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

1,1

Comments

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

Links

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

Formula

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

Example

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

Mathematica

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

Crossrefs

Cf. A001358, A005385, A111153.

Sequence in context: A118130 A144482 A251212 * A347502 A134717 A216980

Adjacent sequences: A176893 A176894 A176895 * A176897 A176898 A176899

Keyword

nonn

Author

Juri-Stepan Gerasimov, Apr 28 2010

Extensions

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

Status

approved