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Photonic Altermagnets: Magnetic Symmetries in Photonic Structures

T0 review · 4 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash

Pith's one-line read The paper establishes a photonic analogue of altermagnetism: a 2D photonic crystal with alternating-handed elliptical chiral cylinders shows helicity-split bands and helicity-dependent refraction.

desk verdict New photonic analogue of altermagnetism with a concrete design and convincing numerics, but the pseudo-time-reversal symmetry that protects the effect is asserted rather than derived in the main text. read the letter →

arxiv 2506.23497 v1 pith:7KW3PHFB submitted 2025-06-30 physics.optics cond-mat.mtrl-sci

classification physics.opticscond-mat.mtrl-sci
keywords altermagnetismphotoniccrystalhelicitysplittingpseudo-time-reversalsymmetrychiralityspin-splitbandsspinsplitterisofrequencycontour
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 argues that the symmetry structure of altermagnets—magnets with zero net magnetization but spin-split electronic bands—can be transplanted into a two-dimensional photonic crystal, giving light analogues of spin-split bands. It does this by pairing a pseudo-time-reversal operator, built from parity, bosonic time reversal, and duality, with an arrangement of alternating-handed elliptical chiral cylinders rotated by 90 degrees. In the computed band structure, left- and right-circularly polarized photons form Kramers pairs that split everywhere except along symmetry directions, where four nodal points survive in the isofrequency contours. These split bands produce a photonic spin-splitter effect: a linearly polarized beam entering along a nodal direction refracts into two beams of opposite helicity, and in some propagation windows only one helicity is transmitted. If the framework holds, photonic crystals become a testbed for altermagnetic phenomena and a route to circular-polarization routing that does not rely on spin-orbit-like geometric phases.

What carries the argument

The load-bearing object is the pseudo-time-reversal symmetry operator Tp = P Tb D, the product of parity P, bosonic time reversal Tb, and duality exchange D, defined from the symmetries of free-space Maxwell equations. It is the one operator that converts an arbitrary polarization into its orthogonal counterpart and simultaneously flips the handedness of chiral media, making it the photonic analogue of fermionic time reversal. The other essential element is the crystal design: alternating-handed chiral cylinders shaped as ellipses with one handedness rotated by C4, so that applying the pTR operator is equivalent to a 90-degree rotation—exactly the spin-space-group condition of d-wave altermagnets. This machinery produces helicity-split bands with four nodal points in the isofrequency contours and dictates the helicity-dependent group velocities behind PSSE and photonic spin filtering.

What would settle it

Measure or compute the helicity-resolved isofrequency contours of the proposed crystal at normalized frequency ωa0/2πc = 0.62; if the four nodal points along ΓX and ΓY do not appear, or if the two helicity bands become degenerate when the permittivity-to-permeability ratio is slightly detuned from the duality condition, the claimed symmetry protection is not exact.

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Extended reading notes

Core claim

The central claim is that a photonic analogue of altermagnetism is realized by a 2D photonic crystal whose unit cell contains chiral cylinders of opposite handedness, made elliptical and related by a C4 rotation. Under the pseudo-time-reversal operator Tp = P Tb D, which flips both photon helicity and the handedness of chiral objects, the structure behaves like a d-wave altermagnet: the two sublattices cannot be superimposed by translation alone, so the Kramers degeneracy that pairs orthogonal polarization states is lifted. Numerical band calculations show helicity-polarized bands with broken degeneracy along the ΓM/M′ intervals and four symmetry-protected nodal points in the isofrequency contours. The resulting transport, termed the photonic spin-splitter effect (PSSE), deflects opposite helicities in opposite directions without any Berry curvature, and the same crystal acts as a lossless spin filter and helicity-preserving mirror in other propagation directions. The authors conclude these effects follow from broken Kramers degeneracy, not from strong spin-orbit interaction.

Load-bearing premise

The argument rests on Tp = P Tb D remaining an exact symmetry of the periodic chiral crystal, so that flipping the handedness of the chiral inclusions behaves exactly like reversing electron spin in an altermagnet.

