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 A279420 #16 Mar 29 2025 15:26:06
%S A279420 10,20,30,100,105,138,145,155,179,195,200,205,217,226,241,243,245,247,
%T A279420 249,251,253,255,257,259,274,283,295,300,305,1000,1005,1010,1015,1020,
%U A279420 1025,1030,1049,1054,1068,1082,1091,1100,1114,1127,1136,1149,1158,1162,1175
%N A279420 Numbers k such that k^2 has an odd number of digits and the middle digit is 0.
%H A279420 Lars Blomberg, <a href="/A279420/b279420.txt">Table of n, a(n) for n = 1..10000</a>
%e A279420 10^2 = 1(0)0, 195^2 = 38(0)25, 1000^2 = 100(0)000.
%e A279420 The sequences of squares starts: 100, 400, 900, 10000, 11025, 19044, 21025, 24025, 32041, 38025, 40000, ...
%t A279420 Select[Range@ 1175, Function[w, And[OddQ@ Length@ w, First@ Take[w, {Ceiling[Length[w]/2]}] == 0]]@ IntegerDigits[#^2] &] (* _Michael De Vlieger_, Dec 12 2016 *)
%t A279420 Select[Range[1200],With[{len=IntegerLength[#^2]},OddQ[len]&&IntegerDigits[#^2][[(len+1)/2]]==0&]] (* _Harvey P. Dale_, Mar 29 2025 *)
%o A279420 (PARI) isok(n) = my(d=digits(n^2)); (#d % 2) && (d[#d\2 + 1] == 0); \\ _Michel Marcus_, Dec 18 2016
%Y A279420 Cf. A279421-A279429.
%K A279420 nonn,base,easy
%O A279420 1,1
%A A279420 _Lars Blomberg_, Dec 12 2016