A178661 - OEIS

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A178661

Semiprimes of the form m*k such that (m+1)/(k-1) is also semiprime.

6, 10, 21, 26, 33, 51, 57, 74, 87, 115, 122, 123, 129, 146, 161, 201, 291, 295, 303, 314, 327, 339, 371, 386, 411, 413, 415, 489, 515, 519, 543, 554, 581, 611, 623, 626, 633, 649, 655, 687, 695, 723, 755, 771, 794, 842, 843, 849, 914, 917, 923, 951, 979, 993

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

EXAMPLE

a(1)=6 because semiprime=6=3*2 and (3+1)/(2-1)=4=semiprime.

MATHEMATICA

spQ[n_]:=Module[{fs=Transpose[FactorInteger[n]][[1]]}, PrimeOmega[(Last[fs] + 1)/(First[fs] - 1)] == 2]; Select[Select[Range[1000], PrimeOmega[#] == 2&], spQ] (* Vincenzo Librandi, Sep 22 2012 *)

CROSSREFS

Cf. A178379.

Sequence in context: A254028 A085274 A175648 * A190273 A030007 A323729

Adjacent sequences: A178658 A178659 A178660 * A178662 A178663 A178664

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Jun 01 2010

EXTENSIONS

Corrected (339, 515, 755 inserted) by R. J. Mathar, Jun 04 2010

STATUS

approved