Editorial extensions

If this is right

  • A linearly polarized beam incident along the ΓX or ΓY nodal directions splits into two beams of opposite helicity that refract at different angles, an effect the paper calls the photonic spin-splitter effect (PSSE).
  • In propagation windows where only one helicity band exists, the crystal transmits that helicity and reflects the other while preserving its handedness, making it a lossless photonic spin filter and helicity-preserving mirror.
  • The transmitted helicity and the splitting direction can be switched by rotating the propagation direction, because the helicity-selective response is tied to the anisotropic band structure.
  • Because the band splitting survives for small chirality parameters at higher frequencies and the structure is uniaxial, the effect works for in-plane propagation and is compatible with integrated photonic circuitry.
  • The framework reproduces not only altermagnetic bands but also the antiferromagnetic and ferromagnetic photonic limits by arranging uniform or alternating handedness, so the same symmetry language covers all collinear magnetic phases.

Reading between the lines

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

  • Not developed in the paper: other rotation patterns of chiral inclusions should realize even-parity pseudochirality analogues beyond the d-wave case, which the paper only names as a future direction.
  • Because photonic crystals are fabricated lithographically, the domain structure of the alternating-chirality pattern can be engineered and switched in ways solid-state altermagnet domains currently resist; one could test this by patterning adjacent domains rotated by 90 degrees and observing helicity routing at the domain wall.
  • The Tp operator's dependence on duality suggests a testable design rule: maintaining a constant permittivity-to-permeability ratio across the unit cell may be necessary for exact Kramers protection, so deviations should reintroduce degeneracy in a measurable way.
  • A direct application not developed in the paper is polarization-division multiplexing: the nodal-direction splitting could separate two circular channels in an integrated photonic circuit, and the helicity-preserving reflection could serve as a circular-polarization filter in backscattering isolation.
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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

4 major / 6 minor

Summary. The manuscript proposes a photonic analogue of altermagnetism in a two-dimensional photonic crystal made of alternating-handedness elliptical chiral cylinders arranged with C4 rotations. The authors introduce a pseudo-time-reversal operator Tp = P Tb D, argue that it pairs orthogonal polarization states into Kramers doublets, and present numerical band structures showing helicity-split bands with four nodal points in the isofrequency contours. They further report full-wave beam simulations of a photonic spin-splitter effect (PSSE) and photonic spin filtering (PSF), interpreting these phenomena as arising from broken Kramers degeneracy rather than from spin-orbit interaction. The main text places the analytic derivations (effective-medium equivalence, symmetry proofs, and the Berry-curvature calculation) in the Supplemental Material, which was not part of the reviewed manuscript.

Significance. If the symmetry arguments hold, this is a valuable conceptual bridge between altermagnetism and photonics, with concrete and falsifiable predictions: helicity-split bands, symmetry-protected nodal points, PSSE, and PSF. The numerical portion is transparent: the band structures and beam simulations solve Maxwell's equations directly for the stated parameters (epsilon_r = mu_r = 2, kappa = ±1.5, alpha = 1.3, D/a0 = 3√2/10) with no fitting to a target, and the parameter choices are explicitly justified by duality requirements. The paper also carefully distinguishes the proposed mechanism from geometric-phase-based helicity splitting, which is an important conceptual point. However, the central symmetry argument is asserted rather than proved in the main text, and the supporting derivations are not available for review; the current manuscript is therefore not yet verifiable as a theoretical framework.

