Efficiency and Random Walk

This chapter, which corresponds to the second subdivision of the part, attempts to characterize stock markets. The stock markets are known as efficient and are representative of the perfect competition. They are also known as advanced indicators of the global economic activity. The evolution of stock market indexes show however trajectories whose amplitudes are often considerably higher than those of gross domestic product. The concept and the models of rational expectations (arbitrage and stock price fixing) will give a first argument to try to explain these differences between trajectories. One will consider the efficiency concept, in the sense of Fama, about the stock markets and their instability.