From the beginning of the XX century, in a theoretical paper regarding stock speculation, Louis Bachelier shows that the prices of goods on the stock market fallow a random movement, drawing, from here the conclusion that speculation is an actual game, meaning that neither the buyers or the sellers don't make in whole a net profit. Further statistic analysis seems to confirm the hypothesis of the random movement of exchange rates. So, it was appreciated that the successive changes in the prices at the stock market are practically independent, and the dependents are practically negligible. This means that the fact of knowing these correlations cannot be used to boost profits resulting from operations, the eventual supplemental earnings being canceled by costs. From these reasons, a conception regarding the price movements on the stock market, the so called theory of the efficient market.)

efficient market, portfolio theory, informational ensemble