%I #48 Aug 04 2022 05:18:57
%S 0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,
%T 0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,
%U 0,1,0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,0,1,0,1,0,1,0,0,1,0,1,0,0
%N Sturmian word: limit S(infinity) where S(0) = 0, S(1) = 0,1 and for n>=1, S(n+1) = S(n)S(n)S(n-1).
%C Fixed point of morphism 0 -> 0,1; 1 -> 0,1,0.
%C This sequence corresponds to the case k = 1 of the Sturmian word S_k(infinity) as defined in A080764. See A171588 for the case k = 2. - _Peter Bala_, Nov 22 2013
%C This sequence is the {1->01}-transform of the Sturmian word A080764. - _Clark Kimberling_, May 17 2017
%H Reinhard Zumkeller, <a href="/A159684/b159684.txt">Table of n, a(n) for n = 0..10000</a>
%H W. W. Adams and J. L. Davison, <a href="http://www.jstor.org/stable/2041889">A remarkable class of continued fractions</a>, Proc. Amer. Math. Soc. 65 (1977), 194-198.
%H P. G. Anderson, T. C. Brown, and P. J.-S. Shiue, <a href="http://people.math.sfu.ca/~vjungic/tbrown/tom-28.pdf">A simple proof of a remarkable continued fraction identity</a> Proc. Amer. Math. Soc. 123 (1995), 2005-2009.
%H Jeffrey Shallit, <a href="http://cs.uwaterloo.ca/~shallit/papers.html">Characteristic words as fixed points of homomorphisms</a>, University of Waterloo Technical Report CS-91-72, 1991. See Example 1.
%H Jeffrey Shallit, <a href="/A005614/a005614.ps">Characteristic words as fixed points of homomorphisms</a>. See Example 1. [Cached copy, with permission]
%H N. J. A. Sloane, <a href="/A115004/a115004.txt">Families of Essentially Identical Sequences</a>, Mar 24 2021 (Includes this sequence)
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Sturmian_word">Sturmian word</a>
%F From _Peter Bala_, Nov 22 2013: (Start)
%F a(n) = floor((n + 2)*(sqrt(2) - 1)) - floor((n + 1)*(sqrt(2) - 1)).
%F If we read the sequence as the decimal constant C = 0.01010 01010 01010 10010 10010 ... then C = sum {n >= 1} 1/10^floor(n*(1 + sqrt(2))).
%F The real number 9*C has the simple continued fraction expansion [0; 11, 1010, 10000100, 100000000000100000, 100000000000000000000000000001000000000000, ...], the partial quotients having the form 10^Pell(n)*(1 + 10^Pell(n+1)) = 10^A001333(n+1) + 10^A000129(n) (see Adams and Davison).
%F A rapidly converging series for C is C = 9*sum {n >= 1} 10^Pell(2*n-1)*(1 + 10^Pell(2*n))/( (10^Pell(2*n-1) - 1)*(10^Pell(2*n+1) - 1) ): for example, the first 10 terms of the series give a rational approximation to C accurate to more than 130 million decimal places. Compare with the Fibonacci words A005614 and A221150. (End)
%e 0 -> 0,1 -> 0,1,0,1,0 -> 0,1,0,1,0,0,1,0,1,0,0,1 ->...
%t Nest[ Flatten[ # /. {0 -> {0, 1}, 1 -> {0, 1, 0}}] &, {1}, 6] (* _Robert G. Wilson v_, May 02 2009 *)
%t SubstitutionSystem[{0->{0,1},1->{0,1,0}},{1},{6}][[1]] (* _Harvey P. Dale_, Dec 25 2021 *)
%o (Haskell)
%o a159684 n = a159684_list !! n
%o a159684_list = 0 : concat (iterate (concatMap s) [1])
%o where s 0 = [0,1]; s 1 = [0,1,0]
%o -- _Reinhard Zumkeller_, Oct 26 2013
%o (Python)
%o def aupto(nn):
%o Snm1, Sn = [0], [0, 1]
%o while len(Sn) < nn+1: Snm1, Sn = Sn, Sn + Sn + Snm1
%o return Sn[:nn+1]
%o print(aupto(104)) # _Michael S. Branicky_, Jul 23 2022
%o (Python)
%o from math import isqrt
%o def A159684(n): return -isqrt(m:=(n+1)**2<<1)+isqrt(m+(n<<2)+6)-1 # _Chai Wah Wu_, Aug 03 2022
%Y See A188037 for another version of this sequence. - _N. J. A. Sloane_, Mar 22 2011
%Y Cf. A000129, A001333, A005614, A080764, A119812, A171588, A221150, A221151, A221152, A096270.
%Y The following sequences are all essentially the same, in the sense that they are simple transformations of each other, with A003151 as the parent: A003151, A001951, A001952, A003152, A006337, A080763, A082844 (conjectured), A097509, A159684, A188037, A245219 (conjectured), A276862. - _N. J. A. Sloane_, Mar 09 2021
%K nonn,easy
%O 0,1
%A _Philippe Deléham_, Apr 19 2009
%E More terms from _Robert G. Wilson v_, May 02 2009