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 A273361 #17 Sep 08 2022 08:46:16
%S A273361 4,25,76,241,400,561,724,889,1009,1504,2001,2500,3001,3504,4009,4516,
%T A273361 5025,5536,6049,6564,7081,7600,8121,8644,9169,9696,11524,13089,14656,
%U A273361 16225,17796,19369,20944,22521,24100,25681,27264,28849,30436
%N A273361 Numbers n such that the decimal number concat(6,n) is a square.
%C A273361 Elements are squares of integers in (sqrt(61), sqrt(70)) * sqrt(10)^k without the leading 6 elements for nonnegative k. - _David A. Corneth_, May 20 2016
%H A273361 Nathan Fox, <a href="/A273361/b273361.txt">Table of n, a(n) for n = 1..10000</a>
%e A273361 76 is a member because 676 = 26^2 is a square.
%e A273361 0 is not a member because 60 is not a square.
%p A273361 t1:=[];
%p A273361 for k from 1 to 50000 do
%p A273361 if issqr(k+6*10^length(k)) then t1:=[op(t1), k]; fi;
%p A273361 od;
%p A273361 t1;
%t A273361 Select[Range[31000],IntegerQ[Sqrt[FromDigits[Join[{6}, IntegerDigits[ #]]]]]&] (* _Harvey P. Dale_, Feb 09 2019 *)
%o A273361 (Magma) [n: n in [1..20000 ] | IsSquare(Seqint(Intseq(n) cat Intseq(6)))]; // _Marius A. Burtea_, Mar 21 2019
%Y A273361 Cf. A272671, A273357, A273358, A273359, A273360, A273362, A273363, A273364.
%K A273361 nonn,base
%O A273361 1,1
%A A273361 _Nathan Fox_, _Brooke Logan_, and _N. J. A. Sloane_, May 20 2016