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 A003583 a(n) = (n+2)*2^(2*n-1) - (n/2)*binomial(2*n,n). 6

%I

%S 1,5,26,130,628,2954,13612,61716,276200,1223002,5367676,23383100,

%T 101215576,435712580,1866667448,7963424104,33846062544,143373104378,

%U 605518549660,2550438016812,10716162617336

%N a(n) = (n+2)*2^(2*n-1) - (n/2)*binomial(2*n,n).

%C a(n) gives the number of open partitions of a tree made of two chains with n points each, that share an added root. (An open partition pi of a tree T is a partition of the vertices of T with the property that, for each block B of pi, the upset of B is a union of blocks of pi.) - _Pietro Codara_, Jan 14 2015

%D Pietro Codara, Partitions of a finite partially ordered set, From Combinatorics to Philosophy: The Legacy of G.-C. Rota, Springer, New York (2009), 45-59.

%H G. C. Greubel, <a href="/A003583/b003583.txt">Table of n, a(n) for n = 0..1000</a>

%H N. J. Calkin, <a href="http://dx.doi.org/10.1016/0012-365X(94)90394-8">A curious binomial identity</a>, Discr. Math., 131 (1994), 335-337.

%H Pietro Codara, Ottavio D'Antona, Francesco Marigo, Corrado Monti, <a href="http://arxiv.org/abs/1307.1348">Making simple proofs simpler</a>, arXiv:1307.1348 [cs.MS], 2013.

%H Zachary Hamaker, Eric Marberg, <a href="https://arxiv.org/abs/1802.09805">Atoms for signed permutations</a>, arXiv:1802.09805 [math.CO], 2018.

%H M. Hirschhorn, <a href="http://dx.doi.org/10.1016/0012-365X(95)00086-C">Calkin's binomial identity</a>, Discr. Math., 159 (1996), 273-278.

%H Jun Wang and Zhizheng Zhang, <a href="http://dx.doi.org/10.1016/S0012-365X(03)00206-1">On extensions of Calkin's binomial identities</a>, Discrete Math., 274 (2004), 331-342.

%F Main diagonal of correlation matrix of A055248. a(n) = Sum_{k=0..n} ( Sum_{m=k..n} binomial(n, m))^2 ). - _Paul Barry_, Jun 05 2003

%F Let S2 := (n, t)->add( k^t * (add( binomial(n, j), j=0..k))^2, k=0..n); a(n) = S2(n, 0).

%F From _Robert Israel_, Jan 13 2015: (Start)

%F G.f.: (1-2*x)/(1-4*x)^2 - x/(1 - 4*x)^(3/2).

%F E.g.f.: (2*x+1)*exp(4*x) - x*exp(2*x)*(I_0(2*x)+I_1(2*x)) where I_0 and I_1 are modified Bessel functions.

%F a(n) ~ 4^n*(n/2 - sqrt(n)/(2*sqrt(Pi)) + 1 + O(n^(-1/2))).

%F (End)

%p seq((n+2)*2^(2*n-1)-(n/2)*binomial(2*n,n), n=0..50); # _Robert Israel_, Jan 13 2015

%t Table[(n+2)*2^(2*n-1)-(n/2)*Binomial[2*n,n], {n,0,50}] (* _Pietro Codara_, Jan 14 2015 *)

%t Table[Sum[Sum[Binomial[n-1,k-1]Binomial[n-1,j-1]Min[k,j],{j,1,n}],{k,1 n}],{n,1,51}] (* _Pietro Codara_, Jan 14 2015 *)

%o (PARI) x='x+O('x^50); Vec((1-2*x)/(1-4*x)^2 - x/(1 - 4*x)^(3/2)) \\ _G. C. Greubel_, Feb 15 2017

%Y If the exponent E in a(n) = Sum_{m=0..n} (Sum_{k=0..m} C(n,k))^E is 1, 2, 3, 4, 5 we get A001792, A003583, A007403, A294435, A294436 respectively.

%K nonn

%O 0,2

%A _N. J. A. Sloane_

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Last modified June 5 12:47 EDT 2020. Contains 334840 sequences. (Running on oeis4.)