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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS 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 Adjacent sequences:  A177207 A177208 A177209 * A177211 A177212 A177213 KEYWORD nonn AUTHOR Vladimir Joseph Stephan Orlovsky, May 04 2010 STATUS approved

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Last modified December 1 22:12 EST 2021. Contains 349435 sequences. (Running on oeis4.)