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 A188949 #21 Dec 22 2020 18:29:14
%S A188949 1,3,12,46,120,597,2035,6554,28560,86158,341525,1315911,3455641,
%T A188949 17021162,58317492,186118929,815616479,2474152797,9719139348,
%U A188949 37641223154,99498527400,485257533003,1671083125805,5284814079046,23290743888720,71041880304722,276564805068235
%N A188949 Values of y such that x^2 + y^2 = 13^n with x and y coprime and 0 < x < y.
%C A188949 The x values are in A188948.
%H A188949 Vincenzo Librandi, <a href="/A188949/b188949.txt">Table of n, a(n) for n = 0..200</a>
%H A188949 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).
%t A188949 Table[Select[PowersRepresentations[13^n, 2, 2], CoprimeQ @@ # &][[1,2]], {n, 0, 30}]
%Y A188949 Cf. A158936, A188948.
%K A188949 nonn
%O A188949 0,2
%A A188949 _Zak Seidov_, Apr 10 2011
%E A188949 Edited by _T. D. Noe_, Apr 14 2011