We generalize the usual screening approach and conditions for efficiency-at-the-top and acyclic property from linear to fixed-plus-separable or concave costs and multidimensional commodities. But under non-concave costs, like capacity constraints, an example shows a cycle in the solution graph....

Using linear demands for two types of consumers, this note shows why discounts are ubiquitous and premia are rare. By observing premia, one can infer that the package sold at premium is undistorted and the single-crossing condition must hold.

Examining a standard monopolistic competition model with unspecified utility/cost functions, we find necessary and sufficient conditions on their elasticities for welfare losses to arise from trade or market expansion. Two numerical examples explain the losses (under unrealistic elasticities).

We modify Paul Krugman’s (1991) ‘Core–Periphery’ model by replacing the traditional competitive sector with a monopolistically competitive one. We show that the structure of spatial equilibria remains the same as in the original model. This result continues to hold true under Cournot or...