A123910 Numbers k such that k!!-4 is prime.

5, 7, 9, 11, 13, 15, 19, 25, 35, 79, 81, 105, 171, 243, 271, 295, 355, 523, 591, 1211, 3073, 11157, 12887, 19825

Offset: 1

Author

Alexander Adamchuk, Oct 28 2006

Keywords

• hard
• more
• nonn

Comments

Corresponding primes of the form k!!-4 = a(n)!!-4 are {11, 101, 941, 10391, 135131, 2027021, 654729071, 7905853580621, 221643095476699771871, ...}.

a(25) > 50000. - Robert Price, May 08 2015

Links

• Joe McLean, Interesting Sources of Probable Primes

Crossrefs

Cf. A007749, A094144.

Programs

Mathematica

Do[f=n!!-4;If[PrimeQ[f],Print[{n,f}]],{n,1,355}]

Extensions

More terms from Farideh Firoozbakht, Nov 19 2006

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008

a(23)-a(24) from Robert Price, May 08 2015