A278931 Semiprimes whose ternary representations are also semiprime when read as a decimal number.

25, 49, 65, 82, 106, 115, 118, 121, 142, 143, 155, 187, 209, 235, 254, 259, 262, 265, 274, 289, 299, 314, 319, 326, 334, 335, 341, 355, 361, 382, 398, 415, 445, 451, 454, 458, 469, 493, 511, 515, 538, 551, 562, 566, 583, 586, 589, 614, 622, 634, 649, 667, 679

Offset

1,1

Links

K. D. Bajpai, Table of n, a(n) for n = 1..9646

Wikipedia, Ternary numeral system

Example

65 is in the sequence because 5*13 = 65 (semiprime) and its ternary representation, 2102 = 2*1051, when read as a decimal number, is also semiprime.
115 is in the sequence because 5*23 = 115 (semiprime) and its ternary representation, 11021 = 103*107, when read as a decimal number, is also semiprime.

Mathematica

Select[Range[5000], PrimeOmega[#] == 2 && PrimeOmega[FromDigits[ IntegerDigits[ #, 3]]] == 2 &]

Crossrefs

Subsequence of A001358.

Cf. A001358, A005935, A007089, A065721, A089981, A236537.

Sequence in context: A245272 A091300 A112771 * A181782 A322121 A020146

Adjacent sequences: A278928 A278929 A278930 * A278932 A278933 A278934

Keyword

nonn,base

Author

K. D. Bajpai, Dec 04 2016

Status

approved