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A097801 (2*n)!/(n!*2^(n-1)). 7
2, 2, 6, 30, 210, 1890, 20790, 270270, 4054050, 68918850, 1309458150, 27498621150, 632468286450, 15811707161250, 426916093353750, 12380566707258750, 383797567925021250, 12665319741525701250, 443286190953399543750, 16401589065275783118750, 639661973545755541631250 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Equals 2*A001147.

REFERENCES

Right-hand edge of triangle in A097749.

LINKS

Table of n, a(n) for n=0..20.

Guo-Niu Han, Enumeration of Standard Puzzles

Guo-Niu Han, Enumeration of Standard Puzzles [Cached copy]

FORMULA

a(n) = 2*(2*n-1)!!. - Johannes W. Meijer, Nov 12 2009

E.g.f.: 2/sqrt(1-2*x). - Sergei N. Gladkovskii, Jul 06 2012

G.f.: G(0), where G(k)= 1 + 1/(1 - x*(2*k+1)/(x*(2*k+1) + 1/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, May 26 2013

a(n) = 2 * prod_{i=1..n} denominator( i!/(2i-1) ). - Wesley Ivan Hurt, Oct 12 2013

MAPLE

a:= proc(n) a(n):= `if`(n=0, 2, a(n-1)*(2*n-1)) end:

seq(a(n), n=0..25);  # Alois P. Heinz, May 27 2013

MATHEMATICA

FoldList[Times, 2, Range[1, 39, 2]] (* Arkadiusz Wesolowski, May 08 2012 *)

2(2*Range[0, 20]-1)!! (* Harvey P. Dale, Apr 22 2013 *)

CROSSREFS

Appears in A167576. - Johannes W. Meijer, Nov 12 2009

Sequence in context: A278258 A067644 A184312 * A164347 A052584 A094303

Adjacent sequences:  A097798 A097799 A097800 * A097802 A097803 A097804

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Sep 21 2004

STATUS

approved

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Last modified September 22 00:25 EDT 2017. Contains 292326 sequences.