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Search: id:A133123
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| A133123 |
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Double Sophie Germain semiprimes: semiprimes s such that s1=2s+1 and s2=2s1+1 are also semiprimes. |
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| 38, 46, 106, 129, 133, 145, 169, 201, 203, 235, 289, 291, 298, 334, 335, 381, 407, 417, 458, 489, 497, 529, 538, 579, 583, 597, 623, 626, 649, 685, 689, 694, 707, 758, 767, 781, 815, 898, 899, 921, 926, 959, 995, 1073, 1079, 1082, 1094, 1099, 1139, 1142
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OFFSET
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1,1
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COMMENT
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Numbers n such that both n and 2n+1 are in A111153.
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FORMULA
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n, n1=2n+1 and n2=2n1+1 are semiprimes.
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EXAMPLE
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38=2*19, 2*38+1=77=7*11, and 2*77+1=155=5*31;
129=3*43, 2*129+1=259=7*37, and 2*259+1=519=3*173.
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MATHEMATICA
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fQ[n_]:=2==Plus@@Last/@FactorInteger[n]; Select[Range[2000], fQ[ # ]&&fQ[2#+1]&&fQ[4#+3]&]
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CROSSREFS
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Cf. A111153.
Sequence in context: A066473 A031409 A078550 this_sequence A078539 A125970 A026047
Adjacent sequences: A133120 A133121 A133122 this_sequence A133124 A133125 A133126
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)yahoo.com), Sep 19 2007
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