A128811 A113551(n)/A006882(n).

1, 4, 35, 240, 3003, 26880, 415701, 4435200, 79676025, 968647680, 19535112675, 263472168960, 5837993287875, 85816649318400, 2058476433304725, 32567418416332800, 836589039864843825, 14112547980410880000

Offset: 1

Author

Jonathan Vos Post, Apr 10 2007

Keywords

• easy
• nonn

Comments

a(n) is to A001700 as n!! is to n!.

Examples

			a(1) = (1)/(1) = 1.
a(2) = (2*4)/(2) = 4.
a(3) = (3*5*7)/(3*1) = 35.
a(4) = (4*6*8*10)/(4*2) = 240.
a(5) = (5*7*9*11*13)/(5*3*1) = 3003.
a(6) = (6*8*10*12*14*16)/(6*4*2) = 26880.
		

Crossrefs

Cf. A001700, A006882, A113551.

Formula

a(n) = n * (n+2) * ... * ((3*n)-2) / n!!.

Extensions

Corrected and extended by Klaus Brockhaus, Apr 15 2007