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On-shell Lagrangians as total derivatives and the generalized Komar charge

T0 review · 0 major / 6 minor · reviewed 2026-08-15 · deepseek-v4-flash

Pith's one-line read A homogeneous Lagrangian is on-shell an exact form, and this identity builds generalized Komar charges.

desk verdict A clean field-theoretic Euler identity that powers a worked Komar-charge construction; minor presentational gaps only, so it deserves a serious referee. read the letter →

arxiv 2506.14024 v2 pith:4ODDPTRZ submitted 2025-06-16 hep-th gr-qc

classification hep-thgr-qc
keywords KomarchargeNoether–Waldon-shellLagrangianEulertheoremforhomogeneousfunctionsSmarrformulablackholethermodynamicsmomentummapsquasi-homogeneousLagrangians
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper proves a functional version of Euler's theorem for homogeneous functions: if the fields rescale with weights $\omega_i$ and the Lagrangian rescales with weight $\omega_L$, then $L=\frac{\omega_i}{\omega_L}E_i\wedge\phi^i+d\Theta(\phi,\frac{\omega}{\omega_L}\phi)$ holds identically, with $E_i$ the Euler–Lagrange forms and $\Theta$ the pre-symplectic potential, the boundary term generated by varying the action. On solutions, $E_i=0$, so the Lagrangian is the total derivative of a universal expression that depends only on the fields and the fixed weights, not on the particular solution. Because the generalized Komar construction needs the on-shell pullback of the Lagrangian by a Killing vector to be exact, this identity supplies exactly the boundary term needed to close the charge. The authors demonstrate the method on Einstein–Maxwell–Dilaton gravity, minimal five-dimensional supergravity, and a self-interacting scalar, where nonlinearity is handled by promoting the coupling constant to a constrained field. The result is a systematic route to conserved surface charges of the form needed for Smarr formulas in black-hole thermodynamics.

What carries the argument

The load-bearing identity is the functional Euler theorem (Eq. (2.7)): $L=\frac{\omega_i}{\omega_L}E_i\wedge\phi^i+d\Theta(\phi,\frac{\omega}{\omega_L}\phi)$, where $E_i$ are the Euler–Lagrange forms and $\Theta$ is the pre-symplectic potential, the boundary term generated by an arbitrary field variation. On solutions it becomes $L\doteq dJ_0$ with $J_0=\Theta(\phi,\frac{\omega}{\omega_L}\phi)$. This identity is the engine of the paper: it turns the on-shell Lagrangian into a total derivative without solving the field equations, and when combined with the generalized-Komar requirement $i_k L+B(\Lambda_k,\phi)\doteq d\omega(k,\phi)$, it yields the exact $(d-2)$-form $\omega(k,\phi)$ and hence the closed charge $K[k]$.

What would settle it

Take the proposed generalized Komar charge for Einstein–Maxwell–Dilaton theory (Eq. (4.23)) and evaluate its exterior derivative on a known non-extremal black-hole solution; if $dK[k]$ does not vanish on-shell, or if the horizon integral differs from the integral at infinity, the exactness assumption is false. A simpler algebraic check is to substitute arbitrary field configurations into Eq. (2.7) and verify the identity term by term; any counterexample would falsify the Euler-theorem claim.

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Extended reading notes

Core claim

The central claim is that for any Lagrangian $d$-form that transforms homogeneously under a global transformation of the fields, the Lagrangian can be written identically as a sum of the equations of motion contracted with the fields plus an exact piece: $L=\frac{\omega_i}{\omega_L}E_i\wedge\phi^i+d\Theta(\phi,\frac{\omega}{\omega_L}\phi)$. On-shell this reduces to $L\doteq dJ_0$, where $J_0=\Theta(\phi,\frac{\omega}{\omega_L}\phi)$ is solution-independent. The paper shows that $J_0$ is unique only up to conserved currents $J_n$ associated with scaling symmetries of the theory, and that the same Euler-theorem logic works for any global transformation with nonzero weight, not just rescalings. It then plugs this identity into the generalized Komar algorithm: given a Killing vector $k$, the on-shell interior product $i_k L$ becomes an exact $(d-2)$-form, and the resulting closed charge $K[k]$ reproduces and organises the known charges of Einstein–Maxwell–Dilaton theory and minimal $d=5$ supergravity, and gives a potential-times-charge form for the self-interacting scalar.

