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A157342
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Semiprimes that are the product of two non-Sophie Germain primes.
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13
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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
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OFFSET
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1,1
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LINKS
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EXAMPLE
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49 = 7*7, 2*7 + 1 = 15 (not prime);
91 = 7*13, 2*7 + 1 = 15 (not prime), 2*13 + 1 = 27 (not prime); ...
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MATHEMATICA
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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 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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