A092558 Numbers k such that 2^k +- 1 are both semiprimes.

11, 23, 101, 167, 199, 347

Offset: 1

Author

Zak Seidov, Feb 27 2004

Keywords

• nonn

Comments

Intersection of A092559 and A085724.

a(7), if it exists, is at least 41519. - Charles R Greathouse IV, Jun 05 2013

2^41519 + 1 is the product of 3 and a composite number, so if a(7) exists, it exceeds 41519. - Jon E. Schoenfield, Feb 22 2022

Examples

			11 is a term because 2^11 - 1 = 23*89 and 2^11 + 1 = 3*683.
		

Crossrefs

Subsequence of A000040.

Cf. A092559, A085724, A092561, A092562.

Programs

PARI

is(n)=isprime(n) && n>7 && ispseudoprime((2^n+1)/3) && bigomega(2^n-1)==2 \\ Charles R Greathouse IV, Jun 05 2013

Extensions

a(6) from Robert G. Wilson v, Apr 18 2006