Fama (1970) defined an efficient market as one in which prices always “fully reflect” available information. This paper formalizes this definition and provides various characterizations relating to equilibrium models, profitable trading strategies, and equivalent martingale measures. These...

Aggregation is an often used tool in finance and macroeconomics, whereby economic equilibrium in a heterogeneous trader economy is characterized by means of the first order optimality conditions of a representative agent. In this paper we study the conditions under which a representative agent...

Given a market with a price process S populated by heterogeneous traders with differential information, beliefs, and trading constraints, let the smallest information set containing all of the traders' information be denoted F. This market is defined to be informationally efficient (Fama [2])...

The martingale theory of price bubbles defines an asset bubble to exist when the asset's price process is a strict local martingale, that is, a local martingale that is not a martingale. Using this definition of a price bubble, for continuous semimartingales, we characterize the conditions under...

A constrained informationally efficient market is defined to be one whose price process arises as the outcome of some equilibrium where agents face restrictions on trade. This paper investigates the case of short sale constraints, a setting which despite its simplicity, generates new insights....