nLab
variational calculus

Context

Differential geometry

synthetic differential geometry

Introductions

from point-set topology to differentiable manifolds

geometry of physics: coordinate systems, smooth spaces, manifolds, smooth homotopy types, supergeometry

Differentials

V-manifolds

smooth space

Tangency

The magic algebraic facts

Theorems

Axiomatics

cohesion

  • (shape modality \dashv flat modality \dashv sharp modality)

    (ʃ)(ʃ \dashv \flat \dashv \sharp )

  • dR-shape modality\dashv dR-flat modality

    ʃ dR dRʃ_{dR} \dashv \flat_{dR}

  • tangent cohesion

    • differential cohomology diagram
    • differential cohesion

      • (reduction modality \dashv infinitesimal shape modality \dashv infinitesimal flat modality)

        (&)(\Re \dashv \Im \dashv \&)

      • graded differential cohesion

        • fermionic modality\dashv bosonic modality \dashv rheonomy modality

          (Rh)(\rightrightarrows \dashv \rightsquigarrow \dashv Rh)

        • id id fermionic bosonic bosonic Rh rheonomic reduced infinitesimal infinitesimal & étale cohesive ʃ discrete discrete continuous *

          \array{ && id &\dashv& id \ && \vee && \vee \ &\stackrel{fermionic}{}& \rightrightarrows &\dashv& \rightsquigarrow & \stackrel{bosonic}{} \ && \bot && \bot \ &\stackrel{bosonic}{} & \rightsquigarrow &\dashv& Rh & \stackrel{rheonomic}{} \ && \vee && \vee \ &\stackrel{reduced}{} & \Re &\dashv& \Im & \stackrel{infinitesimal}{} \ && \bot && \bot \ &\stackrel{infinitesimal}{}& \Im &\dashv& \& & \stackrel{\text{étale}}{} \ && \vee && \vee \ &\stackrel{cohesive}{}& ʃ &\dashv& \flat & \stackrel{discrete}{} \ && \bot && \bot \ &\stackrel{discrete}{}& \flat &\dashv& \sharp & \stackrel{continuous}{} \ && \vee && \vee \ && \emptyset &\dashv& \ast }

          </semantics></math></div>

          Models

          Lie theory, ∞-Lie theory

          differential equations, variational calculus

          Chern-Weil theory, ∞-Chern-Weil theory

          Cartan geometry (super, higher)

          Variational calculus

          Physics

          physics, mathematical physics, philosophy of physics

          Surveys, textbooks and lecture notes


          theory (physics), model (physics)

          experiment, measurement, computable physics

          Contents

          Idea

          Variational calculus – sometimes called secondary calculus – is a version of differential calculus that deals with local extremization of nonlinear functionals: extremization of differentiable functions on non-finite dimensional spaces such as mapping spaces, spaces of sections and hence spaces of histories of fields in field theory.

          Specifically, it studies the critical points , i.e. the points where the first variational derivative of a functional vanishes, for functionals on spaces of sections of jet bundles. The kinds of equations specifying these critical points are Euler-Lagrange equations.

          This applies to, and is largely motivated from, the study of action functionals in physics. In classical physics the critical points of a specified action functional on the space of field configurations encode the physically observable configurations.

          There are strong cohomological tools for studying variational calculus, such as the variational bicomplex and BV-BRST formalism.

          In terms of smooth spaces

          We discuss some basics of variational calculus of functional in terms of smooth spaces and in particular in terms of diffeological spaces.

          Smooth functionals

          Let XX be a smooth manifold. Let Σ\Sigma be a smooth manifold with boundary ΣΣ\partial \Sigma \hookrightarrow \Sigma.

          Write

          [Σ,X]Smooth0Type [\Sigma, X] \in Smooth0Type

          for the smooth space (a diffeological space) which is the mapping space from Σ\Sigma to XX, hence such that for each UU \in CartSp we have

          [Σ,X](U)=C (U×Σ,X). [\Sigma, X](U) = C^\infty(U \times \Sigma, X) \,.
          Definition

          Write

          [Σ,X] Σ[Σ,X]× [Σ,X][Σ,X] [\Sigma, X]_{\partial \Sigma} \coloneqq [\Sigma, X] \times_{[\partial \Sigma,X]} \flat [\partial \Sigma,X]

          for the pullback in smooth spaces

          [Σ,X] Σ [Σ,X] [Σ,X] ()| Σ [Σ,X], \array{ [\Sigma,X]_{\partial \Sigma} &\to& \flat [\partial \Sigma, X] \\ \downarrow && \downarrow \\ [\Sigma,X] &\stackrel{(-)|_{\partial \Sigma}}{\to}& [\partial \Sigma,X] } \,,

          where

          • the bottom morphism is the restriction [ΣΣ,X][\partial \Sigma \hookrightarrow \Sigma, X] of configurations to the boundary;

          • the right vertical morphism is the counit of the (DiscΓ)(Disc \dashv \Gamma)-adjunction on smooth spaces.

