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 A240129 #10 Apr 03 2014 05:41:55 %S A240129 0,1,36 %N A240129 Triangular numbers that are squares of triangular numbers. %C A240129 Ljunggren used p-adic methods to prove that 0, 1, and 36 are the only triangular numbers that are squares of triangular numbers. %C A240129 Same as: the only positive integer solutions of (x(x-1))^2 = 2y(y-1) are (x,y) = (1,1), (2,2), and (4,9). %C A240129 Cassels used elliptic curves to simplify Ljunggren's proof. %C A240129 Subsequence of A001110 (triangular numbers that are squares). %H A240129 W. Ljunggren, <a href="http://zbmath.org/scans/134/275.gif">Review of "Integral points on certain elliptic curves" by J.W.S. Cassels, Proc. Lond. Math. Soc., III. Ser. 14 A (1965), 55-57</a>, zbMATH 0134.27501. %e A240129 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 = 36 = 6^2 = (1 + 2 + 3)^2. %Y A240129 Cf. A000217, A001110. %K A240129 nonn,fini,full,bref %O A240129 1,3 %A A240129 _Jonathan Sondow_, Apr 02 2014