1,1

a(n) is even for n>1. a(n) = 2*A091415(n-1) for n>1, where A091415(n) = {2, 3, 4, 8, 13, 32, 41, 45, 59, 97, 107, 364, 421, 444, 1164, 1738, 3202, 4335, 4841, ...} Numbers n such that n!*2^n - 1 is prime. Corresponding primes of the form n!!-1 are listed in A117141[n] = {2,7,47,383,10321919,51011754393599,...}. - Alexander Adamchuk (alex(AT)kolmogorov.com), Nov 19 2006

The Top Ten (a Catalogue of Primal Configurations) from the unpublished collections of R. Ondrejka, assisted by C. Caldwell and H. Dubner, March 11, 2000, Page 61.

R. Ondrejka, The Top Ten: a Catalogue of Primal Configurations

Index entries for sequences related to factorial numbers

Ken Davis, Status of Search for Multifactorial Primes.

Ken Davis, Results for n!2-1.

a(n) = 2*A091415(n-1) for n>1. - Alexander Adamchuk (alex(AT)kolmogorov.com), Nov 19 2006

a(1) = 3, for n>1 k=2; f=2; Do[k=k+2; f=f*k; If[PrimeQ[f-1], Print[k]], {n, 2, 5000}] - Alexander Adamchuk (alex(AT)kolmogorov.com), Nov 19 2006

Cf. A091415 = numbers n such that n!*2^n - 1 is prime. Cf. A117141 = Primes of the form n!! - 1.

Sequence in context: A050475 A025073 A134580 this_sequence A139452 A063506 A084438

Adjacent sequences: A007746 A007747 A007748 this_sequence A007750 A007751 A007752

nonn,hard,nice

N. J. A. Sloane (njas(AT)research.att.com).

Entry updated by Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 18 2000

Corrected and extended by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008