          Proposition

          The smooth space [Σ,X] Σ[\Sigma, X]_{\partial \Sigma} is a diffeological space whose underlying set is C (Σ,X)C^\infty(\Sigma,X) and whose UU-plots for UU \in CartSp are smooth functions ϕ:U×ΣX\phi \colon U \times \Sigma \to X such that ϕ(,s):UX\phi(-,s) \colon U \to X is the constant function for all sΣΣs \in \partial \Sigma \hookrightarrow \Sigma.

          Definition

          A functional on the mapping space [Σ,X][\Sigma, X] is a homomorphism of smooth spaces

          S:[Σ,X] Σ. S \colon [\Sigma, X]_{\partial \Sigma} \to \mathbb{R} \,.

          Functional derivative

          Write

          d:Ω 1 \mathbf{d} \colon \mathbb{R} \to \Omega^1

          for the de Rham differential incarnated as a homomorphism of smooth spaces from the real line to the smooth moduli space of differential 1-forms.

          Definition

          The functional derivative

          dSΩ 1([Σ,X] Σ) \mathbf{d}S \in \Omega^1([\Sigma,X]_{\partial \Sigma})

          of a functional SS, def. , is simply its de Rham differential as a homomorphism of smooth spaces, hence the composite

          dS:[Σ,X] ΣSdΩ 1. \mathbf{d} S \colon [ \Sigma, X]_{\partial \Sigma} \stackrel{S}{\to} \mathbb{R} \stackrel{\mathbf{d}}{\to} \Omega^1 \,.
          Definition

          This means that for each smooth parameter space UU \in CartSp and for each smooth UU-parameterized collection

          ϕ:U×ΣX \phi \colon U \times \Sigma \to X

          of smooth functions ΣX\Sigma \to X, constant in the parameter UU in some neighbourhood of the boundary of Σ\Sigma,

          S ϕ:[Σ,X] Σ(U)C (U,) S_\phi \colon [\Sigma,X]_{\partial \Sigma}(U) \to C^\infty(U,\mathbb{R})

          is the function that sends each such UU-collection of configurations to the UU-collection of their values under SS. Then

          (dS) ϕΩ 1(U) (\mathbf{d}S)_\phi \in \Omega^1(U)

          is the smooth differential 1-form

          (dS) ϕ=d(S(ϕ)). (\mathbf{d}S)_\phi = \mathbf{d}(S(\phi)) \,.
          Example

          Let Σ=[0,1]\Sigma = [0,1] \hookrightarrow \mathbb{R} be the standard interval. Let

          L(,)dtΩ 1([0,1],C ( 2)) L(-,-) \mathbf{d}t \in \Omega^1([0,1], C^\infty(\mathbb{R}^2))

          and consider the functional

          S:([0,1]γX) 0 1L(γ(t),γ˙(t))dt. S \colon ([0,1] \stackrel{\gamma}{\to} X) \mapsto \int_{0}^1 L(\gamma(t), \dot \gamma(t)) d t \,.

          Then for U=U = \mathbb{R} and any smooth UU-parameterized collection {γ u:ΣX} uI\{\gamma_{u} \colon \Sigma \to X\}_{u \in I} the functional derivative takes the value

          dS γ () =(ddu 0 1L(γ u(t),γ˙ u(t))dt)du = 0 1(Lγ(γ u(t),γ˙ u(t))dγ u(t)du+Lγ˙(γ u(t),γ˙ u(t))γ˙ u(t)u)du = 0 1(Lγ(γ u(t),γ˙ u(t))dγ u(t)du+Lγ˙(γ u(t),γ˙ u(t))tγ u(t)u)du = 0 1(Lγ(γ u(t),γ˙ u(t))tLγ˙(γ u(t),γ˙ u(t)))γ u(s)udu. \begin{aligned} \mathbf{d}S_{\gamma_{(-)}} & = \left( \frac{d}{d u} \int_0^1 L(\gamma_u(t), \dot \gamma_u(t)) dt \right) \mathbf{d}u \\ & = \int_{0}^1 \left( \frac{\partial L}{\partial \gamma}(\gamma_u(t), \dot \gamma_u(t)) \frac{d \gamma_u(t)}{d u} + \frac{\partial L}{\partial \dot \gamma}(\gamma_u(t), \dot \gamma_u(t)) \frac{\partial \dot \gamma_u(t)}{\partial u} \right) \mathbf{d} u \\ & = \int_{0}^1 \left( \frac{\partial L}{\partial \gamma}(\gamma_u(t), \dot \gamma_u(t)) \frac{d \gamma_u(t)}{d u} + \frac{\partial L}{\partial \dot \gamma}(\gamma_u(t), \dot \gamma_u(t)) \frac{\partial }{\partial t}\frac{\partial \gamma_u(t)}{\partial u} \right) \mathbf{d} u \\ & = \int_{0}^1 \left( \frac{\partial L}{\partial \gamma}(\gamma_u(t), \dot \gamma_u(t)) - \frac{\partial}{\partial t}\frac{\partial L}{\partial \dot \gamma}(\gamma_u(t), \dot \gamma_u(t)) \right) \frac{\partial \gamma_u(s)}{\partial u} \mathbf{d}u \end{aligned} \,.