Load-bearing premise

The load-bearing assumption is that a symmetry that leaves all fields fixed also makes the on-shell Lagrangian pullback exact: there must always exist some $(d-2)$-form $\omega(k,\phi)$ with $i_k L+B(\Lambda_k,\phi)\doteq d\omega(k,\phi)$. The paper takes this exactness from the earlier generalized-Komar framework rather than proving it; if it fails, the constructed charge is not conserved on-shell and the Gauss law behind Smarr formulas is lost.

Editorial extensions

If this is right

  • In any theory with a homogeneous rescaling of the fields, the action evaluated on solutions is a pure boundary term, $S_{\mathrm{on-shell}}=\int dJ_0$.
  • For every Killing vector $k$, the on-shell interior product $i_k L$ is exact, so the generalized Komar charge $K[k]$ is closed on-shell and satisfies a Gauss law.
  • The ambiguity in $J_0$ by conserved currents is physical: in Einstein–Maxwell–Dilaton theory, adding $\alpha J_1$ shifts the Komar charge by a term proportional to the conserved scalar charge $Q_k$, while leaving Smarr formulas unchanged when the scalar-charge Gauss law is used.
  • Promoting a dimensionful coupling constant to a constrained scalar field extends the Euler-theorem identity to nonlinear Lagrangians, as demonstrated for a self-interacting scalar; the authors state that the same trick applies to nonlinear electrodynamics coupled to gravity.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The paper does not pursue it, but Eq. (2.7) also gives a shortcut for Euclidean on-shell actions: the boundary term is read off from the pre-symplectic potential before solving equations, which could simplify free-energy computations for stationary black holes.
  • Because the identity is algebraic in the fields, it should extend to higher-curvature or Chern–Simons theories; testing it on Lovelock gravity or generalized Proca models would be a direct next step.
  • Applying the coupling-promotion trick to a cosmological constant rather than a scalar coupling should yield a Komar charge whose boundary integral is the pressure–volume term of extended black-hole thermodynamics.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 6 minor

Summary. The paper proves a field-theoretic analogue of Euler's homogeneous-function theorem: if a Lagrangian d-form L is homogeneous under a global rescaling of the fields with weights ω_i and transforms with weight ω_L, then L = (ω_i/ω_L) E_i∧φ_i + dΘ(...), so that on-shell L is an exact form with a solution-independent representative (Eq. 2.7). The paper then combines this identity with the generalized Komar charge algorithm of Refs. [12–16] to construct Komar charges in three examples: Einstein–Maxwell–Dilaton theory, the bosonic sector of minimal 5-dimensional supergravity, and a self-interacting scalar field coupled to gravity (using a Lagrange-multiplier extension for the non-linear potential). It also discusses the non-uniqueness of the total-derivative representative coming from additional scaling symmetries.

Significance. The central identity in Section 2 is clean, self-contained, and parameter-free; it follows directly from the variational identity and does not depend on the earlier Komar-charge algorithm. The application sections show that this identity supplies the needed on-shell total-derivative form in three non-trivial theories and reproduce known generalized Komar charges, which is a valuable consistency check. The Lagrange-multiplier trick in Section 6 extends the method to non-linear potentials. If correct, the paper provides a systematic and practical tool for constructing Smarr formulas and generalized Komar charges.

minor comments (6)
  1. [Section 2, Eq. (2.8)] The sign in Eq. (2.8) appears incorrect: from Eq. (2.6) with ω_L = 0 one obtains dJ_n = -ω^n_i E_i∧φ_i, as used in Eq. (4.11), not dJ_n = +ω^n_i E_i∧φ_i.
  2. [Section 3, Eq. (3.24b)] The trailing '= 0' in Eq. (3.24b) is inconsistent with the definition of the generalized Komar charge and with the non-trivial results in the subsequent examples; it should be removed, or replaced by a statement that dK[k] ≑ 0.
  3. [Section 4, Eq. (4.22)] The placement of the parentheses in Eq. (4.22) gives the αQ_k term a factor of 1/2; the derivation via Eq. (4.21a) gives the term +αQ_k outside the factor 1/2, matching Eq. (4.23).
  4. [Section 5, Eqs. (5.9)–(5.10)] The sign of τ(χ,φ) in Eq. (5.10) appears inconsistent with a direct explicit computation of δχJ0 from J0 = (1/3)⋆G∧V using Eq. (5.3b); please check and correct the sign.
  5. [Section 6, Eq. (6.3)] The on-shell equivalence of the extended action (6.3) to the original theory is asserted but not proven; a short proof (integrating out C and g, or a brief citation plus explanation) would make the example self-contained.
  6. [Throughout] There are several small typos: 'generalized generalized Komar charges' before Eq. (4.23), 'a a matter of fact' after Eq. (2.10), 'Let as assume' at the start of Section 3, and 'equtions' in Section 3.1.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the Euler-theorem identity is derived from first principles; earlier Komar-charge work is cited contextually, not as load-bearing input.

