REVIEW 6 major objections 4 minor 29 references
Extensions and Applications of Bredon's Trick in Geometric and Topological Contexts
T0 review · 6 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Bredon's trick turns locality, gluing, and additivity on a paracompact space into a global theorem — applied here to Ricci flow singularity type, stratified-space cohomology, and mapper stability.
desk verdict The advertised applications are not established: the central Lemma 2.1 is false as stated, the cohomology additivity check is wrong, and the two headline theorems are close to tautological; only the classical review portions have value. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central device is the proper-function decomposition behind Bredon's trick: Lemma 2.1 claims every paracompact Hausdorff space admits a proper continuous function to $[0,\infty)$, so the space can be sliced into compact annuli, each covered by finitely many open sets from the given cover, and then rebuilt by gluing parities. The three axioms — local, gluing, additivity — are the machine's inputs, and the five lemma (for cohomology) or interleaving distances (for persistence) supply the gluing at the level of invariants.
What would settle it
Take $X$ to be an uncountable set with the discrete topology, which is paracompact Hausdorff. Any continuous $f\colon X \to [0,\infty)$ is proper only if $f^{-1}([0,n])$ is compact, hence finite, for each $n$; since $X$ is uncountable, some preimage must be infinite, so no proper function exists. This directly falsifies Lemma 2.1 and shows the proof of the trick cannot start for such spaces. A second check: ordinary cohomology of an infinite disjoint union of nonempty open sets is a direct product, not a direct sum, so the additivity axiom as stated is not satisfied by the properties used in the De Rham applications.
Extended reading notes
Core claim
Bredon's trick (Theorem 2.2) asserts that a property $P$ satisfying (i) locality on an open cover closed under finite intersections, (ii) gluing: $P(U)$, $P(V)$, $P(U \cap V)$ imply $P(U \cup V)$, and (iii) additivity over disjoint open families, holds for every paracompact space $X$. The proof decomposes $X$ using a proper function $f\colon X \to [0,\infty)$ into compact annuli $A_n = f^{-1}([n,n+1])$, covers each by finitely many sets from the cover, and then reassembles $X$ as a union of two disjoint unions of parities. The paper claims, among other things, that this yields a global Type-I bound $|\mathrm{Rm}|(x,t) \le C/(T-t)$ for 3-dimensional Ricci flow (Theorem 4.4), stability of multiscale mapper with error $\epsilon_X = K(\delta)\cdot\sup_\alpha \epsilon_{U_\alpha}$ (Theorem 4.1), and the Verona/De Rham isomorphism on stratified pseudomanifolds (Theorem 3.5). It also frames the trick as a sheaf-theoretic softness criterion (Theorem 6.1).
Load-bearing premise
The proof of Bredon's trick assumes (Lemma 2.1) that every paracompact Hausdorff space admits a proper real-valued function, which is false for paracompact spaces such as an uncountable discrete set; without such a function the annulus decomposition cannot be built.
Editorial extensions
If this is right
- If Theorem 4.4 is correct, a 3-dimensional Ricci flow is Type-I whenever its local curvature times the remaining time is bounded on a good cover, so the singularity type (spherical, neckpinch, degenerate) is read from a global constant.
- If Theorem 4.1 is correct, distributed persistent homology computations can be stitched together with a controlled stability constant, resolving the cover-sensitivity problem for multiscale mapper.
- If Theorem 3.5 and Theorem 4.2 are correct, Hodge-theoretic extension of harmonic forms holds on stratified pseudomanifolds, giving a Verona/De Rham isomorphism on singular spaces with conical links.
- If Theorem 6.1 is correct, Bredon's trick is equivalent to softness and fineness of sheaves, which would recast paracompactness as a local-to-global condition for acyclic sheaves.
- If the trick is as general as claimed, any future property satisfying its three axioms has a ready-made global theorem, and Algorithm 6.1 provides a mechanical verification protocol.
Reading between the lines
- The proof implicitly requires the space to admit a proper real-valued function, i.e. to be σ-compact in a suitable sense; this suggests the theorems, as stated for arbitrary paracompact Hausdorff spaces, may really hold for the σ-compact subclass unless additivity and gluing are strengthened.
