  We show that the mechanism-design problem for a monopolist selling multiple,
heterogeneous objects to a buyer with ex ante symmetric and additive values is
equivalent to the mechanism-design problem for a monopolist selling identical
objects to a buyer with decreasing marginal values. We derive three new results
for the identical-objects model: (i) a sufficient condition on priors, such
that prices in optimal deterministic mechanism are not increasing, (ii) a
simplification of incentive constraints for deterministic mechanisms, and (iii)
a new condition for revenue monotonicity of stochastic mechanisms. We use the
equivalence to establish corresponding results in the heterogeneous-objects
model.