major comments (4)
  1. [Main text, p. 4–5] The paper states that 'the chirality-switching operator is none other than Tp' and that 'the pTR pair of the constructed PhC can be obtained through simple C4 rotation,' but it never exhibits the action of Tp = P Tb D on the periodic constitutive profiles (epsilon_r, mu_r, kappa) of the engineered unit cell with elliptical chiral cylinders (alpha = 1.3, alternating kappa = ±1.5). Because Tp contains spatial inversion, invariance of the full Maxwell operator under Tp requires a precise compatibility among the ellipse orientations, the cylinder positions, and the chirality signs; the main text gives no argument that this compatibility holds at finite frequency. If Tp is only an approximate symmetry, the four nodal points in Fig. 2c are not symmetry-protected, and the interpretation of PSSE as broken Kramers degeneracy (p. 8) loses its basis. Please provide the explicit transformation of the unit cell under Tp and a numerical test (e.g., perturbing alpha or kappa) showing that the degeneracies persist as exact crossings rather than anticrossings.
  2. [Main text, p. 5 and p. 8] The equivalence between the altermagnetic photonic crystal and an effectively homogeneous pseudochiral medium, as well as the 'detailed theoretical proofs of altermagnetic Kramers degeneracy lifting in both effective material and photonic crystal limits,' are placed entirely in Supplemental Material Secs. I–III. The Supplemental Material was not included in the reviewed manuscript, so the central derivation of the framework cannot be checked. Since this equivalence is the bridge from the heuristic analogy to the quantitative band-structure and transport predictions, the SM (or an equivalent main-text derivation) is essential for evaluation; please supply it in full for review.
  3. [Main text, p. 6 and Fig. 2b] The bands are labeled as helicity-polarized, but for a periodic medium with position-dependent chirality parameter kappa(r), the helicity operator does not in general commute with the Maxwell operator. The manuscript should specify the operator used to assign helicity to each Bloch mode and justify that the labels are meaningful away from high-symmetry directions; without this, the 'helicity-split bands' could be an artifact of a projection choice rather than an intrinsic symmetry property.
  4. [Main text, p. 8] The claim that 'the Berry curvature does not exist in the altermagnetic PhC' and hence that PSSE is unrelated to spin-orbit interaction is delegated to Supplemental Material Sec. VII, with no calculation in the main text. Because this claim is what distinguishes PSSE from geometric-phase-based helicity splitting, it is load-bearing. Please include the Berry-curvature calculation or a precise symmetry argument in the main text or in the reviewed SM.
minor comments (6)
  1. [Abstract and p. 8] The phrases 'geometrodynamic spin-orbit interaction' and 'unprecedent' appear to be typos for 'geometric-phase' and 'unprecedented'; please correct them.
  2. [Fig. 2a] The field maps are described as normalized but no color scale is provided; adding a colorbar would make the field-localization claim quantitatively verifiable.
  3. [Fig. 2c] The four nodal points are not explicitly marked; please mark them and state the wavevectors at which they occur.
  4. [Main text, p. 7] The sentence 'the symmetry also impels helicity-degeneracy within the ΓM(M′) interval' is unclear; 'impels' should likely be 'implies,' and the notation ΓM(M′) should be defined.
  5. [Main text, p. 8 and Fig. 3b] The statement that the helicity of the reflected light is opposite to the transmitted light, followed by the sentence 'the reflected beam retains the same handedness as the rejected portion of the incident light,' is confusing; please clarify the helicity bookkeeping in the PSF simulation.
  6. [Data Availability] The statement that all study data are available in the main text or the Supplemental Material is difficult to verify because the Supplemental Material was not included in the reviewed text; please make the SM available.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the photonic-altermagnet construction is verified by independent full-wave Maxwell simulations; the symmetry mapping is an analogy, not a fitted input.

full rationale

The paper's central claim is that a photonic crystal with alternating chiral elliptic cylinders obeying C4-related sublattice symmetry exhibits helicity-split bands and the photonic spin-splitter effect. The derivation chain is not circular: the band structures and transport simulations are obtained by directly solving Maxwell's equations in the engineered structure, with material parameters (kappa = ±1.5, alpha = 1.3) chosen from symmetry considerations rather than fitted to the predicted splittings. The pseudo-time-reversal operator Tp = P Tb D is defined from free-space symmetries, and its action on chiral objects follows from the known transformation of the chirality pseudoscalar under parity; it is not defined in terms of the band structure it is used to explain. Citations to prior work (He et al., Bliokh et al., etc.) are external and serve as background, not as the load-bearing proof; the symmetry-protected degeneracies and nodal points are computed, not imported by self-citation. The comparison with pseudochiral media is presented as a validation of the effective-medium limit, not as the source of the simulated photonic-crystal results. Any concern that the structure was deliberately built to satisfy the symmetry, making the outcome expected, is standard constructive verification rather than circularity; at most, the identification of Tp with the chirality-switching operator is definitional in the sense that Tp contains parity, but this does not reduce the numerical predictions to the input.

