# Envelope theorem

Envelope theorem This article may be too technical for most readers to understand. Please help improve it to make it understandable to non-experts, without removing the technical details. (November 2021) (Learn how and when to remove this template message) In mathematics and economics, the envelope theorem is a major result about the differentiability properties of the value function of a parameterized optimization problem.[1] As we change parameters of the objective, the envelope theorem shows that, in a certain sense, changes in the optimizer of the objective do not contribute to the change in the objective function. The envelope theorem is an important tool for comparative statics of optimization models.[2] The term envelope derives from describing the graph of the value function as the "upper envelope" of the graphs of the parameterized family of functions {displaystyle left{fleft(x,cdot right)right}_{xin X}} that are optimized.

Contents 1 Statement 2 For arbitrary choice sets 3 Applications 3.1 Applications to producer theory 3.2 Applications to mechanism design and auction theory 3.3 Applications to multidimensional parameter spaces 3.4 Applications to parameterized constraints 3.5 Other applications 4 See also 5 References Statement Let {displaystyle f(x,alpha )} and {displaystyle g_{j}(x,alpha ),j=1,2,ldots ,m} be real-valued continuously differentiable functions on {displaystyle mathbb {R} ^{n+l}} , where {displaystyle xin mathbb {R} ^{n}} are choice variables and {displaystyle alpha in mathbb {R} ^{l}} are parameters, and consider the problem of choosing {displaystyle x} , for a given {displaystyle alpha } , so as to: {displaystyle max _{x}f(x,alpha )} subject to {displaystyle g_{j}(x,alpha )geq 0,j=1,2,ldots ,m} and {displaystyle xgeq 0} .

The Lagrangian expression of this problem is given by {displaystyle {mathcal {L}}(x,lambda ,alpha )=f(x,alpha )+lambda cdot g(x,alpha )} where {displaystyle lambda in mathbb {R} ^{m}} are the Lagrange multipliers. Now let {displaystyle x^{ast }(alpha )} and {displaystyle lambda ^{ast }(alpha )} together be the solution that maximizes the objective function f subject to the constraints (and hence are saddle points of the Lagrangian), {displaystyle {mathcal {L}}^{ast }(alpha )equiv f(x^{ast }(alpha ),alpha )+lambda ^{ast }(alpha )cdot g(x^{ast }(alpha ),alpha ),} and define the value function {displaystyle V(alpha )equiv f(x^{ast }(alpha ),alpha ).} Then we have the following theorem.[3][4] Theorem: Assume that {displaystyle V} and {displaystyle {mathcal {L}}} are continuously differentiable. Then {displaystyle {frac {partial V(alpha )}{partial alpha _{k}}}={frac {partial {mathcal {L}}^{ast }(alpha )}{partial alpha _{k}}}={frac {partial {mathcal {L}}(x^{ast }(alpha ),lambda ^{ast }(alpha ),alpha )}{partial alpha _{k}}},k=1,2,ldots ,l} where {displaystyle partial {mathcal {L}}/partial alpha _{k}=partial f/partial alpha _{k}+lambda cdot partial g/partial alpha _{k}} .

For arbitrary choice sets Let {displaystyle X} denote the choice set and let the relevant parameter be {displaystyle tin lbrack 0,1]} . Letting {displaystyle f:Xtimes lbrack 0,1]rightarrow R} denote the parameterized objective function, the value function {displaystyle V} and the optimal choice correspondence (set-valued function) {displaystyle X^{ast }} are given by: {displaystyle V(t)=sup _{xin X}f(x,t)}         (1) {displaystyle X^{ast }(t)={xin X:f(x,t)=V(t)}}         (2) "Envelope theorems" describe sufficient conditions for the value function {displaystyle V} to be differentiable in the parameter {displaystyle t} and describe its derivative as {displaystyle V^{prime }left(tright)=f_{t}left(x,tright){text{ for each }}xin X^{ast }left(tright),}         (3) where {displaystyle f_{t}} denotes the partial derivative of {displaystyle f} with respect to {displaystyle t} . Namely, the derivative of the value function with respect to the parameter equals the partial derivative of the objective function with respect to {displaystyle t} holding the maximizer fixed at its optimal level.

Traditional envelope theorem derivations use the first-order condition for (1), which requires that the choice set {displaystyle X} have the convex and topological structure, and the objective function {displaystyle f} be differentiable in the variable {displaystyle x} . (The argument is that changes in the maximizer have only a "second-order effect" at the optimum and so can be ignored.) However, in many applications such as the analysis of incentive constraints in contract theory and game theory, nonconvex production problems, and "monotone" or "robust" comparative statics, the choice sets and objective functions generally lack the topological and convexity properties required by the traditional envelope theorems.

Paul Milgrom and Segal (2002) observe that the traditional envelope formula holds for optimization problems with arbitrary choice sets at any differentiability point of the value function,[5] provided that the objective function is differentiable in the parameter: Theorem 1: Let {displaystyle tin left(0,1right)} and {displaystyle xin X^{ast }left(tright)} . If both {displaystyle V^{prime }left(tright)} and {displaystyle f_{t}left(x,tright)} exist, the envelope formula (3) holds.

Proof: Equation (1) implies that for {displaystyle xin X^{ast }left(tright)} , {displaystyle max _{sin left[0,1right]}left[fleft(x,sright)-Vleft(sright)right]=fleft(x,tright)-Vleft(tright)=0.} Under the assumptions, the objective function of the displayed maximization problem is differentiable at {displaystyle s=t} , and the first-order condition for this maximization is exactly equation (3). Q.E.D.