REVIEW 2 major objections 5 minor 35 references
The paper claims that, after removing constant adjoint modes, the first Gribov horizon on a torus is located exactly when −1 enters the spectrum of a dimensionless Birman-Schwinger operator K_A, a criterion that preserves inertia rather tha
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-02 02:10 UTC pith:HSZ6UFMT
load-bearing objection Exact congruence criterion is solid; the advertised analytic scaffolding has gaps that should be fixed or relabeled before publication. the 2 major comments →
Birman-Schwinger Formulation of the Faddeev-Popov Zero-Mode Problem
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
On T^d, with constant adjoint scalar modes removed, the paper establishes the exact equivalence 0∈spec(M) ⇔ −1∈spec(K_A), via the congruence M = H_0^{1/2}(1+K_A)H_0^{1/2}. Because this congruence preserves inertia (n_± and n_0), horizon crossings are located by the first appearance of −1 in the spectrum of K_A, without equating the numerical spectra of M and 1+K_A. The companion analytic claim is Theorem 1: for d≥3 and transverse A∈L^d, K_A lies in S^{d,∞} with s_n(K_A) ≤ C_d κ_N g ∥A∥_{L^d} n^{−1/d}. The two-dimensional endpoint is characterized on both sides: the plain L^2 estimate fails, while the sharp result holds in L^{2,1}. The same formulation gives the fixed-background ghost dressin
What carries the argument
The central object is the Birman-Schwinger operator K_A = H_0^{−1/2} V_A H_0^{−1/2}, where V_A = −g ad(A_i)∂_i is the first-order background perturbation. K_A is dimensionless and self-adjoint when A is transverse. The load-bearing identity is the congruence M = H_0^{1/2}(1+K_A)H_0^{1/2}, which converts the zero-mode equation into a spectral condition on a bounded operator and preserves the positive, negative, and null dimensions of the quadratic form. Quantitative control comes from reducing K_A to a matrix-valued Cwikel-type operator and applying periodic Cwikel-type estimates at coefficient regularity A∈L^d, with the L^{2,1} endpoint at d=2.
Load-bearing premise
The quantitative singular-value bounds rest on transferring a local scalar Cwikel-type estimate to periodic matrix-valued multipliers 'without change' and on a quoted Lorentz-space endpoint whose per-layer estimate is asserted rather than derived; if either fails, the bounds lose support, while the elementary congruence criterion would survive.
What would settle it
Compute s_n(K_A) on T^d for a periodic transverse matrix-valued A∈L^d with d≥3 and check the predicted n^{−1/d} decay; slower decay or noncompactness would refute Theorem 1. For d=2, take f∈L^{2,1} with finite Lorentz norm, for example a smoothed version of the annulus family with ||f||_{L^{2,1}} finite, and test whether M_fΛ has finite S^{2,∞} norm; if the norm diverges, Proposition 2 fails.
If this is right
- Horizon crossings are governed by a fixed threshold: the first Gribov horizon occurs exactly when −1 becomes an eigenvalue of K_A, independent of volume normalization.
- The exact ghost dressing at fixed background is the diagonal resolvent d(k)=⟨k|(1+K_A)^{−1}|k⟩; along amplitude scaling the dressing is meromorphic with simple poles whose residues are spectral overlaps of the critical mode.
- For d≥3, K_A is compact with quantified singular-value decay n^{−1/d} controlled by ∥A∥_{L^d}, so the analytic framework holds at critical regularity.
- In d=2 the plain L^2 estimate fails; the sharp class is L^{2,1}, which still contains every bounded and smooth background.
- The periodic SU(2) family yields a solvable sector: the threshold satisfies the Mathieu condition a_0(2a_crit)=−4, the no-pole/Born estimate gives a_np²=2(Q²+m²), and the critical amplitude falls as 1/L with action scaling L^{d−4}.
Where Pith is reading between the lines
- A lattice test should diagonalize the normalized operator directly: equation (42) implies that the observed drift of Faddeev-Popov eigenvalues toward zero could arise from the free infrared factor H_0^{−1} rather than from K_A approaching −1, so the two mechanisms are numerically distinguishable.