Assumptions & free parameters 5 free parameters · 4 assumptions · 0 invented entities

The central claim rests on Maxwell equations solved numerically, so no fitted parameters are needed to make the phenomenon appear. The chosen parameters (material constants, geometry, frequency) are illustrative demonstration values, not fitted to match a target. The key conceptual assumptions are the validity of the pseudo-time-reversal operator Tp inside the photonic crystal and the transferability of spin-space-group classification from electronic to photonic systems.

free parameters (5)
  • Relative permittivity/permeability of chiral cylinders (epsilon_r = mu_r) = 2
    Chosen to maintain duality symmetry (eta_r = 1). Not fitted to data; an example input.
  • Chirality parameter magnitude |kappa| = 1.5
    Chosen to make helicity splitting visible in the band structure; not fitted to a target.
  • Cylinder diameter to lattice constant ratio (D/a0) = 3*sqrt(2)/10 ~ 0.424
    Geometric parameter set for the numerical simulation; not fitted.
  • Ellipticity/asymmetry parameter alpha = 1.3
    Introduces the in-plane anisotropy needed for the altermagnetic symmetry; chosen as a demonstration value.
  • Normalized frequency (omega*a0/(2*pi*c)) = 0.62 (also 0.24)
    Sample frequencies used to demonstrate the effects; not fitted.
assumptions (4)
  • standard math Maxwell's equations with bianisotropic constitutive relations apply to the photonic crystal.
    All calculations are based on classical electrodynamics; this is the foundational model.
  • domain assumption Chiral metamaterials with the specified permittivity, permeability, and chirality parameter can be fabricated or modeled.
    The design requires cylinders with epsilon = mu = 2 and chirality magnitude 1.5; experimental realization is not demonstrated in this paper.
  • domain assumption The pseudo-time-reversal operator Tp = P Tb D remains a valid symmetry for the engineered photonic crystal, and duality symmetry holds at each point.
    The Kramers pair construction and the altermagnetic classification rely on this correspondence (main text, section on Kramers doublets).
  • ad hoc to paper Spin-space-group classification for electrons can be transplanted to photonic band structures via the analogy between spin and helicity.
    This is the paper's central mapping; it is an assumption because the spin space group formalism (ref [7]) is developed for electronic systems, not Maxwell modes.

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Cite this review

Pith. "Pith review of Photonic Altermagnets: Magnetic Symmetries in Photonic Structures." pith.science (2026). https://pith.science/paper/7KW3PHFB

@misc{pith2026250623497,
  author       = {Pith},
  title        = {Pith review of: Photonic Altermagnets: Magnetic Symmetries in Photonic Structures},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7KW3PHFB}},
  note         = {Machine review of arXiv:2506.23497}
}
read the original abstract

The unique physical properties of altermagnets, when transplanted to photonic systems, are anticipated to offer a new degree of freedom for engineering electromagnetic waves. Here, we show that a photonic analogue of altermagnetism can be mimicked in photonic crystals, where engineered photonic crystals can host spin space group symmetries. Our approach allows for the creation of spin-split bands and the corresponding transport properties provide an effective platform for circularly polarized light isolation without the need of geometrodynamic spin-orbit interaction. Beyond the concurrent solid-state materials, we anticipate our work to offer photonic crystals as a versatile platform to test the spin-split band properties and inspire optical designs for photospintronic applications.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Spin Splitter without Spin-Split Bands: A Reconfigurable Altermagnetic Texture

    cond-mat.str-el 2026-08 conditional novelty 8.0 of 10

    A counter-spiral magnetic texture generates a pure transverse spin current without spin-split bands, with polarization fixed by a helicity mirror and rotatable in 120-degree steps.