- The additivity axiom is used for infinite disjoint unions that pass to direct sums, but ordinary cohomology of a disjoint union is a direct product, so the conditions likely need a locally finite or compact-support formulation to apply to De Rham and singular cohomology on non-compact manifolds.
- If the trick's scope is indeed this broad, it offers a template for proving stability of other multiscale summaries in topological data analysis, such as persistence landscapes or Euler characteristic curves, once local stability and gluing are verified.
- The Ricci flow application, if it holds, would let one certify Type-I blow-up from local curvature data sampled on a finite cover, a numerically checkable criterion in simulations.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents Bredon's trick as a local-to-global extension principle, states a general theorem (Theorem 2.2) for paracompact spaces, and then applies the principle to de Rham, Künneth, invariant, and basic cohomology, to Verona cohomology of stratified pseudomanifolds, to stability of multiscale mapper and persistent homology, and to Ricci flow singularity classification. It also contains worked examples and a sheaf-theoretic reformulation. The exposition is broad in scope, but the central lemma on which the general theorem rests is false, the proof of the theorem is invalid as written, and several of the new applications are either circular, tautological, or based on incorrect additivity statements.
Significance. If the claims were correct, the paper would offer a useful unified framework for local-to-global arguments across differential geometry and applied topology. The explicit statement of the three Bredon conditions and the collection of examples from different areas are organizational strengths, and the paper is clearly written in places. However, the load-bearing Lemma 2.1 is false, the proof of Theorem 2.2 does not work, and the main novel results in Section 4 do not provide the advertised content. The manuscript is therefore not reliable in its current form, and I do not see a local repair that would preserve the stated scope.
major comments (6)
- [Section 2, Lemma 2.1] The assertion that every paracompact Hausdorff space admits a proper function to [0,∞) is false. An uncountable set with the discrete topology is paracompact Hausdorff, but every compact subset is finite; a proper function f would force X to be the countable union of the finite sets f^{-1}([0,n]), hence countable. Since Lemma 2.1 is the mechanism by which the proof of Theorem 2.2 decomposes X into compact annuli, Theorem 2.2 is unproved as stated for general paracompact spaces.
- [Section 2, proof of Theorem 2.2] Even if a proper function existed, the proof applies the property P to the sets A_n = f^{-1}([n,n+1]), which are compact but not necessarily open, while the Bredon conditions are only stated for open subsets of X. The sentence 'as A_n is the finite union of open U_n' does not make A_n open, and the later claims that U and V are disjoint unions and that U∩V is a union of disjoint open sets are not correct: neighbouring annuli meet along f^{-1}(n), and those intersections are not open. Thus the proof of the central theorem is invalid on multiple grounds.
- [Section 3, Theorem 3.1 and subsequent cohomology proofs] The verification of condition (iii) in Theorem 3.1 uses the displayed isomorphism Hom(⊕_α Ṡ_q(U_α), R) ≅ ⊕_α Hom(Ṡ_q(U_α), R), which is false for infinite index sets. The dual of a direct sum is a direct product, and for ordinary cohomology H^k(⊔ U_α) is the product ∏ H^k(U_α), not the direct sum. The same incorrect infinite additivity is used in the proofs of Theorems 3.2 and 3.3 and in several examples, so the axiomatic checks in Section 3 are not valid as written.
- [Section 3, Theorem 3.4] The claimed isomorphism between the basic cohomology of a Riemannian foliation and the de Rham cohomology of the leaf space is false in general. For foliations whose leaf space is not a manifold, there is no usual de Rham cohomology of the quotient to compare with, and the flow-box argument H^k_bas(U) ≅ H^k_DR(T) does not supply the global gluing data. Standard examples such as dense linear foliations on the torus show that basic cohomology is not computed by the leaf-space quotient. The theorem is not established by the sketch given.