- The linearity of V_A in A gives K_{aA}=aK_A with fixed eigenvectors; this suggests using the residue R(k)=|⟨k|η_min⟩|² of the ghost dressing as a direct probe of which momenta feel an approaching horizon.
- The entropy-action competition in the periodic model becomes volume-independent at d=4; one could test whether this marginality persists for multi-mode or SU(3) backgrounds as a heuristic for the special role of four spacetime dimensions.
- Because the congruence preserves only inertia, not spectra, approximate methods such as Born/no-pole and Feshbach truncations can be compared to the exact criterion only through the extremal eigenvalue of K_A; this frames a systematic truncation hierarchy in the solvable sector.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops a Birman-Schwinger formulation of the Landau-gauge Faddeev-Popov operator on a torus after removal of constant adjoint modes. The central object is K_A=H_0^{-1/2}V_A H_0^{-1/2}, whose spectral value -1 is shown, via a congruence rather than a similarity, to be equivalent to a Faddeev-Popov zero mode. The paper claims a quantitative weak-Schatten bound for K_A in d>=3, a borderline positive result for d=2 in L^{2,1}, an expression for the fixed-background ghost dressing as the diagonal resolvent of (1+K_A)^{-1}, a solvable periodic SU(2) background reducing to a Mathieu recurrence, and volume scaling laws for the critical amplitude and the number of near-critical modes. The exact congruence criterion and the explicit solvable sector are cleanly presented, but the advertised analytic scaffolding for the quantitative bounds relies on two unproved technical transfers.
Significance. If the quantitative estimates are fully established, the paper would provide a useful normalized spectral criterion for the first Gribov horizon and a concrete bridge to the ghost dressing function. The exact congruence (3)/(11), the inertia identity (12), and the explicit Mathieu characterization (47) are valuable and independently checkable. The paper is also appropriately cautious in separating fixed-background statements from ensemble statements, and it explicitly labels the entropy model as approximate. The main risk is not the central equivalence but the spectral-theoretic scaffolding: Theorem 1 and Proposition 2 are presented as settled while their proofs contain gaps that are load-bearing for the n^{-1/d} and n^{-1/2} decay claims.
major comments (2)
- [§4.1, Theorem 1 (Eq. (22))] Step four of §4.1 asserts that the periodic Cwikel–Birman–Solomyak theorem transfers from R^d to T^d 'without change' for matrix-valued multipliers, and that this delivers (21). This is the step that converts the scalar L^d estimate into the S^{d,\infty} bound for K_A, so it is load-bearing for (22). No argument or precise theorem statement is supplied for the torus transfer, nor for the matrix-valued case beyond the FLS reduction in (20). Please provide a self-contained proof or an exact statement with reference for \|M_F\Lambda\|_{S^{d,\infty}}\le C_d\||F|\|_{L^d} on T^d, or explicitly mark Theorem 1 as conditional.
- [§4.2, Proposition 2 and Eq. (28)] The endpoint L^{2,1} estimate is presented as proved, but the proof is only a sketch. The per-layer bound (28) uses p_j=2+1/\log(2^j/\sigma_j), but no specification is given for the regime where \log_+(2^j/\sigma_j) is nonpositive; the dyadic/Yano summation is delegated to refs. [25,26]; and the notation N_{M_{f_j}\Lambda}(\sigma_j) is undefined. Since the Conclusion explicitly relies on Proposition 2 as settled analytic scaffolding, this is a genuine gap. The proof should be completed, or the proposition should be stated with an exact theorem-and-version citation covering this form. As written, the endpoint is asserted rather than demonstrated.
minor comments (5)
- [Abstract] 'The resulting normalized operator, is dimensionless' contains a stray comma after 'operator'.
- [§4.2, Eq. (28)] Define the counting function N_T(\sigma)=\#\{n: s_n(T)>\sigma\} before first use.
- [§5, Fig. 1] The fitted log-log slopes (-1.99, 1.00, 0.76, 0.00) are quoted without error bars or grid-convergence analysis. Since these figures are used as verifications of (33) and (55), please include resolution/uncertainty information or state explicitly that they are illustrative.
- [§7, Eq. (47)] State the Mathieu-equation convention used for a_0(q), since conventions for characteristic values differ and 'a' is also used for the background amplitude.