Reference graph

Works this paper leans on

46 extracted references · 45 canonical work pages · cited by 1 Pith paper

  1. [1]

    Šmejkal, R

    L. Šmejkal, R. González-Hernández, T. Jungwirth, and J. Sinova, Sci. Adv. 6, eaaz8809 (2020)

  2. [2]

    González-Hernández, L

    R. González-Hernández, L. Šmejkal, K. Výborný, Y. Yahagi, J. Sinova, T. Jungwirth, and J. Železný, Phys. Rev. Lett. 126, 127701 (2021)

  3. [3]

    I. I. Mazin, K. Koepernik, M. D. Johannes, R. González-Hernández, and L. Šmejkal, Proc. Natl. Acad. Sci. U. S. A. 118, e2108924118 (2021)

  4. [4]

    K.-H. Ahn, A. Hariki, K.-W. Lee, and J. Kuneš, Phys. Rev. B Condens. Matter 99, 184432 (2019)

  5. [5]

    Hayami, Y

    S. Hayami, Y. Yanagi, and H. Kusunose, J. Phys. Soc. Jpn. 88, 123702 (2019)

  6. [6]

    Šmejkal, J

    L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X. 12, 040501 (2022)

  7. [7]

    Šmejkal, J

    L. Šmejkal, J. Sinova, and T. Jungwirth, Phys. Rev. X. 12, 031042 (2022)

  8. [8]

    Kampfrath et al., Nature Photonics 5, 31 (2011)

    T. Kampfrath et al., Nature Photonics 5, 31 (2011)

Show all 46 references
  1. [9]

    Jungwirth, X

    T. Jungwirth, X. Marti, P. Wadley, and J. Wunderlich, Nature Nanotechnology 11, 231 (2016)

  2. [10]

    Železný, P

    J. Železný, P. Wadley, K. Olejník, A. Hoffmann, and H. Ohno, Nature Physics 14, 220 (2018)

  3. [11]

    J. R. Hortensius, D. Afanasiev, M. Matthiesen, R. Leenders, R. Citro, A. V. Kimel, R. V. Mikhaylovskiy, B. A. Ivanov, and A. D. Caviglia, Nature Physics 17, 1001 (2021)

  4. [12]

    Šmejkal, A

    L. Šmejkal, A. B. Hellenes, R. González-Hernández, J. Sinova, and T. Jungwirth, Phys. Rev. X. 12, 011028 (2022)

  5. [13]

    Han et al., Sci

    L. Han et al., Sci. Adv. 10, eadn0479 (2024)

  6. [14]

    Lee et al., Phys

    S. Lee et al., Phys. Rev. Lett. 132, 036702 (2024)

  7. [15]

    Krempaský et al., Nature 626, 517 (2024)

    J. Krempaský et al., Nature 626, 517 (2024)

  8. [16]

    Zhu et al., Nature 626, 523 (2024)

    Y.-P. Zhu et al., Nature 626, 523 (2024)

  9. [17]

    Reimers et al., Nature Communications 15, 2116 (2024)

    S. Reimers et al., Nature Communications 15, 2116 (2024)

  10. [18]

    Osumi, S

    T. Osumi, S. Souma, T. Aoyama, K. Yamauchi, A. Honma, K. Nakayama, T. Takahashi, K. Ohgushi, and T. Sato, Phys. Rev. B Condens. Matter 109, 115102 (2024)

  11. [19]

    Yablonovitch, Phys

    E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987)

  12. [20]

    John, Phys

    S. John, Phys. Rev. Lett. 58, 2486 (1987)

  13. [21]

    J. D. Joannopoulos, S. G. Johnson, J. N. Winn, and R. D. Meade, Molding the Flow of Light (Princeton Univ. Press, 2008)

  14. [22]

    S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and H. A. Haus, Opt. Express 3, 4 (1998)

  15. [23]

    S. G. Johnson, P. R. Villeneuve, S. Fan, and J. D. Joannopoulos, Phys. Rev. B Condens. Matter 62, 8212 (2000)

  16. [24]

    Vučković, M

    J. Vučković, M. Lončar, H. Mabuchi, and A. Scherer, Phys. Rev. E 65, 016608 (2001)

  17. [25]

