“Lagrange multiplier”的意思、由来-开放百科全书

In mathematical optimization, the method of Lagrange multipliers (or method of Lagrange's undetermined multipliers, named after Joseph-Louis Lagrange^[1]) is a strategy for finding the local maxima and minima of a function subject to equality constraints (i.e., subject to the condition that one or more equations have to be satisfied exactly by the chosen values of the variables). The great advantage of this method is that it allows the optimization to be solved without explicit parameterization in terms of the constraints. As a result, the method of Lagrange multipliers is widely used to solve challenging constrained optimization problems.

Single constraint

For the case of only one constraint and only two choice variables (as exemplified in Figure 1), consider the optimization problem

(Sometimes an additive constant is shown separately rather than being included in g, in which case the constraint is written {{math|g(x, y) {{=}} c}}, as in Figure 1.) We assume that both {{mvar|f}} and {{mvar|g}} have continuous first partial derivatives. We introduce a new variable ({{mvar|λ}}) called a Lagrange multiplier (or Lagrange undetermined multiplier) and study the Lagrange function (or Lagrangian or Lagrangian expression) defined by

where the {{mvar|λ}} term may be either added or subtracted. If {{math|f(x₀, y₀)}} is a maximum of {{math|f(x, y)}} for the original constrained problem, then there exists {{math|λ₀}} such that {{math|(x₀, y₀, λ₀)}} is a stationary point for the Lagrange function (stationary points are those points where the first partial derivatives of

are zero). Also, it must be assumed that

≠ 0. However, not all stationary points yield a solution of the original problem, as the method of Lagrange multipliers yields only a necessary condition for optimality in constrained problems.^[3]^[4]^[5]^[6]^[7] Sufficient conditions for a minimum or maximum also exist, but if a particular candidate solution satisfies the sufficient conditions, it is only guaranteed that that solution is the best one locally – that is, it is better than any permissible nearby points. The global optimum can be found by comparing the values of the original objective function at the points satisfying the necessary and locally sufficient conditions.

The method of Lagrange multipliers relies on the intuition that at a maximum, {{math|f(x, y)}} cannot be increasing in the direction of any neighboring point where {{math|g {{=}} 0}}. If it were, we could walk along {{math|g {{=}} 0}} to get higher, meaning that the starting point wasn't actually the maximum.

We can visualize contours of {{mvar|f}} given by {{math|f(x, y) {{=}} d}} for various values of {{mvar|d}}, and the contour of {{mvar|g}} given by {{math|g(x, y) {{=}} 0}}.

Suppose we walk along the contour line with {{math|g {{=}} 0}}. We are interested in finding points where {{mvar|f}} does not change as we walk, since these points might be maxima.

To check the first possibility (we are following a contour line of {{mvar|f}}), notice that since the gradient of a function is perpendicular to the contour lines, the contour lines of {{mvar|f}} and {{mvar|g}} are parallel if and only if the gradients of {{mvar|f}} and {{mvar|g}} are parallel. Thus we want points {{math|(x, y)}} where {{math|g(x, y) {{=}} 0}} and

are the respective gradients. The constant {{mvar|λ}} is required because although the two gradient vectors are parallel, the magnitudes of the gradient vectors are generally not equal. This constant is called the Lagrange multiplier. (In some conventions {{mvar|λ}} is preceded by a minus sign).

Notice that this method also solves the second possibility, that {{mvar|f}} is level: if {{mvar|f}} is level, then its gradient is zero, and setting {{math|λ {{=}} 0}} is a solution regardless of

To incorporate these conditions into one equation, we introduce an auxiliary function

Note that this amounts to solving three equations in three unknowns. This is the method of Lagrange multipliers. Note that

implies {{math|g(x, y) {{=}} 0}}. To summarize

The constrained extrema of {{mvar|f}} are critical points of the Lagrangian

, but they are not necessarily local extrema of

(see Example 2 below).

One may reformulate the Lagrangian as a Hamiltonian, in which case the solutions are local minima for the Hamiltonian. This is done in optimal control theory, in the form of Pontryagin's minimum principle.

The fact that solutions of the Lagrangian are not necessarily extrema also poses difficulties for numerical optimization. This can be addressed by computing the magnitude of the gradient, as the zeros of the magnitude are necessarily local minima, as illustrated in the numerical optimization example.

Multiple constraints

The method of Lagrange multipliers can be extended to solve problems with multiple constraints using a similar argument. Consider a paraboloid subject to two line constraints that intersect at a single point. As the only feasible solution, this point is obviously a constrained extremum. However, the level set of

is clearly not parallel to either constraint at the intersection point (see Figure 3); instead, it is a linear combination of the two constraints' gradients. In the case of multiple constraints, that will be what we seek in general: the method of Lagrange seeks points not at which the gradient of

is multiple of any single constraint's gradient necessarily, but in which it is a linear combination of all the constraints' gradients.