- [Conclusion] The phrase 'settled in every dimension except the two-dimensional endpoint' is confusing because Proposition 2 addresses the two-dimensional endpoint. Rephrase to distinguish the endpoint S^{2,\infty} estimate from the unproved endpoint Weyl law in (33).
Circularity Check
No circular derivation; the Birman-Schwinger criterion is a self-contained identity, while the cited analytic estimates are external and the numerical fits are post-hoc checks.
full rationale
The central claim, Prop. 1 / eq. (11), follows from the exact congruence M = H0^{1/2}(1+K_A)H0^{1/2} with K_A = H0^{-1/2} V_A H0^{-1/2}; this is an algebraic identity, not an assumption containing the conclusion, and the inertia/nullity transfer (12) is standard Sylvester-law form theory. No parameter is fitted and then called a prediction: the exponents in Figs. 1 and 3 (e.g., -1.99 vs -2, and 1.00/0.76/0.00 vs d(4-d)/4) are checked against independently derived formulas. The paper contains no self-citations by the author; all load-bearing analytic input (Cwikel, Birman-Solomyak, Frank-Lieb-Seiringer, Orland-Semenoff) is external. The genuine limitations are analytic, not circular: §4.1 step four asserts the periodic CBS transfer 'without change' for matrix-valued multipliers; Prop. 2 delegates the endpoint Yano-type summation to Solomyak [25,26] with the per-layer estimate (28) sketched rather than fully derived; and the paper itself disclaims a proved endpoint Weyl law for (33) ('not, at this stage, a proved endpoint Weyl law for coefficients of critical L^2 regularity') and labels (55) 'an entropy estimate, stated as an approximation and not as a theorem.' If those external estimates fail, Theorem 1 and Prop. 2 lose support, but that is a correctness/rigor risk about borrowed theorems, not a reduction of a prediction to its own input. The congruence criterion (11) is elementary and survives independently of those estimates. Score 1 reflects the mild structural oversell in the Conclusion of calling Prop. 2 'settled' when its proof is delegated, without any circularity.
Axiom & Free-Parameter Ledger
free parameters (2)
- fitted log-log slopes of the counting and volume laws =
−1.99 (d=2, Fig. 1); 1.00, 0.76, 0.00 (d=2,3,4, Fig. 3)
- per-mode threshold constant c in the entropy model (Section 8) =
unspecified (drops out of the leading exponent)
axioms (6)
- standard math Periodic matrix-valued Cwikel-Birman-Solomyak estimate: for d≥3, ∥M_fΛ∥_{S^{d,∞}} ≤ C_d∥f∥_{L^d}, given the ℓ^{d,∞} symbol condition {p: |p|^{−1} > t} = O(t^{−d})
- standard math L^{2,1} two-dimensional endpoint: M_fΛ ∈ S^{2,∞} with ∥M_fΛ∥_{S^{2,∞}} ≤ C∥f∥_{L^{2,1}} (Prop. 2), via off-endpoint Kato-Seiler-Simon bounds and Solomyak's Yano extrapolation
- standard math Frank-Lieb-Seiringer matrix-to-scalar reduction for weak Schatten ideals, with color factor r^{1/d} (eq. 20)
- domain assumption Principal-symbol semiclassical counting for the order-(−1) operator K_A (eqs. 31-33): negative eigenvalues counted by symbol phase-space volume
- domain assumption Exact Landau transversality of the background (q_iA_i(q)=0) is a spectral hypothesis making K_A self-adjoint
- domain assumption First Gribov region is the set of transverse backgrounds with positive-definite reduced FP operator; constant adjoint modes are removed as the kernel of H_0
read the original abstract
After removing the constant adjoint modes associated with global gauge rotations, we formulate the Landau-gauge Faddeev-Popov zero-mode problem in Birman-Schwinger form. The resulting normalized operator, is dimensionless and self-adjoint for transverse backgrounds, and the first Gribov horizon is identified with the appearance of the spectral value $-1$. Because this reduction is a congruence rather than a similarity transformation, it preserves the inertia and nullity relevant to horizon crossings without identifying the numerical spectra of the two operators. We study these analytic properties on a periodic domain, using matrix-valued Cwikel estimates to control its singular-value behavior at the regularity scale selected by the first-order Faddeev-Popov interaction, with separate attention to the two-dimensional endpoint. The same formulation expresses the fixed-background ghost Green function through the diagonal resolvent, providing a common framework in which the exact spectral condition can be compared with the Born expansion and Gribov's no-pole construction while remaining distinct from statements involving the functional average over gauge fields. As an explicit application, we consider a periodic transverse $SU(2)$ background for which the zero-mode equation reduces to a Mathieu-type recurrence and admits a systematic analysis through finite-channel and Feshbach reductions. This solvable sector also permits an examination of the volume dependence of horizon-touching configurations, without assigning them a statistical weight in the Yang-Mills measure.
