Generating potentials via difference equations

The condition for pressure isotropy, for spherically symmetric gravitational fields with charged and uncharged matter, is reduced to a recurrence equation with variable, rational coefficients. This difference equation is solved, in general, using mathematical induction leading to an exact solution to the Einstein field equations which extends the isotropic model of John and Maharaj. The metric functions, energy density and pressure are well behaved, which suggests that this model could be used to describe a relativistic sphere. The model admits a barotropic equation of state, which approximates a polytrope close to the stellar centre. Copyright (C) 2006 John Wiley & Sons, Ltd.