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%I #14 Sep 30 2013 17:32:27
%S 1,1,13,239
%N Positive integer solutions y1, x1, y2, x2 to Ljunggren's equation x^2 + 1 = 2y^4.
%C See the Wikipedia links for other references.
%C The only square stella octangula numbers are A007588(1) = (a(1)*a(2))^2 = 1 and A007588(169) = (a(3)*a(4))^2 = 9653449.
%D W. Ljunggren, Zur Theorie der Gleichung x^2 + 1 = Dy^4, Avh. Norske Vid. Akad. Oslo. I. 1942 (5): 27.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Stella_octangula_number">Stella octangula number</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Wilhelm_Ljunggren">Wilhelm Ljunggren</a>
%e 239^2 + 1 = 57122 = 2*13^4.
%Y Cf. A007588.
%K nonn,fini,full
%O 1,3
%A _Jonathan Sondow_, Sep 30 2013
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