    Park, S.-H

    H.-G. Park, S.-H. Kim, S.-H. Kwon, Y.-G. Ju, J.-K. Yang, J.-H. Baek, S.-B. Kim, and Y.-H. Lee, Science 305, 1444 (2004)

  18. [26]

    Y. A. Vlasov, M. O'Boyle, H. F. Hamann, and S. J. McNab, Nature 438, 65 (2005)

  19. [27]

    L. Lu, Z. Wang, D. Ye, L. Ran, L. Fu, J. D. Joannopoulos, and M. Soljačić, Science 349, 622 (2015)

  20. [28]

    Z. Wang, Y. Chong, J. D. Joannopoulos, and M. Soljačić, Nature 461, 772 (2009)

  21. [29]

    A. B. Khanikaev, S. Hossein Mousavi, W.-K. Tse, M. Kargarian, A. H. MacDonald, and G. Shvets, Nat. Mater. 12, 233 (2013). 13

  22. [30]

    Dong, X.-D

    J.-W. Dong, X.-D. Chen, H. Zhu, Y. Wang, and X. Zhang, Nat. Mater. 16, 298 (2017)

  23. [31]

    E. Plum, V. A. Fedotov, and N. I. Zheludev, Appl. Phys. Lett. 93 (2008)

  24. [32]

    E. Plum, X. X. Liu, V. A. Fedotov, Y. Chen, D. P. Tsai, and N. I. Zheludev, Phys. Rev. Lett. 102, 113902 (2009)

  25. [33]

    He, X.-C

    C. He, X.-C. Sun, X.-P. Liu, M.-H. Lu, Y. Chen, L. Feng, and Y.-F. Chen, Proc. Natl. Acad. Sci. U. S. A. 113, 4924 (2016)

  26. [34]

    Zhang et al., Sci

    Y. Zhang et al., Sci. Adv. 8, eabq8246 (2022)

  27. [35]

    Zhang, J

    Y. Zhang, J. C. Arias-Muñoz, X. Cui, and Z. Sun, Appl. Phys. Lett. 123, 24 (2023)

  28. [36]

    V. K. Valev, J. J. Baumberg, C. Sibilia, and T. Verbiest, Advanced Materials 25, 2517 (2013)

  29. [37]

    K. Y. Bliokh, A. Y. Bekshaev, and F. Nori, New J. Phys. 15, 033026 (2013)

  30. [38]

    Alpeggiani, K

    F. Alpeggiani, K. Y. Bliokh, F. Nori, and L. Kuipers, Phys. Rev. Lett. 120, 243605 (2018)

  31. [39]

    Serdyukov, I

    A. Serdyukov, I. Semchenko, S. Tretyakov, and A. Sihvola, Electromagnetics of Bi- anisotropic Materials: Theory and Applications (Gordon and Breach Science Publishers, 2001)

  32. [40]

    W. J. Padilla, Opt. Express 15, 1639 (2007)

  33. [41]

    V. S. Asadchy, A. Dí az-Rubio, and S. A. Tretyakov, Nanophotonics 7, 1069 (2018)

  34. [42]

    Bialynicki-Birula and Z

    I. Bialynicki-Birula and Z. Bialynicka-Birula, J. Phys. A Math. Theor. 46, 053001 (2013)

  35. [43]

    Białynicki-Birula, Acta Phys

    I. Białynicki-Birula, Acta Phys. Pol, A 86, 97 (1994)

  36. [44]

    Feng et al., Nat

    Z. Feng et al., Nat. Electron. 5, 735 (2022)

  37. [45]

    Q. Guo, W. Gao, J. Chen, Y. Liu, and S. Zhang, Phys. Rev. Lett. 115, 067402 (2015)

  38. [46]

    A. J. Schofield, Physics 2, 93 (2009). 14 Fig. 1. Illustration of the corresponding entities between condensed matter and photonic systems. (a) The spin quantum numbers relate the electron spins to the photon helicity, where positive and negative spin numbers are color -coded ...

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