Figures
Reference graph
Works this paper leans on
-
[1]
Quantization of non-Abelian gauge theories,
V. N. Gribov, “Quantization of non-Abelian gauge theories,” Nucl. Phys. B139, 1-19 (1978), doi:10.1016/0550-3213(78)90175-X
-
[2]
Local and renormalizable action from the Gribov horizon,
D. Zwanziger, “Local and renormalizable action from the Gribov horizon,” Nucl. Phys. B323, 513-544 (1989), doi:10.1016/0550-3213(89)90122-3
-
[3]
The Gribov problem and QCD dynamics,
N. Vandersickel and D. Zwanziger, “The Gribov problem and QCD dynamics,” Phys. Rep. 520, 175-251 (2012), arXiv:1202.1491, doi:10.1016/j.physrep.2012.07.003. 18
Pith/arXiv arXiv 2012
-
[4]
An all-order proof of the equivalence between Gribov’s no-pole and Zwanziger’s horizon conditions,
M. A. L. Capri, D. Dudal, M. S. Guimaraes, L. F. Palhares, and S. P. Sorella, “An all-order proof of the equivalence between Gribov’s no-pole and Zwanziger’s horizon conditions,” Phys. Lett. B719, 448-453 (2013), arXiv:1212.2419, doi:10.1016/j.physletb.2013.01.039
Pith/arXiv arXiv 2013
-
[5]
D. Dudal, R. F. Sobreiro, S. P. Sorella, and H. Verschelde, “The Gribov parameter and the dimension two gluon condensate in Euclidean Yang-Mills theories in the Landau gauge,” Phys. Rev. D72, 014016 (2005), arXiv:hep-th/0502183, doi:10.1103/PhysRevD.72.014016
Pith/arXiv arXiv 2005
-
[6]
Local and BRST-invariant Yang-Mills theory within the Gribov hori- zon,
M. A. L. Capriet al., “Local and BRST-invariant Yang-Mills theory within the Gribov hori- zon,” Phys. Rev. D94, 025035 (2016), arXiv:1605.02610, doi:10.1103/PhysRevD.94.025035
Pith/arXiv arXiv 2016
-
[7]
The universal character of Zwanziger’s horizon function in Euclidean Yang-Mills theories,
M. A. L. Capriet al., “The universal character of Zwanziger’s horizon function in Euclidean Yang-Mills theories,” Phys. Lett. B781, 48-54 (2018), arXiv:1802.04582, doi:10.1016/j.physletb.2018.03.058
Pith/arXiv arXiv 2018
-
[8]
B. W. Mintz, L. F. Palhares, G. Peruzzo, and S. P. Sorella, “Infrared massive gluon propagator from a BRST-invariant Gribov horizon in a family of covariant gauges,” Phys. Rev. D99, 034002 (2019), arXiv:1812.03166, doi:10.1103/PhysRevD.99.034002
Pith/arXiv arXiv 2019
-
[9]
The BRST- invariant vacuum state of the Gribov-Zwanziger theory,
D. Dudal, C. P. Felix, L. F. Palhares, F. Rondeau, and D. Vercauteren, “The BRST- invariant vacuum state of the Gribov-Zwanziger theory,” Eur. Phys. J. C79, 731 (2019), arXiv:1901.11264, doi:10.1140/epjc/s10052-019-7235-0
Pith/arXiv arXiv 2019
-
[10]
Toward background field indepen- dence within the Gribov horizon,
I. F. Justo, A. D. Pereira, and R. F. Sobreiro, “Toward background field indepen- dence within the Gribov horizon,” Phys. Rev. D106, 025015 (2022), arXiv:2206.04103, doi:10.1103/PhysRevD.106.025015
Pith/arXiv arXiv 2022
-
[11]
M. S. Guimaraes and S. P. Sorella, “A few remarks on the zero modes of the Faddeev-Popov operator in the Landau and maximal Abelian gauges,” J. Math. Phys.52, 092302 (2011), arXiv:1106.3944, doi:10.1063/1.3641892
Pith/arXiv arXiv 2011
-
[12]
