 Lagrangian Dynamics

In General > s.a. Euler-Lagrange Equations; formulations of quantum theory; lagrangian systems.
* Idea: A formulation of dynamics based on the principle of stationary action (often, least action), in turn based on variational calculus.
* History: The term "action", and the concept that physical laws are a consequence of "economy of means" were introduced by Maupertuis in 1744, with W = $$int$$p dq, and improved by Euler and Lagrange; In 1832 Hamilton proposed the action S = L(q, q·) dt.
* Motivation: (1) It is an elegant way of expressing the dynamical content of a physical theory; (2) Convenient for the study of symmetries of the theory; (3) Important for the path integration quantization program (> but see path integrals); (4) For a field theory, it is a "spacetime covariant" formulation, and for relativistic quantum field theory it is preferable to a Hamiltonian formulation.
* Action: (1) Choose a configuration space, with coordinates q depending on x; (2) Given a region X of the xs, find a Lagrangian (density) $$\cal L$$ or an action functional S[q, ∂q, x], written as

S[q, ∂q, x] = M $$\cal L$$(q, ∂q, x) dx ;

(3) Derive the equations of motion using variational principles; Physical trajectories are extrema of the action.
* For Newtonian mechanics: The true trajectory between two spacetime events is a minimum of the action if the final event occurs before the kinetic focus of the initial event; Otherwise, it is a saddle point of the action.
* For a field theory: A Lagrangian of order k is a horizontal form L: J kY → ΛnM, where (Y, π, M) is the configuration bundle for the theory, (JkY, πk, M) its k-jet prolongation, and M the spacetime manifold; A variation of the fields is a vertical vector field X = δyi (∂/∂yi), under which the Lagrangian varies by

δXL = e(L, X) + dF(L, X) ,  with  e(L, X) = Euler-Lagrange form ,  and  F(L, X) = Poincaré-Cartan form .

* Normalization constants: They do not affect the equations of motion, but they do appear in the conjugate momenta.
* Interacting systems: To get the Lagrangian, just add the ones for each system and (if they are needed) the extra interaction terms, but we have to be careful with the relative coefficients for each term.

And Canonical Formalism > s.a. constrained systems.
* Legendre transformation: The map F: TQ → T*Q (Q is configuration space) defined, for a Lagrangian L, by $$F(w)v$$:= dL($$w+tv$$)/dt$$|_{t=0}$$.
@ General references: Kastrup PRP(87); Bertin et al MPLA(05)ht [first-order actions, Hamilton-Jacobi analysis]; Compère PhD(07)-a0708 [covariant Hamiltonian formulation, symmetries, and black holes]; Arcuş a1108 [Legendre duality between Lagrangian and Hamiltonian Mechanics].
@ For field theories: Giachetta et al 97; Echeverría-Enríquez et al IJMMS(02)mp/01, JMP(04) [unified formalism]; Geyer et al JPA(03)ht/02 [and gauge symmetries].
@ Related topics: Duplij JMPAG-a1002 [degenerate Lagrangian theories, Clairaut-type formalism]; Gallardo IJMPA(12) [timelike boundary terms].
& Related topics: R Skinner and R Rusk, Lagrangian-Hamiltonian unified formalism.

Choice of Action and Ambiguities > s.a. variational principles.
* Idea: In practice, in particular for field theories, we get the invariances from nature (e.g., gauge symmetries), and this often is very restrictive; However, classically one can always add a boundary term to the action, or a divergence to $$\cal L$$, that depends on the fields; This has the effect of changing the definitions of the canonical momenta and energy but not the equations of motion – the Lagrangians are in the same variational cohomology class; In the quantum theory, it is not so simple.
* Field redefinitions: For example, by conformal transformations.
@ Ambiguity in Lagrangian: Hojman & Shepley in(88), Lunev TMP(91), TMP(92) [for field theories]; in Matzner & Shepley 91; Faraoni CQG(94); Cisło & Łopuszański JMP(01)mp/00 [and quantization]; Nucci JPCS(12)-a1202 [Jacobi's method for finding Lagrangians, and choices that can be used for quantization].
@ Lagrangian cohomology classes: in Marathe & Martucci 92; in Kolár et al 93.
@ Related topics: Frieden 98 [Lagrangians from Fisher information]; Buniy & Kephart PLA(08) [higher-order topological actions and quantum theory].

