Fig. 12.
Quarter ellipses (left) define the regions up to R2 (green line) and up to R4 (blue line) according to the biquadratic approximation: Eq. (21). The red dotted lines are for the linear model, Eq. (18). The black dashed lines are approximation of the biquadratic model by an ellipse, Eq. (23). Right: corresponding regions in terms of the squared eccentricities.
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