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 A031608 #17 Sep 18 2021 18:33:53
%S A031608 2729,5233,8362,8917,24874,29873,31973,37997,38777,46237,54818,56698,
%T A031608 57173,58129,64129,64634,65141,66161,66674,67189,67706,68746,74713,
%U A031608 87277,88457,89645,90841,100186,101450,104002,104645,115333,116689,126541
%N A031608 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 20.
%t A031608 cf20Q[n_]:=Module[{s=Sqrt[n],cf,len},cf=If[IntegerQ[s],{1,1},ContinuedFraction[ s][[2]]];len=Length[cf];OddQ[len]&&cf[[Floor[len/2]]] == cf[[Ceiling[len/2]]] ==20]; Select[ Range[ 130000],cf20Q] (* _Harvey P. Dale_, Sep 18 2021 *)
%Y A031608 Subsequence of A003814.
%K A031608 nonn
%O A031608 1,1
%A A031608 _David W. Wilson_
%E A031608 First term 401 removed by _Georg Fischer_, Jun 16 2019