Random price discrimination

When a monopolist randomly sorts customers, price discrimination "concavifies" the revenue function of the firm, so that it may be optimal for a monopolist to divide customers into groups that have the same demand function and charge them different prices. It is impossible to rule out this type of result whenever the revenue function is somewhere convex in the "economically relevant" set of quantities, because there always exists a non-decreasing cost function that leads to that conclusion. It is also impossible to rule out the case where, with respect to monopoly, the firm raises or lowers price to all classes and, accordingly, the case where the social welfare decreases or increases.