The minimum cost for a consumer of achieving a given utility level. Consider a consumer choosing the quantities, x1 and x2, of two goods to minimize expenditure subject to a utility constraint. The cost minimization problem is
The solution is described by the two compensated demand functions x1 = h1(p1,p2,U) and x2 = h2(p1,p2,U). Substituting the optimal choices back into the objective gives the minimized level of expenditure as
The function E(p1,p2,U) is the expenditure function. Shephard’s lemma states that ∂E/∂pi = hi(p1,p2,U), a result that is useful for calculating the welfare consequences of a price change. See also indirect utility function.