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 A053889 #27 Jul 18 2025 20:39:50
%S A053889 1,9,121,29929,9991921,1212919929,1929229929,1991212129,2929299129,
%T A053889 1229129212921,92222199291121,219992999929921,99221129991291219129,
%U A053889 2291192212129911919221121,9992221299191112291912929211222129,211192199129121999192121999211919121,21112999921929291129929211912291229929191119991929921
%N A053889 Squares composed of digits {1,2,9}.
%H A053889 Patrick De Geest, <a href="https://www.worldofnumbers.com/threedigits.htm">Squares containing at most three distinct digits, Index entries for related sequences</a>.
%H A053889 Hisanori Mishima, <a href="https://www.asahi-net.or.jp/~KC2H-MSM/mathland/math02/math0210.htm#129">Squares consisting of 3 different digits</a>.
%H A053889 Author?, <a href="http://web.archive.org/web/20080708203024/http://blue.kakiko.com/mmrmmr/htm/eqtn06.html">Source</a>(<a href="http://web.archive.org/web/20060426155831/http://blue.kakiko.com/mmrmmr/txt/eqtn06.txt">txt</a>)
%F A053889 a(n) = A053888(n)^2. - _Elmo R. Oliveira_, Jul 18 2025
%t A053889 Flatten[Table[Select[FromDigits/@Tuples[{1,2,9},n], IntegerQ[Sqrt[#]]&],{n,12}]] (* This program generates only the first 9 terms of the sequence *) (* _Harvey P. Dale_, Oct 19 2011 *)
%Y A053889 Cf. A053888.
%K A053889 nonn,base
%O A053889 1,2
%A A053889 _Patrick De Geest_, Mar 15 2000
%E A053889 a(13) from C. Ronaldo (aga_new_ac(AT)hotmail.com), Jan 04 2005
%E A053889 a(14) from _Jon E. Schoenfield_, Sep 03 2006
%E A053889 a(15)-a(16) from Mishima's webpage added by _Max Alekseyev_, Jun 17 2011
%E A053889 a(17) from _Zhao Hui Du_, Feb 29 2024