A091424 Numbers m such that m#*2^m + 1 is prime, where m# = A002110(m).

1, 3, 4, 6, 10, 30, 31, 98, 156, 230, 432, 490, 1623, 1666, 9324, 9693

Offset: 1

Author

Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 02 2004

Keywords

• nonn
• hard
• more

Comments

1# = 2, 2# = 2*3 = 6, 3# = 2*3*5 = 30.

No more terms < 5000. - L. Joris Perrenet, Mar 17 2020

Examples

			a(1) = 1 because 1#*2^1 + 1 = 5 is prime
a(2) = 3 because 3#*2^3 + 1 = 241 is prime
		

Crossrefs

Cf. A002110, A091421.

Programs

PARI

pp(n)=my(s=1);for(i=1,n,s=s*prime(i));return(s);
f(n)=pp(n)*2^n +1;
for (i=1,500,if(isprime(f(i)),print1(i, ", ")))

Extensions

a(13)-a(14) from Chai Wah Wu, Dec 23 2019

a(15)-a(16) from Michael S. Branicky, Aug 31 2024