Axisymmetric surfaces generally differ by the third term from the (1.38b) expansion. In this case the shape is called a spheroid and is represented by the power series z = X Anrn, (1.39)
where if having A2 = 1/2R and A4 = (1 + k)/8R3, then A6/A4 = (1 + k)/2R2 .
If A6/A4 = (1 + k)/2R2 and if the next term differs from that of a conicoid, then the spheroid may be called a deformed conicoid whose shape departs from a conicoid by the 7-th order term (i.e. the A8r8 term).
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