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Definition: An ellipse is a curve that is the locus of all points in the plane the sum of whose distances r_1 and r_2 from two fixed points F_1 and F_2 (the foci) separated by a distance of 2c is a given positive constant 2a (Hilbert and Cohn-Vossen 1999, p. 2). This results in the two-center bipolar coordinate equation r_1+r_2=2a, where a is the semimajor axis and the origin of the coordinate system is at one of the foci. The corresponding parameter b is known as the semiminor axis.