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A078678
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Number of binary strings with n 1's and n 0's avoiding zigzags, that is avoiding the substrings 101 and 010.
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6
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1, 2, 4, 8, 18, 42, 100, 242, 592, 1460, 3624, 9042, 22656, 56970, 143688, 363348, 920886, 2338566, 5949148, 15157874, 38674978, 98803052, 252701484, 646990518, 1658066668, 4252908542, 10917422860, 28046438252, 72099983802, 185469011130, 477383400300
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OFFSET
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0,2
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COMMENTS
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Also number of Grand Dyck paths of length 2*n with no zigzags, that is, with no factors UDU or DUD. [Emanuele Munarini, Jul 07 2011]
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LINKS
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FORMULA
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G.f.: sqrt( ( 1 + x + x^2 ) / ( 1 - 3*x + x^2 ) ).
a(n) = sum( binomial( n - k + 2*floor(k/3), floor(k/3) )^2, k=0..n+floor(n/2)).
a(n) = sum( binomial(n-k,k)^2*( 2*n^2 - 6*n*k + 6*k^2 )/(n-k)^2 ), k=0..floor(n/2) ).
a(n) ~ 2 * ((3+sqrt(5))/2)^n / (5^(1/4)*sqrt(Pi*n)). - Vaclav Kotesovec, Mar 21 2014
a(n) = [x^n y^n](1+x*y+x^2*y^2)/(1-x-y+x*y-x^2*y^2). - Gheorghe Coserea, Jul 18 2016
D-finite with recurrence: n*a(n) -2*n*a(n-1) +(-n+2)*a(n-2) +2*(-n+4)*a(n-3) +(n-4)*a(n-4)=0. [Doslic] - R. J. Mathar, Jun 21 2018
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EXAMPLE
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For n = 2 : 0011, 0110, 1001, 1100.
For n = 3 : 000111, 011001, 100011, 110001, 001110, 011100, 100110, 111000.
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MAPLE
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a:= proc(n) option remember; `if`(n<5, [1, 2, 4, 8, 18][n+1],
(2*n*a(n-1)+(n-2)*a(n-2)+(2*n-8)*a(n-3)-(n-4)*a(n-4))/n)
end:
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MATHEMATICA
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Table[SeriesCoefficient[Series[Sqrt[(1 + x + x^2)/(1 - 3 x + x^2)], {x, 0, n}], n], {n, 0, 40}]
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PROG
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(Maxima) a(n):=coeff(taylor((1+x+x^2)/sqrt(1-2*x-x^2-2*x^3+x^4), x, 0, n), x, n);
makelist(a(n), n, 0, 12); [Emanuele Munarini, Jul 07 2011]
(PARI) x='x+O('x^99); Vec(((1+x+x^2)/(1-3*x+x^2))^(1/2)) \\ Altug Alkan, Jul 18 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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