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 A031601 #24 Aug 18 2021 00:10:32
%S A031601 409,1181,2258,3562,3802,3925,5305,5897,8269,9829,10834,11041,12389,
%T A031601 15737,16745,17785,21757,23029,23633,24554,25178,26869,27194,27521,
%U A031601 28181,28514,28849,30554,30901,32413,33853,36073,36185,40841,41362,41765,42170
%N A031601 Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted the two central terms are both 13.
%H A031601 Vincenzo Librandi, <a href="/A031601/b031601.txt">Table of n, a(n) for n = 1..150</a>
%t A031601 cf13Q[n_]:=Module[{per=ContinuedFraction[Sqrt[n]][[2]]},OddQ[Length[ per]]&&per[[Floor[Length[per]/2]+1]]==13]; nn=43000;With[{terms = Complement[Range[nn], Range[Floor[Sqrt[nn]]]^2]}, Select[terms, cf13Q]] (* _Harvey P. Dale_, Nov 23 2011 *)
%Y A031601 Subsequence of A003814.
%K A031601 nonn
%O A031601 1,1
%A A031601 _David W. Wilson_
%E A031601 Corrected by _Harvey P. Dale_, Nov 23 2011