A175626 Semiprimes s such that 2^(s-1) mod s is also a semiprime.

9, 15, 21, 33, 35, 39, 49, 51, 55, 57, 69, 77, 87, 93, 111, 115, 123, 129, 141, 159, 177, 183, 201, 203, 213, 219, 237, 249, 267, 291, 295, 303, 309, 321, 327, 329, 339, 361, 377, 381, 393, 411, 417, 437, 447, 451, 453, 471, 473, 489, 501, 515, 519

Offset

1,1

Comments

Semiprimes s such that A062173(s) is also a semiprime.

Links

François Huppé, Table of n, a(n) for n = 1..10000

Example

9 is in the sequence because 9 and (2^(9-1) mod 9)=4 are both semiprime.

Mathematica

semiPrimeQ[x_] := Plus @@ Last /@ FactorInteger@ x == 2; fQ[n_] := semiPrimeQ@ n && semiPrimeQ@ PowerMod[2, n - 1, n]; Select[ Range@ 525, fQ]

Crossrefs

Cf. A001358, A062173, A175168.

Sequence in context: A331662 A007962 A061254 * A096788 A050991 A033553

Adjacent sequences: A175623 A175624 A175625 * A175627 A175628 A175629

Keyword

nonn

Author

Juri-Stepan Gerasimov, Dec 04 2010

Status

approved