A177210 Numbers k that are the products of two distinct primes such that 2*k-1 are also products of two distinct primes.

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

Links

Table of n, a(n) for n=1..54.

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