Lagrange multipliers on Banach spaces

Let X and Y be real Banach spaces. Let U be an open subset of X and let f : U → R be a continuously differentiable function. Let g : U → Y be another continuously differentiable function, the constraint: the objective is to find the extremal points (maxima or minima) of f subject to the constraint that g is zero.

Suppose that u₀ is a constrained extremum of f, i.e. an extremum of f on

${\displaystyle g^{-1}(0)=\{x\in U\mid g(x)=0\in Y\}\subseteq U.}$

Suppose also that the Fréchet derivative Dg(u₀) : X → Y of g at u₀ is a surjective linear map. Then there exists a Lagrange multiplier λ : Y → R in Y^∗, the dual space to Y, such that

${\displaystyle \mathrm {D} f(u_{0})=\lambda \circ \mathrm {D} g(u_{0}).\quad {\mbox{(L)}}}$

Since Df(u₀) is an element of the dual space X^∗, equation (L) can also be written as

${\displaystyle \mathrm {D} f(u_{0})=\left(\mathrm {D} g(u_{0})\right)^{*}(\lambda ),}$

where (Dg(u₀))^∗(λ) is the pullback of λ by Dg(u₀), i.e. the action of the adjoint map (Dg(u₀))^∗ on λ, as defined by

${\displaystyle \left(\mathrm {D} g(u_{0})\right)^{*}(\lambda )=\lambda \circ \mathrm {D} g(u_{0}).}$

In the case that X and Y are both finite-dimensional (i.e. linearly isomorphic to R^m and Rⁿ for some natural numbers m and n) then writing out equation (L) in matrix form shows that λ is the usual Lagrange multiplier vector; in the case n = 1, λ is the usual Lagrange multiplier, a real number.

In many optimization problems, one seeks to minimize a functional defined on an infinite-dimensional space such as a Banach space.

Consider, for example, the Sobolev space ${\textstyle X=H_{0}^{1}([-1,+1];\mathbb {R} )}$ and the functional ${\textstyle f:X\rightarrow \mathbb {R} }$ given by

${\displaystyle f(u)=\int _{-1}^{+1}u'(x)^{2}\,\mathrm {d} x.}$

Without any constraint, the minimum value of f would be 0, attained by u₀(x) = 0 for all x between −1 and +1. One could also consider the constrained optimization problem, to minimize f among all those u ∈ X such that the mean value of u is +1. In terms of the above theorem, the constraint g would be given by

${\displaystyle g(u)={\frac {1}{2}}\int _{-1}^{+1}u(x)\,\mathrm {d} x-1.}$

However this problem can be solved as in the finite dimensional case since the Lagrange multiplier ${\displaystyle \lambda }$ is only a scalar.

• Pontryagin's minimum principle, Hamiltonian method in calculus of variations

• Luenberger, David G. (1969). "Local Theory of Constrained Optimization". Optimization by Vector Space Methods. New York: John Wiley & Sons. pp. 239–270. ISBN 0-471-55359-X.
• Zeidler, Eberhard (1995). Applied functional analysis: Variational Methods and Optimization. Applied Mathematical Sciences 109. Vol. 109. New York, NY: Springer-Verlag. doi:10.1007/978-1-4612-0821-1. ISBN 978-1-4612-0821-1. (See Section 4.14, pp.270–271.)

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