On the zero modes of the Faddeev-Popov operator in the Landau gauge,
R. R. Landim, L. C. Q. Vilar, O. S. Ventura, and V. E. R. Lemes, “On the zero modes of the Faddeev-Popov operator in the Landau gauge,” J. Math. Phys.55, 022901 (2014), arXiv:1212.4098, doi:10.1063/1.4865424
Pith/arXiv arXiv 2014
-
[13]
Spectral properties of the Landau gauge Faddeev-Popov operator in lattice gluodynamics,
A. Sternbeck, E.-M. Ilgenfritz, and M. Müller-Preussker, “Spectral properties of the Landau gauge Faddeev-Popov operator in lattice gluodynamics,” Phys. Rev. D73, 014502 (2006), arXiv:hep-lat/0510109, doi:10.1103/PhysRevD.73.014502
Pith/arXiv arXiv 2006
-
[14]
Towards the in- frared limit in SU(3) Landau gauge lattice gluodynamics,
A. Sternbeck, E.-M. Ilgenfritz, M. Müller-Preussker, and A. Schiller, “Towards the in- frared limit in SU(3) Landau gauge lattice gluodynamics,” Phys. Rev. D72, 014507 (2005), arXiv:hep-lat/0506007, doi:10.1103/PhysRevD.72.014507
Pith/arXiv arXiv 2005
-
[15]
Faddeev-Popov spectra at the Gribov horizon,
J. Greensite, “Faddeev-Popov spectra at the Gribov horizon,” Phys. Rev. D81, 114011 (2010), arXiv:1001.0784, doi:10.1103/PhysRevD.81.114011
Pith/arXiv arXiv 2010
-
[16]
A. Cucchieri and T. Mendes, “Crossing the Gribov horizon: An unconventional study of geometric properties of gauge-configuration space in Landau gauge,” PoS(LATTICE2013)367 (2014), arXiv:1311.4699, doi:10.22323/1.187.0367
Pith/arXiv arXiv 2014
-
[17]
Bloch waves in minimal Landau gauge and the infinite- volume limit of lattice gauge theory,
A. Cucchieri and T. Mendes, “Bloch waves in minimal Landau gauge and the infinite- volume limit of lattice gauge theory,” Phys. Rev. Lett.118, 192002 (2017), arXiv:1612.01279, doi:10.1103/PhysRevLett.118.192002. 19
Pith/arXiv arXiv 2017
-
[18]
Deconfinement in pure gauge SU(3) Yang-Mills theory: The ghost propagator,
O. Oliveira, V. Paiva, and P. J. Silva, “Deconfinement in pure gauge SU(3) Yang-Mills theory: The ghost propagator,” EPJ Web Conf.274, 05008 (2022), arXiv:2301.01229, doi:10.1051/epjconf/202227405008
Pith/arXiv arXiv 2022
-
[19]
Mass generation in Landau- gauge Yang-Mills theory,
G. Eichmann, J. M. Pawlowski, and J. M. Silva, “Mass generation in Landau- gauge Yang-Mills theory,” Phys. Rev. D104, 114016 (2021), arXiv:2107.05352, doi:10.1103/PhysRevD.104.114016
Pith/arXiv arXiv 2021
-
[20]
On the spectrum of singular boundary-value problems,
M. Sh. Birman, “On the spectrum of singular boundary-value problems,” Mat. Sb. (N.S.) 55(97), no. 2, 125-174 (1961)
1961
-
[21]
On the bound states of a given potential,
J. Schwinger, “On the bound states of a given potential,” Proc. Natl. Acad. Sci. U.S.A.47, 122-129 (1961), doi:10.1073/pnas.47.1.122
-
[22]
Simon,Trace Ideals and Their Applications, 2nd ed
B. Simon,Trace Ideals and Their Applications, 2nd ed. (American Mathematical Society, Providence, RI, 2005), doi:10.1090/surv/120
doi:10.1090/surv/120 2005
-
[23]
The generalized Birman-Schwinger principle,
J. Behrndt, A. F. M. ter Elst, and F. Gesztesy, “The generalized Birman-Schwinger principle,” Trans. Am. Math. Soc.375, 799-845 (2022), arXiv:2005.01195, doi:10.1090/tran/8401
