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 A068875 Expansion of (1+x*C)*C, where C = (1-(1-4*x)^(1/2))/(2*x) is the g.f. for Catalan numbers, A000108. 18
 1, 2, 4, 10, 28, 84, 264, 858, 2860, 9724, 33592, 117572, 416024, 1485800, 5348880, 19389690, 70715340, 259289580, 955277400, 3534526380, 13128240840, 48932534040, 182965127280, 686119227300, 2579808294648, 9723892802904 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS A Catalan transform of A040000 under the mapping g(x)->g(xc(x)). A040000 can be retrieved using the mapping g(x)->g(x(1-x)). A040000(n) = Sum_{k=0..floor(n/2)} C(n-k,k)*(-1)^k*a(n-k). a(n) and A040000 may be described as a Catalan pair. - Paul Barry, Nov 14 2004 a(n) = number of Dyck (n+1)-paths all of whose nonterminal descents to ground level are of odd length. For example, a(2) counts UUUDDD, UUDUDD, UDUUDD, UDUDUD. - David Callan, Jul 25 2005 For n >= 1, a(n) is the number of binary trees with n+1 internal nodes in which one of the subtrees of the root is empty. Cf. A002057. [Sedgewick and Flajolet] - Geoffrey Critzer, Jan 05 2013 Empirical: a(n) is the number of entries of absolute value 1 that appear among all partitions in the canonical basis of the Temperley-Lieb algebra of order n. - John M. Campbell, Oct 17 2017 REFERENCES R. Sedgewick and P Flajolet, Analysis of Algorithms, Addison Wesley, 1996, page 225. LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 Paul Barry, A Catalan Transform and Related Transformations on Integer Sequences, Journal of Integer Sequences, Vol. 8 (2005), Article 05.4.5. Paul Barry, Riordan Pseudo-Involutions, Continued Fractions and Somos 4 Sequences, arXiv:1807.05794 [math.CO], 2018. S. J. Cyvin, J. Brunvoll, E. Brendsdal, B. N. Cyvin and E. K. Lloyd, Enumeration of polyene hydrocarbons: a complete mathematical solution, J. Chem. Inf. Comput. Sci., 35 (1995) 743-751. [Annotated scanned copy] Guo-Niu Han, Enumeration of Standard Puzzles Guo-Niu Han, Enumeration of Standard Puzzles [Cached copy] FORMULA Apart from initial term, twice Catalan numbers. G.f.: (1 - x - sqrt(1 - 4*x)) / x. - Michael Somos, Apr 13 2012 G.f.: (1+x*c(x))/(1-x*c(x)), where c(x) is the g.f. of A000108; a(n)=C(n)*(2-0^n); C(n) as in A000108; a(n) = Sum_{j=0..n} Sum_{k=0..n} C(2*n, n-k)*((2*k+1)/(n+k+1))*binomial(k, j)*(-1)^(j-k)*(2-0^j). - Paul Barry, Nov 14 2004 Assuming offset 1, then series reversion of g.f. A(x) is -A(-x). - Michael Somos, Aug 17 2005 Assuming offset 2, then A(x) satisfies A(x - x^2) = x^2 - x^4 and so A(x) = C(x)^2 - C(x)^4, A(A(x)) = C(x)^4 - C(x)^8, A(A(A(x))) = C(x)^8 - C(x)^16, etc., where C(x) = (1-sqrt(1-4*x))/2 = x + x^2 + 2*x^3 + 5*x^4 + 14*x^5 + ... . - Paul D. Hanna, May 16 2008 Apart from initial term, INVERTi transform of A000984(n+1) = binomial(2*n+2,n+1), also, for n >= 1, a(n) = (1/Pi)*Integral_{x=0..4} x^(n-1)*sqrt(x*(4-x)). - Groux Roland, Mar 15 2011 a(n) = sum of top row terms in M^n, where M is the following infinite square production matrix:   1, 1, 0, 0, 0, 0, ...   