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 A309071 Complete list of solutions to y^2 = x^3 + 20*x; sequence gives x values. 1
 0, 4, 5, 720 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS (x,y) = (0,0) is this solution. Consider x > 0. If x is square, x^2 + 20 is square and we get (x,y) = (4,12). If x is not square, x = i^2*j where j is squarefree. j | x^2 + 20, so j is 2,5 or 10. If j = 2 or 10, there is not such (x,y). If j = 5, (y/5)^2 = i^2*(5*i^4 + 4). So 5*i^4 + 4 = k^2. That is k^2 - 5*i^4 = 4. i^2 is a square Fibonacci number. i^2 = 1 or 144, so x = 5 or 720. LINKS EXAMPLE 0^3 + 20*  0 =         0 =     0^2.   4^3 + 20*  4 =       144 =    12^2.   5^3 + 20*  5 =       225 =    15^2. 720^3 + 20*720 = 373262400 = 19320^2. PROG (PARI) for(k=0, 1e5, if(issquare(k*(k^2+20)), print1(k", "))) (SageMath) [i[0] for i in EllipticCurve([20, 0]).integral_points()] # Seiichi Manyama, Aug 25 2019 CROSSREFS Cf. A029728, A116108, A126203. Sequence in context: A304286 A134463 A298938 * A270975 A058916 A064612 Adjacent sequences:  A309068 A309069 A309070 * A309072 A309073 A309074 KEYWORD nonn,fini,full AUTHOR Seiichi Manyama, Jul 10 2019 STATUS approved

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Last modified February 17 12:14 EST 2020. Contains 331996 sequences. (Running on oeis4.)