  We extend recent work on the resonance structure for post-inflation
reheating, providing an analytic treatment for models in which both the
inflaton and the fields into which it decays are massless. Solutions are
derived which are valid for either a spatially-flat or spatially-open metric.
Closed-form solutions are given for the characteristic exponent, which measures
the rate of particle production during preheating. It is demonstrated that in
certain regions of parameter space, the maximum values of the characteristic
exponent in an open universe are several times greater than the maximum values
in a spatially-flat universe. It is further demonstrated that the solutions
found here by means of a simple algebraic construction match the two
previously-known exact solutions, which were derived in terms of special
functions.
