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 A238753 #22 Jul 29 2023 10:25:04
%S A238753 409,4688329,250545884330641,199554894091303668073201,
%T A238753 231574628370557219024862837633967849,
%U A238753 30240048139343876249762998173442832362876517638009,330823513952828243573122480536077533156064000139119724642295861921
%N A238753 Squarefree integers D such that Q(sqrt(D)) contains a non-constant arithmetic progression of 5 squares.
%C A238753 Note that there exists no D such that Q(sqrt{D}) contains a non-constant arithmetic progression of 6 squares.
%H A238753 Enrique González-Jiménez and Xavier Xarles, <a href="http://arxiv.org/abs/0909.1663">Five squares in arithmetic progression over quadratic fields</a>, arXiv:0909.1663 [math.NT], 2009-2013.
%e A238753 For D=409, such an arithmetic progression is 49, 169, 289, 409, 529.
%K A238753 nonn
%O A238753 1,1
%A A238753 _David Cushing_, Mar 04 2014