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 A128014 Central binomial coefficients C(2n,n) repeated. 13
 1, 1, 2, 2, 6, 6, 20, 20, 70, 70, 252, 252, 924, 924, 3432, 3432, 12870, 12870, 48620, 48620, 184756, 184756, 705432, 705432, 2704156, 2704156, 10400600, 10400600, 40116600, 40116600, 155117520, 155117520, 601080390, 601080390 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Binomial transform is A097893. Hankel transform is A128017. Hankel transform of a(n+1) is A128018. - Paul Barry, Nov 23 2009 Number of 2n-bead balanced binary necklaces that are equivalent to their reverse. - Andrew Howroyd, Sep 29 2017 Number of ballot sequences of length n in which the vote is tied or decided by 1 vote. - Nachum Dershowitz, Aug 12 2020 Number of binary strings of length n that are abelian squares. - Michael S. Branicky, Dec 21 2020 LINKS Table of n, a(n) for n=0..33. FORMULA G.f.: (1+x)/sqrt(1-4*x^2). a(n) = C(n,n/2)*(1+(-1)^n)/2 + C(n-1,(n-1)/2)*(1-(-1)^n)/2. a(n) = (1/Pi)*Integral_{x=-2..2} x^n*(1+x)/(x*sqrt(4-x^2)), as moment sequence. E.g.f. of a(n+1): Bessel_I(0,2*x)+2*Bessel_I(1,2*x). - Paul Barry, Mar 26 2010 n*a(n) +(n-2)*a(n-1) +4*(-n+1)*a(n-2) +4*(-n+3)*a(n-3) = 0. - R. J. Mathar, Nov 26 2012 a(n) = 2^n*Product_{k=0..n-1} ((k/n+1/n)/2)^((-1)^k). - Peter Luschny, Dec 03 2013 From Reinhard Zumkeller, Nov 14 2014: (Start) a(n) = A000984(floor(n/2)). a(n) = A249095(n,n) = A249308(n) / 2^n. (End) MATHEMATICA (1+x)/Sqrt[1-4x^2] + O[x]^34 // CoefficientList[#, x]& (* Jean-François Alcover, Oct 07 2017 *) With[{cb=Table[Binomial[2n, n], {n, 0, 20}]}, Riffle[cb, cb]] (* Harvey P. Dale, Feb 17 2020 *) PROG (Haskell) a128014 = a000984 . flip div 2 -- Reinhard Zumkeller, Nov 14 2014 CROSSREFS Cf. A097893, A128017, A128018. Cf. A000984, A249095, A249308. Sequence in context: A309094 A109859 A128057 * A135401 A129881 A132369 Adjacent sequences: A128011 A128012 A128013 * A128015 A128016 A128017 KEYWORD easy,nonn AUTHOR Paul Barry, Feb 11 2007 STATUS approved

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Last modified February 20 20:53 EST 2024. Contains 370217 sequences. (Running on oeis4.)