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A128588 A007318 * A128587. 12

%I

%S 1,2,4,6,10,16,26,42,68,110,178,288,466,754,1220,1974,3194,5168,8362,

%T 13530,21892,35422,57314,92736,150050,242786,392836,635622,1028458,

%U 1664080,2692538,4356618,7049156,11405774,18454930,29860704,48315634,78176338

%N A007318 * A128587.

%C a(n)/a(n-1) tends to phi, 1.618...

%C Regardless of initial two terms, any linearly recurring sequence with signature (1,1) will yield an a(n)/a(n+1) ratio tending to phi (the Golden Ratio). - _Harvey P. Dale_, Mar 29 2017

%C Apart from the initial term, double the Fibonacci numbers. O.g.f.: x*(1+x+x^2)/(1-x-x^2). a(n) gives the number of binary strings of length n-1 avoiding the substrings 000 and 111. a(n) also gives the number of binary strings of length n-1 avoiding the substrings 010 and 101. - _Peter Bala_, Jan 22 2008

%C From A014217=1,1,2,4,6,. Which leads to A153819=16,34,88,. Inverse binomial transform of A069403=1,3,9,25,67. - _Paul Curtz_, Jan 03 2009

%C Variation on "Narayana's Cows". One cow at step n=1. At any subsequent step any cow generates another one but after two steps dies. The sequence gives the total number of cows at any steps. - _Paolo P. Lava_, Oct 07 2009

%C Row lengths of triangle A232642. - _Reinhard Zumkeller_, May 14 2015

%H Reinhard Zumkeller, <a href="/A128588/b128588.txt">Table of n, a(n) for n = 1..2500</a>

%H J.-P. Allouche and T. Johnson, <a href="http://www.math.jussieu.fr/~allouche/johnson2.pdf"> Narayana's Cows and Delayed Morphisms</a>.

%H Elena Barcucci, Antonio Bernini, Stefano Bilotta, Renzo Pinzani, <a href="http://arxiv.org/abs/1601.07723">Non-overlapping matrices</a>, arXiv:1601.07723 [cs.DM], 2016. See 1st column of Table 2 p. 11.

%H P. Flajolet and R. Sedgewick, <a href="http://algo.inria.fr/flajolet/Publications/AnaCombi/anacombi.html">Analytic Combinatorics</a>, 2009, page 52.

%H B. Winterfjord, <a href="http://www.math.chalmers.se/~einar/exjobb/winterfjord.pdf">Binary strings and substring avoidance</a>.

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

%F G.f.: x(1+x+x^2)/(1-x-x^2).

%F Binomial transform of A128587; a(n+2) = a(n+1) + a(n), n>3.

%F a(n) = A068922(n-1), n>2. - _R. J. Mathar_, Jun 14 2008

%F For n > 1: a(n+1) = a(n) + if a(n) odd then max{a(n),a(n-1)} else min{a(n),a(n-1)}, see also A038754. - _Reinhard Zumkeller_, Oct 19 2015

%e a(4) = 6 = 1*1 + 3*1 + 3*1 + 1*(-1); where A128587 = (1, 1, 1, -1, 3, -5, 9,...).

%e G.f. = x + 2*x^2 + 4*x^3 + 6*x^4 + 10*x^5 + 16*x^6 + 26*x^7 + 42*x^8 + ...

%t nn=20;a=(1-x^3)/(1-x);b=x (1-x^2)/(1-x);CoefficientList[Series[a^2 /(1-b^2),{x,0,nn}],x] (* _Geoffrey Critzer_, Sep 01 2012 *)

%t LinearRecurrence[{1,1},{1,2,4},40] (* _Harvey P. Dale_, Mar 29 2017 *)

%o (Haskell)

%o a128588 n = a128588_list !! (n-1)

%o a128588_list = 1 : cows where

%o cows = 2 : 4 : zipWith (+) cows (tail cows)

%o -- _Reinhard Zumkeller_, May 14 2015

%o (PARI) {a(n) = if( n<2, n==1, 2 * fibonacci(n))}; /* _Michael Somos_, Jul 18 2015 */

%Y Cf. A128587, A128586, A007318.

%Y Cf. A006355, A055389.

%Y Cf. A232642, A242593.

%Y Cf. A038754.

%K nonn,easy

%O 1,2

%A _Gary W. Adamson_, Mar 11 2007

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Last modified September 25 19:48 EDT 2017. Contains 292500 sequences.