          Here we used integration by parts and used that the boundary term vanishes

          0 1t(γ˙L(γ u(s),γ˙ u(s))γ u(s)u)ds =(γ˙L(γ u(1),γ˙ u(1))γ u(1)u)(γ˙L(γ u(0),γ˙ u(0))γ u(0)u) =0 \begin{aligned} \int_{0}^1 \frac{\partial}{\partial t} \left( \frac{\partial}{\partial \dot\gamma} L(\gamma_u(s), \dot \gamma_u(s)) \frac{\partial \gamma_u(s)}{\partial u} \right) d s & = \left( \frac{\partial}{\partial \dot\gamma} L(\gamma_u(1), \dot \gamma_u(1)) \frac{\partial \gamma_u(1)}{\partial u} \right) - \left( \frac{\partial}{\partial \dot\gamma} L(\gamma_u(0), \dot \gamma_u(0)) \frac{\partial \gamma_u(0)}{\partial u} \right) \\ & = 0 \end{aligned}

          since by prop. γ ()(1)\gamma_{(-)}(1) and γ ()(0)\gamma_{(-)}(0) are constant.

          In terms of the variational bicomplex

          In the special case that the functional to be varied comes from a Lagrangian density, then its variational derivative is the image under transgression of the vertical derivative in the variational bicomplex of differential forms on the given jet bundle.

          (…)

          References

          Exposition of variational calculus in terms of jet bundles and Lepage forms and aimed at examples from physics is in

          Fundamental texts on variational calculus include

          • Ian Anderson, The variational bicomplex, (pdf)

          • Hubert Goldschmidt, Shlomo Sternberg, The Hamilton-Cartan formalism in the calculus of variations, Annales de l’institut Fourier 23 no. 1 (1973), p. 203-267 numdam

          • Peter Olver, Applications of Lie groups to differential equations, Springer; Equivalence, invariants, and symmetry, Cambridge Univ. Press 1995.

          • Demeter Krupka, Introduction to global variational geometry, 2015

          • Olga Krupková, The geometry of ordinary variational equations, Springer 1997, 251 p.

          • Robert Hermann, Some differential-geometric aspects of the Lagrange variational problem, Illinois J. Math. 6, 1962, 634–673 MR145457 euclid; Differential geometry and the calculus of variations, Acad. Press 1968

          • J. Jost, X. Li-Jost, Calculus of variations, CUP 1998

          • G. J. Zuckerman, Action Principles and Global Geometry , in Mathematical Aspects of String Theory, S. T. Yau (Ed.), World Scientific, Singapore, 1987, pp. 259€284. (pdf)

          Zuckerman’s ideas are used in

          Examples: Jürgen Jost, Variational problems from physics and geometry, pdf

          • J. J. Duistermaat, On the Morse index in variational calculus, Adv. Math. 21 (1976), 2, 173–195 pdf.

          Some new results are in

          • E. Getzler, A Darboux theorem for Hamiltonian operators in the formal calculus of variations, Duke Math. J. 111, n. 3 (2002), 535-560, MR2003e:32026, doi
          • Alberto De Sole, Victor G. Kac, The variational Poisson cohomology, arxiv/1106.0082

          Geometric extremization problems (e.g. minimal surfaces), see also geometric measure theory:

          • Jürgen Jost, The geometric calculus of variations: a short survey and a list of open problems, Exposition. Math. 6 (1988), no. 2, 111–143, MR89h:58036
          • H. Federer, Geometric measure theory, Springer 1969(especially appendices to Russian transl.)
          • Frederick J., Jr. Almgren, Almgren’s big regularity paper (book form of a 1970s preprint)

          Discussion in the context of BV formalism:

          Other references

          • J. C. P. Bus, The Lagrange multiplier rule on manifolds and optimal control of nonlinear systems, SIAM J. Control and Optimization 22, 5, 1984, 740-757 pdf

          Relation to covariant phase spaces

          • L. Vitagliano, Secondary calculus and the covariant phase space, pdf

          By functorial analysis and 𝒟\mathcal{D}-geometry

          See also references at diffiety.

          A formalism for variational calculus based on functorial analysis (with a precise relation with functional analytic methods and jet formalism) and a long list of examples of variational problems arising in classical mechanics and quantum field theory are collected in

          The formulation of variational calculus in terms of diffeological spaces is mentioned for instance in section 1.65 of

          following section 2.3.20 of

          For variational calculus in nonstandard analysis see survey

          • Vítor Neves, Nonstandard calculus of variations, a survey, pdf
          category: analysisphysics

          Last revised on November 30, 2017 at 07:33:42. See the history of this page for a list of all contributions to it.