In combining operators sequentially, it is useful to insert an arbitrary function after the operator to avoid errors. For example, if the operators Aˆ and Bˆ are x and d /dx, then AˆBˆ f (x) = x df (x)/dx. Derive the operator for kinetic energy using the classical relation for the kinetic energy, p² 2m, and the operator for linear momentum listed in Table 14.1.