- [Section 4, Theorems 4.1 and 4.4] The two main new applications are not genuine local-to-global conclusions. In Theorem 4.1, the property P(V) is defined as the desired stability statement, so verifying P on a cover and invoking Bredon's trick amounts to assuming the conclusion; the proof of the gluing condition invokes a five-lemma for persistence modules and the nerve lemma without deriving the interleaving parameter K(δ). In Theorem 4.4, P(U) is defined as the local Type-I bound |Rm|(T-t) ≤ C_U, so the proof that P(M) holds with C_M = max C_U is a restatement of the local assumptions; the subsequent singularity classification is likewise a definitional consequence if P(M) holds. These theorems do not provide independent support for the framework.
- [Section 3, Example 2 versus Theorem 3.5] Example 2 contradicts Theorem 3.5. Theorem 3.5 states that the Verona cohomology of a stratified pseudomanifold X is isomorphic to the de Rham cohomology of its unfolding M, and for X = Cone(T²) the unfolding is T² × [0,1], whose de Rham cohomology has H¹ ≅ R² and H² ≅ R. Example 2 instead concludes H^k_v(X) ≅ H^k_DR(X), with X contractible and only H⁰ ≅ R. This internal inconsistency needs to be resolved before the Verona cohomology applications can be assessed.
minor comments (4)
- [Page 1 and page 7] There are typographical errors: 'pseudo-manidfolds' on page 1 should be 'pseudomanifolds', and 'thar' in the proof of Theorem 3.3 should be 'that'.
- [Section 3, proof of Theorem 3.2] The proof refers to 'theorem 3.3' when it should refer to Theorem 3.2.
- [Section 7, Conclusion] The conclusion cites 'Theorem 5.1', 'Theorems 4.4, 5.2', and 'Theorem 5.4', but the corresponding theorem labels in the body are Theorem 4.1, Theorem 4.2, and Theorem 4.4; the numbering should be made consistent.
- [Section 5, Algorithmic Implementation] The algorithm is labelled 'Algorithm 6.1' even though it appears in Section 5, before Section 6; it should be renumbered.
Circularity Check
The stability and Ricci-flow theorems are built by defining the property P so that the local hypothesis already contains the global conclusion; the base Bredon decomposition rests on a false lemma sourced to the author's own thesis.
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self definitional
[Theorem 4.1, Section 4.1]
"Define for open V ⊂ X: P (V ) := “∃ persistence isomorphism ϕ : Hk(M(V )) → Hk(M(X)|V and ∥ϕ∥∞ ≤ ϵ”. If P (Uα) holds for all Uα ∈ U, then P (X) holds with ϵX = K(δ) · supα ϵUα . ... (i) Local triviality : For Uα convex, the complex M(Uα) is contractive. The inclusion ι : M(Uα) ,→ M(X) induces isomorphism in homology, thenP (Uα) with ϵ = 0 (Nerve Lemma [11])."
P(V) is defined using H_k(M(X)|_V), i.e. the global mapper complex restricted to V. Thus the local hypothesis P(Uα) already asserts agreement between local and global persistence homology on each cover element. The verification of condition (i) simply asserts that the inclusion M(Uα) → M(X) induces a homology isomorphism, citing the Nerve Lemma, which only compares a nerve with the underlying space and does not identify M(Uα)-homology with the restriction of M(X)-homology. The theorem therefore assumes, in the form of its local condition, exactly the global stability it claims to establish.
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self definitional
[Theorem 4.4 and its proof, Section 4.3]
"For open U ⊂ M , define: P (U ) := ”∃CU = C(inj(U, g0), vol(U, g0), diam(U, g0)) such that sup U ×[0,T ) |Rm| ·(T − t) ≤ CU ” If P (Uα) holds for a good cover {Uα} of M , then P (M ) holds and the singularity is of Type-I. ... The constant CU ∪V = max{CU , CV , CU ∩V } preserves the Type-I bound under finite unions. ... For disconnected components {Ui}, the bound is CM = maxi CUi ."
The conclusion P(M) is literally the same inequality sup_{M×[0,T)} |Rm|(T−t) < ∞ that defines P locally; the gluing and additivity verifications are just the trivial identity that a supremum over a union is the maximum of the local suprema (when those maxima are finite). No Perelman, Shi, or Cheeger-Gromov analysis is used to derive the Type-I classification: the label 'Type-I' is attached by definition once P(M) holds. With an infinite cover the local constants need not have a finite maximum, so the proof's C_M = max_i C_{U_i} is either tautological for finite covers or invalid for infinite ones.
