  It is shown that the expressions for the tangential pressure, the anisotropy
factor and the radial pressure in the Einstein equations may serve as
generating functions for stellar models. The latter can incorporate an equation
of state when the expression for the energy density is also used. Other
generating functions are based on the condition for the existence of conformal
motion (conformal flatness in particular) and the Karmarkar condition for
embedding class one metrics. In all these cases the equations are linear first
order differential equations for one of the metric components and Riccati
equations for the other. The latter may be always transformed into second order
homogenous linear differential equations. These conclusions are illustrated by
numerous particular examples from the study of stellar models.
