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A018191 Number of symmetric chord diagrams of degree n. 2
1, 2, 5, 16, 53, 206, 817, 3620, 16361, 80218, 401501, 2139512, 11641885, 66599846, 388962953, 2367284236, 14700573137, 94523836850, 619674301621, 4186249123808, 28809504493061, 203556335785342, 1463877667140065, 10777146970619636, 80686484464418233 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..25.

Alexander Stoimenow, On the number of chord diagrams, Discr. Math. 218 (2000), 209-233.

FORMULA

Formula due to Valery A. Liskovets.

a(n) = A047974(n-1)+(n-1)*A047974(n-2) = A081126(n-1). - Vladeta Jovovic, Aug 06 2006

MATHEMATICA

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

Array[a, 25] (* Jean-Fran├žois Alcover, Aug 29 2019 *)

CROSSREFS

Sequence in context: A148398 A100442 A081126 * A006191 A149959 A161941

Adjacent sequences:  A018188 A018189 A018190 * A018192 A018193 A018194

KEYWORD

nonn

AUTHOR

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

STATUS

approved

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Last modified November 18 22:26 EST 2019. Contains 329306 sequences. (Running on oeis4.)