Symmetries and Conserved Quantities > s.a. conservation laws; noether theorem; symmetries.
* Symmetric variations: The issue is whether the Principle of Symmetric Criticality holds; It states that for any group-invariant lagrangian the equations obtained by restriction of the Euler-Lagrange equations to group-invariant fields are equivalent to the Euler-Lagrange equations of a canonically defined, symmetry-reduced lagrangian.
* Conserved currents and Lie algebras: One may introduce at least three different Lie algebras in any Lagrangian field theory, (i) the Lie algebra of local BRST cohomology classes with the odd Batalin-Vilkovisky antibracket; (ii) the Lie algebra of local conserved currents with the Dickey bracket; and (iii) the Lie algebra of conserved, integrated charges with the Poisson bracket.
* Question: Are there known examples of systems for which symmetric criticality fails?
@ General references: Gràcia & Pons JPA(92); Grigore JPA(95)ht/94; Barnich & Henneaux JMP(96) [Batalin-Vilkovisky antibracket and Poisson bracket]; Banerjee et al PLB(99)ht, PLB(00)ht/99, Banerjee ht/00 [and Hamiltonian]; Torre AIP(11)-a1011 [symmetric criticality in field theory]; Deriglazov & Rizzuti PRD(11)-a1105 [extended Lagrangian formalism as a systematic procedure to look for local symmetries].
@ Conservation laws: Lutzky JPA(79), JPA(82), JPA(95), JPA(98) [non-Noether]; Sardanashvily mp/03 [Noether]; Accornero & Palese a1710 [and higher-order variations].
@ Symmetric variations: Palais CMP(79); Christodoulakis & Korfiatis NCB(94); Davis GRG(98)gq/96, Fels & Torre CQG(02)gq/01 [gravity]; Wu PRD(09).
@ Lagangians from Lie transformation groups: Paal & Virkepu in(09)-a0706; Nucci & Leach JMP(07)-a0706 [Jacobi last multiplier].

References > s.a. classical mechanics; history of physics.
@ Textbooks: Lagrange 1788; in Sudarshan & Mukunda 75; Calkin 96 [+ solutions 99]; Curry 13, Hamill 13, Brizard 15 [II]; Nolting 16.
@ General: Bailey FP(81) ["more precise statement"]; Cariñena et al PRP(95) [need for lagrangian in physics]; Stöltzner SHPMP(03) [principle of least action]; Evans AJP(03)may [interpretation of particle action]; Toffoli IJTP(03) [meaning?]; Butterfield phy/04 [ontology]; Gray & Taylor AJP(07)may [not minimized in general]; Cuell RPMP(07) [as a skew critical problem]; Gondran & Gondran a1203 [Euler-Lagrange and Hamilton-Jacobi actions]; Lachièze-Rey a1411 [history-based]; in Cortés & Haupt book(17)-a1612 [lecture notes, mathematical].
@ Conceptual: Terekhovich a1511 [and the Humean view of the laws of nature, Leibniz's concept of the possibles].
@ And equations of motion: Grigore NCB(96)ht [conditions for first-order Lagrangian]; Gitman & Kupriyanov JPA(07)-a0710; Nucci & Tamizhmani a0809 [finding L by the method of Jacobi Last Multiplier]; Pons JMP(10) [substituting fields in the action]; Gray & Poisson AJP(11)jan [for geodesic worldlines in a curved spacetime]; Bekenstein & Majhi NPB(15)-a1412 [the least action principle is not necessary].
@ Geometrical: Cariñena et al JMP(88); del Olmo & Santander JPA(89), JGP(90); Sharipov m.DG/01 [in Riemannian manifolds]; Miron a1203 [geometrization]; Meng JPA(15)-a1405 [Tulczyjew's approach, for charged particles in gauge fields].
@ And spacetime transformations: Whiston IJTP(72).
@ Inverse variational principle: Santilli 78; Okubo PRD(80) [non-unique]; Cariñena et al PRP(95); Mestdag et al DG&A(11) [with non-conservative forces]; Saha & Talukdar a1301 [non-standard Lagrangians]; > s.a. variational principles.
@ Complex action: Gocksch PLB(88); Anagnostopoulos & Nishimura PRD(02); Nielsen a0911-proc [and retrocausation]; Nagao & Nielsen PTP(11)-a1104, IJMPA(12)-a1105.
@ Other generalizations, types: Grigore IJMPA(92); Núñez-Yépez et al mp/01-in; Arizmendi et al CSF(03)mp/04; Struckmeier IJMPE(09)-a0811 [extended Hamilton-Lagrange formalism]; Gravanis & Willison JMP(09)-a0901 [distributional fields]; Sardanashvily IJGMP(13) [Grassmann-graded, and reducible degenerate systems]; Polonyi PRD(14)-a1407 [closed-time-path extension of effective theories]; Minguzzi JGM(15)-a1410 [covariant, including friction, non-holonomic constraints and energy radiation]; Surawuttinack et al TMP(16)-a1502 [multiplicative form]; Talamucci a1802; Lazo et al JMP(18)-a1803 [Action-dependent Lagrangians].
@ Related topics: Ferraris et al JMP(00) [dual Lagrangians]; Muñoz Díaz a0801 [and time]; López et al CiM-a1205 [minima vs critical points, and techniques from non-linear analysis]; Neiman a1212 [with null boundary data]; Díaz et al JMP(14)-a1406 [physical degree of freedom count]; > s.a. force; morse theory [on spaces of braids].