Concretely, suppose we have

constraints and are walking along the set of points satisfying

. Every point

on the contour of a given constraint function

has a space of allowable directions: the space of vectors perpendicular to

. The set of directions that are allowed by all constraints is thus the space of directions perpendicular to all of the constraints' gradients. Denote this space of allowable moves by

and denote the span of the constraints' gradients by

. Then

, the space of vectors perpendicular to every element of

We are still interested in finding points where

does not change as we walk, since these points might be (constrained) extrema. We therefore seek

such that any allowable direction of movement away from

is perpendicular to

(otherwise we could increase

by moving along that allowable direction). In other words,

. Thus there are scalars {{math|λ₁, λ₂, ..., λ_M}} such that

These scalars are the Lagrange multipliers. We now have

of them, one for every constraint.

The method of Lagrange multipliers is generalized by the Karush–Kuhn–Tucker conditions, which can also take into account inequality constraints of the form {{math|h(x) ≤ c}}.

Modern formulation via differentiable manifolds

The problem of finding the local maxima and minima subject to constraints can be generalized to finding local maxima and minima on a differentiable manifold

.^[8] In what follows, it is not necessary that

be a Euclidean space, or even a Riemannian manifold. All appearances of the gradient

(which depends on a choice of Riemannian metric) can be replaced with the exterior derivative

Single constraint

Let

be a smooth manifold of dimension

. Suppose that we wish to find the stationary points

of a smooth function

when restricted to the submanifold

defined by

where

is a smooth function for which 0 is a regular value of

Let

and

be the exterior derivatives. Stationarity for the restriction

means

Equivalently, the kernel

contains

In other words,

and

are proportional vectors. For this it is necessary and sufficient that the following system of

equations holds:

where

denotes the exterior product. The stationary points

are the solutions of the above system of equations plus the constraint

Note that the

equations are not independent, since the left-hand side of the equation belongs to the subvariety of

consisting of decomposable elements.

In this formulation, it is not necessary to explicitly find the Lagrange multiplier, a number

such that

Multiple constraints

Let

and

be as in the above section regarding the case of a single constraint. Rather than the function

described there, now consider a smooth function

with component functions

for which

is a regular value. Let

be the submanifold of

defined by

is a stationary point of

if and only if

contains

. For convenience let

and

where

denotes the tangent map or Jacobian

The subspace

has dimension smaller than that of

, namely

and

belongs to

if and only if

belongs to the image of

Computationally speaking, the condition is that

belongs to the row space of the matrix of

or equivalently the column space of the matrix of

(the transpose). If

denotes the exterior product of the columns of the matrix of

the stationary condition for

becomes

Once again, in this formulation it is not necessary to explicitly find the Lagrange multipliers, the numbers

such that

Interpretation of the Lagrange multipliers

Often the Lagrange multipliers have an interpretation as some quantity of interest. For example, if the Lagrangian expression is

So, {{math|λ_k}} is the rate of change of the quantity being optimized as a function of the constraint parameter.

As examples, in Lagrangian mechanics the equations of motion are derived by finding stationary points of the action, the time integral of the difference between kinetic and potential energy. Thus, the force on a particle due to a scalar potential, {{math|F {{=}} −∇V}}, can be interpreted as a Lagrange multiplier determining the change in action (transfer of potential to kinetic energy) following a variation in the particle's constrained trajectory.

Moreover, by the envelope theorem the optimal value of a Lagrange multiplier has an interpretation as the marginal effect of the corresponding constraint constant upon the optimal attainable value of the original objective function: if we denote values at the optimum with an asterisk, then it can be shown that

For example, in economics the optimal profit to a player is calculated subject to a constrained space of actions, where a Lagrange multiplier is the change in the optimal value of the objective function (profit) due to the relaxation of a given constraint (e.g. through a change in income); in such a context {{math|λ*}} is the marginal cost of the constraint, and is referred to as the shadow price.

Sufficient conditions

Sufficient conditions for a constrained local maximum or minimum can be stated in terms of a sequence of principal minors (determinants of upper-left-justified sub-matrices) of the bordered Hessian matrix of second derivatives of the Lagrangian expression.^[9]

Examples

Example 1

Example 1a

Suppose we wish to maximize

subject to the constraint

. The feasible set is the unit circle, and the level sets of {{mvar|f}} are diagonal lines (with slope −1), so we can see graphically that the maximum occurs at

, and that the minimum occurs at

Example 1b

Now we modify the objective function of Example 1a so that we minimize

instead of

again along the circle

. Now the level sets of f are still lines of slope −1, and the points on the circle tangent to these level sets are again

and

. These tangency points are maxima of f.

