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A176896 Semiprimes s such that (s-1)/2 is also semiprime. 3
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 (list; graph; refs; listen; history; text; internal format)
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 * A134717 A216980 A147169

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

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Last modified June 22 13:46 EDT 2021. Contains 345380 sequences. (Running on oeis4.)