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 A031421 #31 Aug 18 2021 00:33:16
%S A031421 509,1450,2237,2425,3946,4778,5189,5473,5618,5914,6445,6761,7417,8185,
%T A031421 9178,9938,10133,10426,10529,10826,10933,11441,11861,12074,12289,
%U A031421 12506,12829,13273,14653,14765,15241,16217,16586,16837,17090,17989,18385,18650,18778
%N A031421 Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted then there are a pair of central terms both equal to 8.
%H A031421 T. D. Noe, <a href="/A031421/b031421.txt">Table of n, a(n) for n = 1..1000</a> [shifted by _Georg Fischer_, Jun 23 2019]
%t A031421 n = 1; t = {}; While[Length[t] < 50, n++; If[! IntegerQ[Sqrt[n]], c = ContinuedFraction[Sqrt[n]]; len = Length[c[[2]]]; If[OddQ[len] && c[[2, (len + 1)/2]] == 8 && c[[2, (len + 1)/2 - 1]] == 8, AppendTo[t, n]]]]; t (* _T. D. Noe_, Apr 04 2014; adapted by _Georg Fischer_, Jun 23 2019 *)
%Y A031421 Cf. A031404-A031423.
%Y A031421 Subsequence of A003814.
%K A031421 nonn
%O A031421 1,1
%A A031421 _David W. Wilson_
%E A031421 a(1) corrected by _T. D. Noe_, Apr 04 2014
%E A031421 Data adapted to new definition by _Georg Fischer_, Jun 23 2019