Pith/arXiv arXiv 2022
-
[24]
Weak type estimates for singular values and the number of bound states of Schrödinger operators,
M. Cwikel, “Weak type estimates for singular values and the number of bound states of Schrödinger operators,” Ann. Math.106, 93-100 (1977), doi:10.2307/1971160
-
[25]
M. Sh. Birman and M. Z. Solomyak, “Estimates for the number of negative eigenvalues of the Schrödinger operator and its generalizations,” inEstimates and Asymptotics for Discrete Spec- tra of Integral and Differential Equations, Adv. Soviet Math.7, 1-55 (American Mathematical Society, Providence, RI, 1991), doi:10.1090/advsov/007/01
-
[26]
Number of bound states of Schrödinger opera- tors with matrix-valued potentials,
R. L. Frank, E. H. Lieb, and R. Seiringer, “Number of bound states of Schrödinger opera- tors with matrix-valued potentials,” Lett. Math. Phys.82, 107-116 (2007), arXiv:0710.1877, doi:10.1007/s11005-007-0211-x
Pith/arXiv arXiv 2007
-
[27]
Mémoire sur le mouvement vibratoire d’une membrane de forme elliptique,
É. Mathieu, “Mémoire sur le mouvement vibratoire d’une membrane de forme elliptique,” J. Math. Pures Appl., 2e série,13, 137-203 (1868)
-
[28]
N. W. McLachlan,Theory and Application of Mathieu Functions(Oxford University Press, London, 1947)
1947
-
[29]
J. Meixner and F. W. Schäfke,Mathieusche Funktionen und Sphäroidfunktionen: Mit Anwendungen auf physikalische und technische Probleme(Springer-Verlag, Berlin, 1954), doi:10.1007/978-3-662-00941-3
-
[30]
Abramowitz and I
M. Abramowitz and I. A. Stegun (eds.),Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, NBS Applied Mathematics Series55(U.S. Government Printing Office, Washington, D.C., 1964)
1964
-
[31]
Infrared instability of the vacuum state of gauge theories and asymptotic freedom,
G. K. Savvidy, “Infrared instability of the vacuum state of gauge theories and asymptotic freedom,” Phys. Lett. B71, 133-134 (1977), doi:10.1016/0370-2693(77)90759-6
-
[32]
Vacuum polarization in uniform non-Abelian gauge fields,
L. S. Brown and W. I. Weisberger, “Vacuum polarization in uniform non-Abelian gauge fields,” Nucl. Phys. B157, 285-326 (1979), doi:10.1016/0550-3213(79)90508-X
-
[33]
The Metric on the Space of Yang-Mills Configurations
P. Orland, “The metric on the space of Yang-Mills configurations,” Report No. NBI-HE-96-35, arXiv:hep-th/9607134 (1996; revised 1997), doi:10.48550/arXiv.hep-th/9607134. 20
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.hep-th/9607134 1996
-
[34]
Extremal curves in(2 + 1)-dimensional Yang-Mills theory,
P. Orland and G. W. Semenoff, “Extremal curves in(2 + 1)-dimensional Yang-Mills theory,” Nucl. Phys. B576, 627-654 (2000), arXiv:hep-th/9912009, doi:10.1016/S0550-3213(00)00134- 6
Pith/arXiv arXiv 2000
-
[35]
Gauge-invariant coordinates on gauge-theory orbit space,
P. Orland, “Gauge-invariant coordinates on gauge-theory orbit space,” Phys. Rev. D70, 045014 (2004), arXiv:hep-th/0402003, doi:10.1103/PhysRevD.70.045014. 21
Pith/arXiv arXiv 2004
discussion (0)
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