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A104255 a(n) = floor( (2n-1)!!/(2n) ). 1
0, 0, 2, 13, 94, 866, 9652, 126689, 1914412, 32736453, 624968662, 13176422634, 304071291562, 7623501667031, 206342778454312, 5996836998828457, 186254702081260312, 6156752652130549218, 215810382437839251562, 7995774669321944270390, 312215963278285442939062 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Asymptotic estimate for A007769(n).

LINKS

Table of n, a(n) for n=1..21.

A. Khruzin, Enumeration of chord diagrams

MATHEMATICA

Table[Floor[(2n-1)!!/(2n)], {n, 20}] (* Harvey P. Dale, Nov 12 2012 *)

CROSSREFS

Sequence in context: A209470 A140636 A282724 * A118352 A320360 A074614

Adjacent sequences:  A104252 A104253 A104254 * A104256 A104257 A104258

KEYWORD

nonn,changed

AUTHOR

Ralf Stephan, Mar 02 2005

STATUS

approved

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Last modified October 20 15:22 EDT 2018. Contains 316388 sequences. (Running on oeis4.)