0, 1, 1, 0, 0, 0, ...   1, 1, 1, 1, 0, 0, ...   0, 1, 1, 1, 1, 0, ...   1, 1, 1, 1, 1, 1, ...   ... For example, the top row of M^3 = (2, 4, 3, 1), sum = 10 = a(3). - Gary W. Adamson, Jul 11 2011 D-finite with recurrence (n+2)*a(n) - 2*(2*n+1)*a(n-1) = 0, n > 1. - R. J. Mathar, Nov 14 2011 For n > 0, a(n) = C(2n+2,n+1) mod 4*C(2n,n-1). - Robert G. Wilson v, May 02 2012 For n > 0, a(n) = 2^(2*n+1)*Gamma(n+1/2)/(sqrt(Pi)*(n+1)!). - Vaclav Kotesovec, Sep 16 2013 G.f.: 1 + 2*x/(Q(0)-x), where Q(k) = 2*x + (k+1)/(2*k+1) - 2*x*(k+1)/(2*k+1)/Q(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Dec 03 2013 G.f.: 3-4*x - 2*S(0), where S(k) = 2*k+1 - x*(2*k+3)/(1 - x*(2*k+1)/S(k+1) ); (continued fraction). - Sergei N. Gladkovskii, Dec 23 2013 0 = a(n)*(16*a(n+1) - 10*a(n+2)) + a(n+1)*(2*a(n+1) + a(n+2)) for all n in Z. - Michael Somos, Jun 18 2014 If A(x)^t = 1 + 2*t*x + Sum_{n >= 2} t*P(n,t)*x^n, then we conjecture that all the zeros of the polynomial P(n,t) lie on the vertical line Re(t) = -n/2 in the complex plane. - Peter Bala, Oct 05 2015 EXAMPLE G.f. = 1 + 2*x + 4*x^2 + 10*x^3 + 28*x^4 + 84*x^5 + 264*x^6 + 858*x^7 + ... For example, the canonical basis of the Temperley-Lieb algebra of order 3 is {{{-3, 1}, {-2, -1}, {2, 3}}, {{-3, 3}, {-2, 2}, {-1, 1}}, {{-3, 3}, {-2, -1}, {1, 2}}, {{-3, -2}, {-1, 1}, {2, 3}}, {{-3, -2}, {-1, 3}, {1, 2}}}, and we see that the total number of entries of absolute value 1 that appear among the partitions in this basis is a(3) = 10. MAPLE Z:=(1-sqrt(1-4*x))/2/x: G:=(2-(1+x)*Z)/Z: Gser:=series(-G, x=0, 30): (1, seq(coeff(Gser, x, n), n=2..26)); # Zerinvary Lajos, Dec 23 2006 Z:=-(1-z-sqrt(1-z))/sqrt(1-z): Zser:=series(Z, z=0, 32): (1, seq(coeff(Zser*4^n, z, n), n=2..26)); # Zerinvary Lajos, Jan 01 2007 MATHEMATICA nn=30; t=(1-(1-4x )^(1/2))/(2x); Prepend[Table[Coefficient[Series[1+x (y t -y+1)^2, {x, 0, nn}], x ^n y], {n, 2, nn}], 1]  (* Geoffrey Critzer, Jan 05 2013 *) a[ n_] := If[ n < 1, Boole[ n == 0], 2 Binomial[ 2 n, n]/(n + 1)]; (* Michael Somos, Jun 18 2014 *) a[ n_] := SeriesCoefficient[ -1 + 4 / (1 + Sqrt[ 1 - 4 x]), {x, 0, n}]; (* Michael Somos, Jun 18 2014 *) Table[If[n==0, 1, 2 CatalanNumber[n]], {n, 0, 25}] (* Peter Luschny, Feb 27 2017 *) PROG (PARI) {a(n) = if( n<1, n==0, 2 * binomial( 2*n, n) / (n + 1))}; /* Michael Somos, Aug 17 2005 */ (PARI) {a(n) = if( n<0, 0, polcoeff( -1 + 4 / (1 + sqrt(1 - 4*x + x * O(x^n))), n))}; /* Michael Somos, Aug 17 2005 */ (MAGMA)  cat [2*Binomial( 2*n, n)/(n+1): n in [1..30]]; // Vincenzo Librandi, Oct 17 2017 CROSSREFS A002420 and A262543 are essentially the same sequence as this. Cf. A000108, A000984, A002057, A040000, A068875. Sequence in context: A331938 A302146 A202135 * A262543 A289709 A192574 Adjacent sequences:  A068872 A068873 A068874 * A068876 A068877 A068878 KEYWORD nonn AUTHOR N. J. A. Sloane, Jun 06 2002 STATUS approved

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Last modified May 27 05:48 EDT 2020. Contains 334649 sequences. (Running on oeis4.)