A174838 Numbers n such that semiprime(n)+1 is prime, where semiprime(n) is A001358.

1, 2, 4, 8, 16, 21, 27, 35, 55, 58, 76, 84, 111, 113, 120, 143, 147, 155, 176, 183, 218, 252, 258, 265, 294, 304, 348, 377, 383, 387, 403, 424, 435, 444, 464, 525, 548, 582, 585, 593, 600, 633, 690, 694, 732, 787, 803, 810, 827, 841, 846, 877, 892, 900, 971

Offset: 1

Author

Juri-Stepan Gerasimov, Mar 30 2010

Keywords

• nonn

Comments

N is in the sequence iff semiprime(n) is of the form 2p where p is a Sophie Germain prime (A005384).

Examples

			a(1)=1 because semiprime(1)+1=5=prime, a(2)=2 because semiprime(2)+1=7=prime, a(3)=4 because semiprime(4)+1=11=prime.
		

Crossrefs

Cf. A000040, A001358, A005384.

Extensions

Edited, corrected and extended by Ray Chandler, Apr 05 2010