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 A031609 #14 Aug 18 2021 00:10:32 %S A031609 1033,5674,9106,9685,13577,14045,19213,19769,20333,25189,26794,33413, %T A031609 33778,40361,41165,42797,43625,51517,53341,59093,59578,61538,63029, %U A031609 64033,69269,70321,71381,71914,72986,74066,74609,75154,75701,93257,94477,96941 %N A031609 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 21. %Y A031609 Subsequence of A003814. %K A031609 nonn %O A031609 1,1 %A A031609 _David W. Wilson_ %E A031609 First term 442 removed by _Georg Fischer_, Jun 16 2019