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 A223077 #10 Jun 10 2018 17:27:09
%S A223077 1012,10212,102212,1022212,10121012,10222212,101201012,102222212,
%T A223077 1012001012,1021210212,1022222212,10120001012,10212010212,10222222212,
%U A223077 101200001012,101210121012,102120010212,102212102212,102222222212,1012000001012,1012102121012,1021200010212,1022120102212,1022222222212
%N A223077 Positive integers, written in base 3, with the property that if the base-3 representation is reversed the result is twice the original number.
%C A223077 For the decimal representations of these same numbers see A173951.
%C A223077 Theorem: The number of terms of length n is equal to A103609(n-2).
%H A223077 N. J. A. Sloane, <a href="/A223077/b223077.txt">Table of n, a(n) for n = 1..108</a>
%t A223077 b3rQ[n_]:=FromDigits[Reverse[IntegerDigits[n]],3]/FromDigits[ IntegerDigits[ n],3] ==2; Select[FromDigits/@Tuples[{0,1,2},13],b3rQ]//Quiet (* _Harvey P. Dale_, Jun 10 2018 *)
%Y A223077 Cf. A173951, A214927.
%K A223077 nonn,base
%O A223077 1,1
%A A223077 _N. J. A. Sloane_, Mar 14 2013