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 A018191 Number of symmetric chord diagrams of degree n. 2

%I

%S 1,2,5,16,53,206,817,3620,16361,80218,401501,2139512,11641885,

%T 66599846,388962953,2367284236,14700573137,94523836850,619674301621,

%U 4186249123808,28809504493061,203556335785342,1463877667140065,10777146970619636,80686484464418233

%N Number of symmetric chord diagrams of degree n.

%H Alexander Stoimenow, <a href="https://doi.org/10.1016/S0012-365X(99)00347-7">On the number of chord diagrams</a>, Discr. Math. 218 (2000), 209-233.

%F Formula due to _Valery A. Liskovets_.

%F a(n) = A047974(n-1)+(n-1)*A047974(n-2) = A081126(n-1). - _Vladeta Jovovic_, Aug 06 2006

%t a[n_] := Sum[Binomial[n-1, k] k! / Floor[k/2]!, {k, 0, n}];

%t Array[a, 25] (* _Jean-François Alcover_, Aug 29 2019 *)

%K nonn

%O 1,2

%A Alexander Stoimenow (stoimeno(AT)math.toronto.edu)

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Last modified July 26 22:10 EDT 2021. Contains 346300 sequences. (Running on oeis4.)