A110371 a(n)=[(n+1)(n+2)(n+3)...(2n)]/(1+2+3+...+n).

2, 4, 20, 168, 2016, 31680, 617760, 14414400, 392071680, 12189864960, 426645273600, 16606346803200, 711700577280000, 33307587016704000, 1690360041097728000, 92472637542405120000, 5425061402487767040000, 339780161524233830400000, 22629358757513973104640000

Offset: 1

Author

Amarnath Murthy, Jul 24 2005

Keywords

• easy
• nonn

Comments

a(n)=2(n-1)!*Catalan(n). a(n)=2*A065866(n-1). - Emeric Deutsch, Aug 05 2005

Examples

			a(4) = 5*6*7*8/10 = 168.
a(5) = 10*9*8*7*6/(5+4+3+2+1) = 2016.
		

Links

• Ron Knott, 2016, January 2016.

Crossrefs

Cf. A065866.

Programs

Maple

seq(2*(2*n)!/(n+1)!/n,n=1..20); # Emeric Deutsch, Aug 05 2005

Mathematica

Table[(Times@@Range[n+1,2n])/((n(n+1))/2),{n,20}] (* or *) Table[ 2(n-1)! CatalanNumber[n],{n,20}] (* Harvey P. Dale, Jul 15 2016 *)

PARI

A110371(n)=binomial(2*n,n-1)/n*(n-1)!*2 \\ M. F. Hasler, Jan 31 2016

Formula

{2*(2n)!}/{(n+1)!*n}

Extensions

More terms from Emeric Deutsch, Aug 05 2005

Edited by M. F. Hasler, Jan 31 2016