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A177210
Numbers k that are the products of two distinct primes such that 2*k-1 are also products of two distinct primes.
11
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
OFFSET
1,1
EXAMPLE
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.
MATHEMATICA
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 *)
CROSSREFS
Cf. A006881.
Sequence in context: A259041 A134252 A153224 * A050770 A050438 A047822
KEYWORD
nonn
AUTHOR
STATUS
approved