Among the assumptions needed to get equation (1) some are inessential and could be relaxed at the cost of increased notational complexity. There are the assumptions of a constant interest rate, no constraints on short sales and risk neutrality. One assumptions is, however, of more consequence: it is that, at least after having observed the price, all agents have the same information. As we shall show, bubbles can exist even in this case and these bubbles would remain even if agents have differential information.