full rationale

The central claim, Eq. (2.7), is obtained directly from the variational identity (2.2) by setting the variation to the assumed rescaling δλφi = ωi λ φi and δλL = ωL λ L. No quantity in this derivation is fitted, nor is the conclusion L = dΘ(φ, (ω/ωL)φ) imported from the references; it is an algebraic consequence of the homogeneity assumption. Eq. (2.10) merely parameterizes the freedom to add on-shell-closed currents Jn, and the coefficients αn are explicitly left as free choices, not tuned to match a precomputed answer. The generalized-Komar discussion in Section 3 is a review of the authors' earlier algorithm (Refs. [12-16]) and supplies the standard Noether-Wald construction; the exactness condition (3.17) is posed as the condition defining the desired ω(k, φ), not as a result secretly supplied by the cited papers. The examples reproduce known charges and thus serve as independent consistency checks rather than circular confirmations. Although self-citations are numerous, they are not load-bearing for the paper's main derivation, so there is no circular step.

Assumptions & free parameters 1 free parameters · 4 assumptions · 1 invented entities

The central Euler-theorem identity requires only the standard action variation formula and the homogeneity assumption. The Komar charge construction adds the inherited exactness assumption Eq. (3.17). The scalar-field example introduces an auxiliary Lagrange-multiplier field and an ad hoc equivalence assumption. No numerical parameters are fitted to data; the only free parameters are the arbitrary coefficients alpha_n in the non-uniqueness analysis.

free parameters (1)
  • alpha_n (and alpha in Section 4) = arbitrary, set to 0 in EMD example
    Coefficients of on-shell closed currents that can be added to the total derivative expression, Eq. (2.10) and Eq. (4.13). They parametrize a family of Komar charges without affecting the Smarr formula when the scalar-charge relation is used.
assumptions (4)
  • standard math The action variation formula (2.2) with Euler-Lagrange forms and pre-symplectic potential
    Standard result in Lagrangian field theory, used to derive Eq. (2.7).
  • domain assumption The Lagrangian is homogeneous under a global rescaling with weights omega_i and weight omega_L (Eq. 2.4)
    Assumed; for non-homogeneous theories such as the scalar example, the coupling constant is promoted to a scalar field to recover homogeneity.
  • domain assumption On-shell exactness of the pulled-back Lagrangian for Killing vectors, Eq. (3.17)
    Inherited from Refs. [12-16]; not proved in this paper, and it is the basis of the generalized Komar charge closure.
  • ad hoc to paper The Lagrange-multiplier extended action (6.3) is on-shell equivalent to the original theory, preserving charges
    Introduced in Section 6; the paper asserts equivalence but does not check boundary terms or charge preservation.
invented entities (1)
  • Auxiliary (d-1)-form Lagrange-multiplier field C
    purpose: Enforces constancy of the promoted coupling constant g in the self-interacting scalar example
    Introduced in Eq. (6.3); a non-dynamical auxiliary field with no independent observable prediction.

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Pith. "Pith review of On-shell Lagrangians as total derivatives and the generalized Komar charge." pith.science (2026). https://pith.science/paper/4ODDPTRZ

@misc{pith2026250614024,
  author       = {Pith},
  title        = {Pith review of: On-shell Lagrangians as total derivatives and the generalized Komar charge},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4ODDPTRZ}},
  note         = {Machine review of arXiv:2506.14024}
}
read the original abstract

Lagrangians which transform homogeneously under a global transformation of the fields (a global rescaling, for instance) can be written on-shell as a total derivative which has a universal, solution-independent expression, using a functional version of the Euler theorem for homogeneous functions. We study the uniqueness of this expression and how this result can be used in the construction of generalized Komar charges.

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