinvary Lajos_, Nov 10 2009]

%o (MAGMA) I:=[1, 7]; [n le 2 select I[n] else 8*Self(n-1) - Self(n-2): n in [1..30]]; // _Vincenzo Librandi_, Jan 26 2013

%Y a(n) = sqrt((3*A057080(n)^2+2)/5) (cf. Richardson comment)

%Y Cf. A057080, A001090, A001091.

%Y Row 8 of array A094954.

%Y Cf. A001090.

%Y Cf. similar sequences listed in A238379.

%Y Cf. A041023

%K nonn,easy

%O 0,2

%A Joe Keane (jgk(AT)jgk.org), May 18 2002

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Last modified September 25 16:31 EDT 2017. Contains 292499 sequences.