If price dynamics is defined by a differential equation then the kind of information required to design a successful price mechanism (which reaches an equilibrium) is known. In general one requires information about z(p) the excess demand and its Jacobian Dz(p). If the price mechanism is iterative, then it is shown that such information is not enough; no such mechanism exist which depend upon the information solely provided by z(p), \(Dz(p),...,D^ Nz(p)\), where N is any positive integer. Thus, for iterative convergent mechanisms a different type of information is required.