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 A230623 #21 Dec 22 2020 18:29:27
%S A230623 4,15,52,240,1121,4888,20047,77280,277441,1093425,5279468,23647519,
%T A230623 99429196,393425745,1457109628,4968639359,24553864319,113193708472,
%U A230623 488133974353,1980778750800,7547952442399,26710380775592,112605054449252
%N A230623 Values of y such that x^2 + y^2 = 17^n with x and y coprime and 0 < x < y.
%C A230623 The corresponding x-values are in A230622.
%H A230623 Vincenzo Librandi, <a href="/A230623/b230623.txt">Table of n, a(n) for n = 1..200</a>
%H A230623 Chris Busenhart, Lorenz Halbeisen, Norbert Hungerbühler, Oliver Riesen, <a href="https://people.math.ethz.ch/~halorenz/publications/pdf/Miniatur.pdf">On primitive solutions of the Diophantine equation x^2+ y^2= M</a>, Eidgenössische Technische Hochschule (ETH Zürich, Switzerland, 2020).
%e A230623 a(2)=15 because 8^2 + 15^2 = 289 = 17^2.
%t A230623 Table[Select[PowersRepresentations[17^n, 2, 2], CoprimeQ@@#&][[1, 2]], {n, 1, 40}] (* _Vincenzo Librandi_, Mar 02 2014 *)
%Y A230623 Cf. A001026, A188949, A230622.
%K A230623 nonn
%O A230623 1,1
%A A230623 _Colin Barker_, Oct 26 2013