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%I #33 Jan 18 2019 16:01:41
%S 0,6,6,12,18,30,48,78,126,204,330,534,864,1398,2262,3660,5922,9582,
%T 15504,25086,40590,65676,106266,171942,278208,450150,728358,1178508,
%U 1906866,3085374,4992240,8077614,13069854,21147468,34217322,55364790,89582112,144946902
%N Fibonacci sequence beginning 0, 6.
%C Starting with a(0)=1, a(1)=3, a(n) = the number of ternary length-2 squarefree words of length n.
%D A. T. Benjamin and J. J. Quinn, Proofs that really count: the art of combinatorial proof, M.A.A. 2003, p. 15.
%H C. Dalfó, M. A. Fiol, <a href="https://arxiv.org/abs/1607.08832">A Note on the Order of Iterated Line Digraphs</a>, Journal of Graph Theory, Volume 85, Issue 2, June 2017, Pages 395-39, 2016; DOI: 10.1002/jgt.22068; arXiv:1607.08832 [math.CO], 2016.
%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>
%H C. Richard and U. Grimm, <a href="https://arxiv.org/abs/math/0302302">On the entropy and letter frequencies of ternary squarefree words</a>, arXiv:math/0302302 [math.CO], 2003.
%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1, 1).
%F a(n) = round( (12*phi-6)/5 * phi^n) for n>3. - _Thomas Baruchel_, Sep 08 2004
%F a(n) = 6F(n) = F(n+3) + F(n+1) + F(n-4), n>3.
%F a(n) = A119457(n+4,n-1) for n>1. - _Reinhard Zumkeller_, May 20 2006
%F G.f.: 6*x/(1-x-x^2). - _Philippe Deléham_, Nov 20 2008
%F a(n) = 6 * A000045(n). - _Alois P. Heinz_, Jan 18 2019
%p a:= n-> 6*(<<0|1>, <1|1>>^n)[1,2]:
%p seq(a(n), n=0..40); # _Alois P. Heinz_, Jan 18 2019
%t a={};b=0;c=6;AppendTo[a,b];AppendTo[a,c];Do[b=b+c;AppendTo[a,b];c=b+c;AppendTo[a,c],{n,1,12,1}];a (* _Vladimir Joseph Stephan Orlovsky_, Jul 23 2008 *)
%t LinearRecurrence[{1,1},{0,6},50] (* _Harvey P. Dale_, Dec 05 2015 *)
%Y Cf. A000032, A000045.
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_
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