This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A278931 #30 Apr 06 2020 13:59:59 %S A278931 25,49,65,82,106,115,118,121,142,143,155,187,209,235,254,259,262,265, %T A278931 274,289,299,314,319,326,334,335,341,355,361,382,398,415,445,451,454, %U A278931 458,469,493,511,515,538,551,562,566,583,586,589,614,622,634,649,667,679 %N A278931 Semiprimes whose ternary representations are also semiprime when read as a decimal number. %H A278931 K. D. Bajpai, <a href="/A278931/b278931.txt">Table of n, a(n) for n = 1..9646</a> %H A278931 Wikipedia, <a href="http://en.wikipedia.org/wiki/Ternary_numeral_system">Ternary numeral system</a> %e A278931 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. %e A278931 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. %t A278931 Select[Range[5000], PrimeOmega[#] == 2 && PrimeOmega[FromDigits[ IntegerDigits[ #, 3]]] == 2 &] %Y A278931 Subsequence of A001358. %Y A278931 Cf. A001358, A005935, A007089, A065721, A089981, A236537. %K A278931 nonn,base %O A278931 1,1 %A A278931 _K. D. Bajpai_, Dec 04 2016