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 A018884 #32 Feb 16 2025 08:32:33
%S A018884 1,4,9,16,25,36,49,64,81,121,144,225,441,484,676,1444,7744,11881,
%T A018884 29929,44944,55225,69696,9696996,6661661161
%N A018884 Squares using at most two distinct digits, not ending in 0.
%C A018884 No other terms below 10^41.
%C A018884 The sequence is probably finite.
%C A018884 The two distinct digits of a term cannot both be in the set {0,2,3,7,8}. Looking at the digits (with leading zeros) of i^2 mod 10^4 for 0 <= i < 10^4 shows that there are no repunit terms > 10 and the two distinct digits of a term must be one of the following 21 pairs: '01', '04', '09', '12', '14', '16', '18', '24', '25', '29', '34', '36', '45', '46', '47', '48', '49', '56', '67', '69', '89'. - _Chai Wah Wu_, Apr 06 2019
%D A018884 Richard K. Guy, Unsolved Problems in Number Theory, Section F24 (at p. 262) (Springer-Verlag, 2d ed. 1994).
%H A018884 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/SquareNumber.html">Square Number</a>.
%t A018884 Flatten[Table[Select[Flatten[Table[FromDigits/@Tuples[{a,b},n],{n,10}]], IntegerQ[ Sqrt[#]]&],{a,9},{b,9}]]//Union (* _Harvey P. Dale_, Sep 21 2018 *)
%Y A018884 Cf. A000290, A016069, A016070, A018885.
%K A018884 nonn,base,more,hard
%O A018884 1,2
%A A018884 _David W. Wilson_