  The exact equation of spin motion in a cylindrical coordinate system with
allowance for electric dipole moments of particles has been derived. This
equation is convenient for analytical calculations of spin dynamics in circular
storage rings when the configuration of main fields is simple enough. The
generalized formula for the influence of a vertical betatron oscillation on the
angular velocity of spin rotation has been found. This formula agrees with the
previously obtained result and contains an additional oscillatory term that can
be used for fitting. The relative importance of terms in the equation of spin
motion is discussed.