On the other hand, the minima occur on the level set for f = 0 (since by its construction f cannot take negative values), at

and

, where the level curves of f are not tangent to the constraint. The condition that

correctly identifies all four points as extrema; the minima are characterized in particular by

Example 2

In this example we will deal with some more strenuous calculations, but it is still a single constraint problem.

with the condition that the {{mvar|x}} and {{mvar|y}} coordinates lie on the circle around the origin with radius {{radic|3}}, that is, subject to the constraint

As there is just a single constraint, we will use only one multiplier, say {{mvar|λ}}.

The constraint {{math|g(x, y)}} is identically zero on the circle of radius {{radic|3}}. See that any multiple of {{math|g(x, y)}} may be added to {{math|f(x, y)}} leaving {{math|f(x, y)}} unchanged in the region of interest (on the circle where our original constraint is satisfied).

Notice that (iii) is just the original constraint. (i) implies {{math|x {{=}} 0}} or {{math|λ {{=}} −y}}. If {{math|x {{=}} 0}} then

by (iii) and consequently {{math|λ {{=}} 0}} from (ii). If {{math|λ {{=}} −y}}, substituting in (ii) we get {{math|x² {{=}} 2y²}}. Substituting this in (iii) and solving for {{mvar|y}} gives {{math|y {{=}} ±1}}. Thus there are six critical points of

Therefore, the objective function attains the global maximum (subject to the constraints) at

and the global minimum at

The point

is a local minimum of f and

is a local maximum of f, as may be determined by consideration of the Hessian matrix of

Given any neighbourhood of

, we can choose a small positive

and a small

of either sign to get

values both greater and less than

. This can also be seen from the fact that the Hessian matrix of

evaluated at this point (or indeed at any of the critical points) is an indefinite matrix. Each of the critical points of

is a saddle point of

Example 3: Entropy

Suppose we wish to find the discrete probability distribution on the points

with maximal information entropy. This is the same as saying that we wish to find the least structured probability distribution on the points

. In other words, we wish to maximize the Shannon entropy equation:

For this to be a probability distribution the sum of the probabilities

at each point

must equal 1, so our constraint is:

We use Lagrange multipliers to find the point of maximum entropy,

, across all discrete probability distributions

. We require that:

This shows that all

are equal (because they depend on {{mvar|λ}} only). By using the constraint

Hence, the uniform distribution is the distribution with the greatest entropy, among distributions on {{mvar|n}} points.

Example 4: Numerical optimization

The critical points of Lagrangians occur at saddle points, rather than at local maxima (or minima).^[10] Unfortunately, many numerical optimization techniques, such as hill climbing, gradient descent, some of the quasi-Newton methods, among others, are designed to find local maxima (or minima) and not saddle points. For this reason, one must either modify the formulation to ensure that it's a minimization problem (for example, by extremizing the square of the gradient of the Lagrangian as below), or else use an optimization technique that finds stationary points (such as Newton's method without an extremum seeking line search) and not necessarily extrema.

As a simple example, consider the problem of finding the value of {{mvar|x}} that minimizes

, constrained such that

. (This problem is somewhat pathological because there are only two values that satisfy this constraint, but it is useful for illustration purposes because the corresponding unconstrained function can be visualized in three dimensions.)

Using Lagrange multipliers, this problem can be converted into an unconstrained optimization problem:

The two critical points occur at saddle points where {{math|x {{=}} 1}} and {{math|x {{=}} −1}}.

In order to solve this problem with a numerical optimization technique, we must first transform this problem such that the critical points occur at local minima. This is done by computing the magnitude of the gradient of the unconstrained optimization problem.

First, we compute the partial derivative of the unconstrained problem with respect to each variable:

If the target function is not easily differentiable, the differential with respect to each variable can be approximated as

Next, we compute the magnitude of the gradient, which is the square root of the sum of the squares of the partial derivatives:

(Since magnitude is always non-negative, optimizing over the squared-magnitude is equivalent to optimizing over the magnitude. Thus, the ''square root" may be omitted from these equations with no expected difference in the results of optimization.)

The critical points of {{mvar|h}} occur at {{math|x {{=}} 1}} and {{math|x {{=}} −1}}, just as in

. Unlike the critical points in

, however, the critical points in {{mvar|h}} occur at local minima, so numerical optimization techniques can be used to find them.

Applications

Control theory

In optimal control theory, the Lagrange multipliers are interpreted as costate variables, and Lagrange multipliers are reformulated as the minimization of the Hamiltonian, in Pontryagin's minimum principle.

Nonlinear programming

The Lagrange multiplier method has several generalizations. In nonlinear programming there are several multiplier rules, e.g., the Carathéodory–John Multiplier Rule and the Convex Multiplier Rule, for inequality constraints.^[11]

Single constraint

Multiple constraints

Modern formulation via differentiable manifolds

Single constraint

Multiple constraints

Interpretation of the Lagrange multipliers

Sufficient conditions

Examples

Example 1

Example 1a

Example 1b

Example 2

Example 3: Entropy

Example 4: Numerical optimization

Applications

Control theory

Nonlinear programming

See also

References

Further reading

External links