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A001002
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Number of dissections of a convex (n+2)-gon into triangles and quadrilaterals by nonintersecting diagonals.
(Formerly M2852 N1146)
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30
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1, 1, 3, 10, 38, 154, 654, 2871, 12925, 59345, 276835, 1308320, 6250832, 30142360, 146510216, 717061938, 3530808798, 17478955570, 86941210950, 434299921440, 2177832612120, 10959042823020, 55322023332420, 280080119609550, 1421744205767418, 7234759677699954
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OFFSET
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0,3
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COMMENTS
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a(n+1) is number of (2,3)-rooted trees on n nodes.
This sequence appears to be a transform of the Fibonacci numbers A000045. This sequence is to the Fibonacci numbers as the Catalan numbers A000108 is to the all ones sequence. See link to Mathematica program. - Mats Granvik, Dec 30 2017
a(n) is the number of parking functions of size n avoiding the patterns 231, 312, and 321. - Lara Pudwell, Apr 10 2023
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REFERENCES
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F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Cambridge, 1998, p. 211 (3.2.73-74)
I. M. H. Etherington, On non-associative combinations, Proc. Royal Soc. Edinburgh, 59 (Part 2, 1938-39), 153-162.
I. M. H. Etherington, Some problems of non-associative combinations (I), Edinburgh Math. Notes, 32 (1940), pp. i-vi. Part II is by A. Erdelyi and I. M. H. Etherington, and is on pages vii-xiv of the same issue.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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I. M. H. Etherington, Some problems of non-associative combinations (I), Edinburgh Math. Notes, 32 (1940), pp. i-vi. [Annotated scanned copy]. Part II [not scanned] is by A. Erdelyi and I. M. H. Etherington, and is on pages vii-xiv of the same issue.
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FORMULA
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G.f. (offset 1) is series reversion of x - x^2 - x^3.
a(n) = (1/(n+1))*Sum_{k=ceiling(n/2)..n} binomial(n+k, k)*binomial(k, n-k). - Len Smiley
D-finite with recurrence 5*n*(n+1) * a(n) = 11*n*(2*n-1) * a(n-1) + 3*(3*n-2)*(3*n-4) * a(n-2). - Len Smiley
G.f.: (4*sin(asin((27*x+11)/16)/3)-1)/(3*x). - Paul Barry, Feb 02 2005
G.f. satisfies: A(x) = 1 + x*A(x)^2 + x^2*A(x)^3. - Paul D. Hanna, Jun 22 2012
Antidiagonal sums of triangle A104978 which has g.f. F(x,y) that satisfies: F = 1 + x*F^2 + x*y*F^3. - Paul D. Hanna, Mar 30 2005
a(n) = Sum_{k=0..floor(n/2)} C(2*n-k, n+k)*C(n+k, k)/(n+1). - Paul D. Hanna, Mar 30 2005
G.f. satisfies: x = Sum_{n>=1} 1/(1+x*A(x))^(2*n) * Product_{k=1..n} (1 - 1/(1+x*A(x))^k). - Paul D. Hanna, Apr 05 2012
G.f.: 1 + (1/x)*Sum_{n>=1} d^(n-1)/dx^(n-1) (x^2+x^3)^n/n!. - Paul D. Hanna, Jun 22 2012
G.f.: exp( Sum_{n>=1} d^(n-1)/dx^(n-1) ((x^2+x^3)^n/x)/n! ). - Paul D. Hanna, Jun 22 2012
a(n) ~ 3^(3*n+3/2) / (2 * sqrt(2*Pi) * 5^(n+1/2) * n^(3/2)). - Vaclav Kotesovec, Mar 09 2014
a(n) = Catalan(n)*hypergeom([1/2-n/2, -n/2], [-2*n], -4) for n>0. - Peter Luschny, Oct 03 2014
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EXAMPLE
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a(3)=10 because a convex pentagon can be dissected in 5 ways into triangles (draw 2 diagonals from any of the 5 vertices) and in 5 ways into a triangle and a quadrilateral (draw any of the 5 diagonals).
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MAPLE
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a:= proc(n) option remember; `if`(n<2, 1, (n*(22*n-11)*
a(n-1) + (9*n-6)*(3*n-4)*a(n-2))/(5*n*(n+1)))
end:
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MATHEMATICA
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Rest[CoefficientList[InverseSeries[Series[y - y^2 - y^3, {y, 0, 30}], x], x]]
a[n_] := a[n] = If[n == 0, 1, Sum[a[i] a[n - 1 - i], {i, 0, n - 1}] + Sum[a[i] a[j] a[n - 2 - i - j], {i, 0, n - 2}, {j, 0, n - 2 - i}]];
Table[a[n], {n, 0, 30}] (* Li Han, Jan 02 2021 *)
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PROG
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(PARI) a(n)=if(n<0, 0, polcoeff(serreverse(x-x^2-x^3+x^2*O(x^n)), n+1))
(PARI) a(n)=if(n<0, 0, sum(k=0, n\2, (2*n-k)!/k!/(n-2*k)!)/(n+1)!)
(PARI) a(n)=sum(k=0, n\2, binomial(2*n-k, n+k)*binomial(n+k, k))/(n+1) \\ Hanna
(PARI) {Dx(n, F)=local(D=F); for(i=1, n, D=deriv(D)); D}
{a(n)=local(A=1); A=1+(1/x)*sum(m=1, n+1, Dx(m-1, (x^2+x^3 +x^2*O(x^n))^m/m!)); polcoeff(A, n)} \\ Paul D. Hanna, Jun 22 2012
(PARI) {Dx(n, F)=local(D=F); for(i=1, n, D=deriv(D)); D}
{a(n)=local(A=1); A=exp(sum(m=1, n+1, Dx(m-1, (x^2+x^3 +x^2*O(x^n))^m/x/m!)+x*O(x^n))); polcoeff(A, n)} \\ Paul D. Hanna, Jun 22 2012
(Maxima)
T(n, k):=if n<0 or k<0 then 0 else if n<k then 0 else if n=k then 1 else if k=0 then 0 else T(n-1, k-1)+T(n, k+1)+T(n, k+2);
(Sage)
A001002 = lambda n: catalan_number(n)*hypergeometric([1/2-n/2, -n/2], [-2*n], -4) if n>0 else 1
(GAP) List([0..25], n->Sum([0..Int(n/2)], k->Binomial(2*n-k, n+k)*Binomial(n+k, k)/(n+1))); # Muniru A Asiru, Mar 30 2018
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CROSSREFS
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KEYWORD
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nonn,easy,nice
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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