Nonlinear mean-reversion to purchasing power parity: exponential smooth transition autoregressive models and stochastic unit root processes

The purpose of this paper is to show an empirical test of subadditivity based on a sufficient condition-namely, transray convexity coupled with global economies of scale-which has not been commonly used in the literature. We use this sufficient condition to test the hypothesis of subadditivity of the cost function in the largest Portuguese Telecommunications firm, CTT, and find that the market for telecommunications services in Portugal was a natural monopoly in the period analysed.