A176896 Semiprimes s such that (s-1)/2 is also semiprime.

9, 21, 51, 69, 77, 93, 111, 115, 155, 183, 187, 213, 237, 247, 259, 267, 287, 291, 319, 323, 339, 355, 371, 403, 407, 411, 413, 427, 437, 453, 471, 519, 535, 579, 583, 591, 597, 611, 629, 655, 669, 671, 679, 717, 723, 731, 755, 763, 789, 791, 807, 815, 831

Offset: 1

Author

Juri-Stepan Gerasimov, Apr 28 2010

Keywords

• nonn

Comments

Semiprime analog of safe primes (A005385). - Zak Seidov, Feb 01 2017

Examples

			a(1)=9 because (3*3 - 1)/2 = 4 = 2*2; a(2)=21 because (3*7 - 1)/2 = 10 = 2*5.
		

Links

• Zak Seidov, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001358, A005385, A111153.

Programs

Mathematica

Select[Range[1000],PrimeOmega[#]==PrimeOmega[(#-1)/2]==2&] (* Harvey P. Dale, Jan 28 2016 *)

Formula

a(n) = 2*A111153(n) + 1. - Zak Seidov, Sep 10 2012

Extensions

Corrected (497 and 745 removed) by R. J. Mathar, May 01 2010