f(x, y) = x / (x^(2) + y^(2))
a = 0.4 (slider from -5 to 5, step 0.1)
p: z = a
b = CurveCartesian(1 / ((2 * a)) + (1 / ((2 * abs(a))) * cos(t)), (1 / ((2 * abs(a))) * sin(t)), a, t, 0, (2 * π))
c = CurveCartesian(1 / ((2 * a)) + (1 / ((2 * abs(a))) * cos(t)), (1 / ((2 * abs(a))) * sin(t)), t, 0, (2 * π))