Biquadrates and Elliptic Curves
  The elliptic curve y^2= x^3-Nx where N=m^4+n^4 has rank at least 2 over Q(m,n). When N can be written in two different ways as sum of two fourth powers, then we prove that the rank is at least 4. 
Juli\'an Aguirre and Juan Carlos Peral
math.NT