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 A370921 #7 Mar 17 2024 04:07:59
%S A370921 7,16,21,22,34,43,48,49,63,64,66,67,70,88,97,102,103,115,124,129,130,
%T A370921 144,145,147,148,151,189,190,192,193,196,198,199,201,202,205,210,211,
%U A370921 250,259,264,265,277,286,291,292,306,307,309,310,313,331,340,345,346
%N A370921 Positive integers in whose ternary representation, 2 occurs at least once, and every 2 is followed by 1.
%e A370921 The ternary representations of 7, 16, and 21 are 21, 121, and 210.
%t A370921 Map[#[[1]] &, Select[Map[{#, #[[1]] > 0 && #[[1]] == #[[2]] &[{Length[
%t A370921 StringCases[#, "2"]], Length[StringCases[#, "21"]]}] &[
%t A370921 IntegerString[#, 3]]} &, Range[500]], #[[2]] &]]
%t A370921 (* _Peter J. C. Moses_, Mar 05 2024 *)
%Y A370921 Cf. A007089, A074940.
%Y A370921 Cf. A370916, A370917, A370918, A370919, A370920, A370922, A370923.
%K A370921 nonn,base
%O A370921 1,1
%A A370921 _Clark Kimberling_, Mar 16 2024