A289488 a(n) = denominator of Sum_{k=1..n} 1/(2*k-1)!!.

1, 3, 5, 105, 945, 3465, 135135, 2027025, 883575, 654729075, 13749310575, 8108567775, 718713961875, 213458046676875, 121378104973125, 191898783962510625, 372509404162520625, 73881031825566590625, 8200794532637891559375, 319830986772877770815625

Offset: 1

Author

Seiichi Manyama, Sep 02 2017

Keywords

• nonn
• frac

Examples

			1, 4/3, 7/5, 148/105, 1333/945, 4888/3465, 190633/135135, 2859496/2027025, 1246447/883575, 923617228/654729075, 19395961789/13749310575, 11438644132/8108567775, ... = A289381(n)/a(n) -> A060196.
		

Links

• Seiichi Manyama, Table of n, a(n) for n = 1..404
• OEIS Wiki, A remarkable formula of Ramanujan

Crossrefs

Cf. A001147, A060196, A289381, A354299.

Programs

Mathematica

Accumulate[Table[1/(2k-1)!!,{k,20}]]//Denominator (* Harvey P. Dale, Mar 01 2023 *)

Formula

Denominators of coefficients in expansion of sqrt(Pi*x*exp(x)/2) * erf(sqrt(x/2)) / (1 - x). - Ilya Gutkovskiy, May 24 2022