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

Author

K. D. Bajpai, Dec 04 2016

Keywords

• nonn
• base

Examples

			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.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..9646
• Wikipedia, Ternary numeral system

Crossrefs

Subsequence of A001358.

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

Programs

Mathematica

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