A157342 Semiprimes that are the product of two non-Sophie Germain primes.

49, 91, 119, 133, 169, 217, 221, 247, 259, 289, 301, 323, 329, 361, 403, 413, 427, 469, 481, 497, 511, 527, 553, 559, 589, 611, 629, 679, 703, 707, 721, 731, 749, 763, 767, 793, 799, 817, 871, 889, 893, 923, 949, 959, 961, 973, 1003, 1027, 1037, 1043, 1057

Offset

1,1

Links

G. C. Greubel, Table of n, a(n) for n = 1..5000

Example

49 = 7*7, 2*7 + 1 = 15 (not prime);
91 = 7*13, 2*7 + 1 = 15 (not prime), 2*13 + 1 = 27 (not prime); ...

Mathematica

lst={}; Do[If[Plus@@Last/@FactorInteger[n]==2, a=First/@FactorInteger[n]; b=a[[1]]; k=0; If[Length[a]==2, c=a[[2]]; If[PrimeQ[2*c+1], k=1]]; If[ !PrimeQ[2*b+1]&&k==0, AppendTo[lst, n]]], {n, 7!}]; lst
With[{nn=60}, Take[Times@@@Tuples[Select[Prime[Range[nn]], !PrimeQ[ 2#+1]&], 2] // Union, nn]] (* Harvey P. Dale, Feb 15 2017 *)

Crossrefs

Cf. A001358, A005384, A111206.

Sequence in context: A158725 A090825 A350704 * A230226 A178705 A118886

Adjacent sequences: A157339 A157340 A157341 * A157343 A157344 A157345

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, Feb 27 2009

Status

approved