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 A187306 Alternating sum of Motzkin numbers A001006. 9
 1, 0, 2, 2, 7, 14, 37, 90, 233, 602, 1586, 4212, 11299, 30536, 83098, 227474, 625993, 1730786, 4805596, 13393688, 37458331, 105089228, 295673995, 834086420, 2358641377, 6684761124, 18985057352, 54022715450, 154000562759, 439742222070, 1257643249141 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Diagonal sums of A089942. Hankel transform is A187307. Also gives the number of simple permutations of each length that avoid the pattern 321 (i.e., are the union of two increasing sequences, and in one line notation contain no nontrivial block of values which form an interval). There are 2 such permutations of length 4, 2 of length 5, etc. - Michael Albert, Jun 20 2012 Convolution of A005043 with itself. - Philippe Deléham, Jan 28 2014 From Gus Wiseman, Nov 15 2022: (Start) Conjecture: Also the number of topologically series-reduced ordered rooted trees with n + 2 vertices. This would imply a(n) = A284778(n-1) + A005043(n). For example, the a(0) = 1 through a(5) = 14 trees are: (o) . (ooo) (oooo) (ooooo) (oooooo) ((oo)) ((ooo)) ((oo)oo) ((oo)ooo) ((oooo)) ((ooo)oo) (o(oo)o) ((ooooo)) (oo(oo)) (o(oo)oo) (((oo)o)) (o(ooo)o) ((o(oo))) (oo(oo)o) (oo(ooo)) (ooo(oo)) (((oo)oo)) (((ooo)o)) ((o(oo)o)) ((o(ooo))) ((oo(oo))) (End) LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..300 M. H. Albert and V. Vatter, Generating and enumerating 321-avoiding and skew-merged simple permutations, arXiv preprint arXiv:1301.3122 [math.CO], 2013. - N. J. A. Sloane, Feb 11 2013 Paul Barry, Jacobsthal Decompositions of Pascal's Triangle, Ternary Trees, and Alternating Sign Matrices, Journal of Integer Sequences, 19, 2016, #16.3.5. Cheyne Homberger, Patterns in Permutations and Involutions: A Structural and Enumerative Approach, arXiv preprint 1410.2657 [math.CO], 2014. FORMULA G.f.: (1-x-sqrt(1-2*x-3*x^2))/(2*x^2*(1+x)). a(n) = sum(k=0..n, A001006(k)*(-1)^(n-k)). D-finite with recurrence -(n+2)*a(n) +(n-1)*a(n-1) +(5*n-2)*a(n-2) +3*(n-1)a(n-3)=0. - R. J. Mathar, Nov 17 2011 a(n) = (2*sum(j=0..n, C(2*j+1,j+1)*(-1)^(n-j)*C(n+2,j+2)))/(n+2). - Vladimir Kruchinin, Feb 06 2013 a(n) ~ 3^(n+5/2)/(8*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Aug 15 2013 a(n) = (-1)^n*(1-hypergeom([1/2,-n-1],[2],4)). - Peter Luschny, Sep 25 2014 a(n) = A005043(n+1) + (-1)^n. - Peter Luschny, Sep 25 2014 G.f.: (1/(1 - x^2/(1 - x - x^2/(1 - x - x^2/(1 - x - x^2/(1 - ...))))))^2, a continued fraction. - Ilya Gutkovskiy, Sep 23 2017 MAPLE a := n -> (-1)^n*(1-hypergeom([1/2, -n-1], [2], 4)); seq(round(evalf(a(n), 99)), n=0..30); # Peter Luschny, Sep 25 2014 MATHEMATICA CoefficientList[Series[(1-x-Sqrt[1-2x-3x^2])/(2x^2(1+x)), {x, 0, 30}], x] (* Harvey P. Dale, Jun 14 2011 *) PROG (PARI) x='x+O('x^66); Vec((1-x-sqrt(1-2*x-3*x^2))/(2*x^2*(1+x))) /* Joerg Arndt, Mar 07 2013 */ (PARI) Vec(serreverse(x*(1-x)/(1-x+x^2) + O(x^30))^2) \\ Andrew Howroyd, Apr 28 2018 (Sage) def A187306(): a, b, n = 1, 0, 1 yield a while True: n += 1 a, b = b, (2*b+3*a)*(n-1)/(n+1) yield b - (-1)^n A187306_list = A187306() [next(A187306_list) for i in range(20)] # Peter Luschny, Sep 25 2014 CROSSREFS Cf. A001006, A005043, A089942. Sequence in context: A228432 A298959 A162460 * A061274 A061575 A306009 Adjacent sequences: A187303 A187304 A187305 * A187307 A187308 A187309 KEYWORD nonn,easy AUTHOR Paul Barry, Mar 08 2011 STATUS approved

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Last modified April 20 15:29 EDT 2024. Contains 371844 sequences. (Running on oeis4.)