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A177210
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Numbers k that are the products of two distinct primes such that 2*k-1 are also products of two distinct primes.
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11
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26, 33, 35, 39, 46, 58, 62, 65, 93, 94, 111, 118, 119, 133, 134, 146, 155, 161, 178, 183, 202, 206, 209, 214, 219, 226, 235, 237, 247, 249, 253, 259, 267, 287, 291, 295, 299, 334, 335, 341, 362, 377, 382, 386, 391, 393, 395, 407, 422, 445, 447, 451, 453, 478
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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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26 is a term because 26 = 2*13 and 2*26 - 1 = 51 = 3*17;
33 is a term because 33 = 3*11 and 2*33 - 1 = 65 = 5*13.
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MATHEMATICA
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f[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; Do[If[f[n]&&f[2*n-1], AppendTo[lst, n]], {n, 0, 4*6!}]; lst
Select[Range[500], PrimeNu[#]==PrimeOmega[#]==PrimeNu[2#-1] == PrimeOmega[ 2#-1] == 2&] (* Harvey P. Dale, May 23 2014 *)
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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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