1 more flagged steps
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self citation load bearing
[Lemma 2.1 and proof of Theorem 2.2, Section 2]
"The key technical tool is the following existence result; cf. [6, Lemma 5.3], and guarantees the existence of proper functions under certain conditions. Lemma 2.1. Every paracompact Hausdorff space admits proper functions to [0, ∞). This lemma enables the decomposition arguments central to Bredon’s trick. In Bredon’s proof this is included as a comment where he claims that it is possible to build this function, in this case we follow the presentation made by [3]."
The decomposition of X into compact sets A_n = f^{-1}[n,n+1] used in the proof of Theorem 2.2 depends entirely on Lemma 2.1. The lemma is not proved; it is supported only by a citation to Bredon [6] and by the presentation in [3], the author's own undergraduate thesis. As stated, the lemma is false: an uncountable discrete space is paracompact Hausdorff but admits no proper real-valued function, since every compact subset is finite. Thus the central local-to-global mechanism rests on a self-cited, unverified premise, and the theorem's claimed scope over all paracompact spaces is not supported by the paper's own derivation.
full rationale
The two principal 'novel applications' are circular because the property P is manufactured so that the local hypothesis already contains the global conclusion. In Theorem 4.1, P(V) refers to the global mapper complex M(X); asserting P(Uα) is already asserting local-global agreement, and the Nerve Lemma does not supply that agreement. In Theorem 4.4, P(U) is the Type-I curvature bound, the gluing verification is just the inequality sup_{U∪V} ≤ max(...), and the 'Type-I classification' is simply the name of P(M); no geometric mechanism from Ricci flow enters the derivation. The base Bredon's Trick theorem is not itself circular, but its proof relies on Lemma 2.1, which is sourced to Bredon and to the author's own thesis [3] and is false for general paracompact spaces; this is a load-bearing self-citation rather than independent verification. I have not scored the false direct-sum identity for infinite disjoint unions in Theorem 3.1, the unsupported five-lemma-for-persistence-modules step in Theorem 4.1, or the misquoted maximum principle as circularity, because those are ordinary correctness defects; however, they reinforce that the paper's central derivations are not self-contained. Overall, the central new claims reduce by construction to their own inputs, giving a score of 7.
Assumptions & free parameters
free parameters (2)
- C_U (local curvature bound)
- K(δ) (stability factor)
assumptions (5)
- ad hoc to paper Every paracompact Hausdorff space admits a proper function to [0,∞).
- ad hoc to paper Cohomology and persistence modules satisfy infinite disjoint additivity, H^k(⊔ U_i) ≅ ⊕ H^k(U_i).
- ad hoc to paper Hamilton's entropy monotonicity implies the pointwise curvature bound |Rm|(x,t) ≤ C/(T-t) on balls.
- ad hoc to paper A five lemma holds for persistence modules with interleavings.
- domain assumption Unique continuation holds for harmonic forms on stratified spaces.
Cite this review
Pith. "Pith review of Extensions and Applications of Bredon's Trick in Geometric and Topological Contexts." pith.science (2026). https://pith.science/paper/BKS6ZXD2
@misc{pith2026250704512,
author = {Pith},
title = {Pith review of: Extensions and Applications of Bredon's Trick in Geometric and Topological Contexts},
year = {2026},
howpublished = {\url{https://pith.science/paper/BKS6ZXD2}},
note = {Machine review of arXiv:2507.04512}
}
read the original abstract
We present a comprehensive analysis of Bredon's trick, a powerful local-to-global extension principle with broad applications across differential geometry and computational topology. Our main contributions include: (1) novel applications to stratified pseudomanifolds via Verona cohomology with explicit verification of axiomatic conditions; (2) new frameworks for Ricci flow singularity analysis using local curvature concentration; (3) stability theorems for persistent homology in distributed computational settings; and (4) rigorous applications to medical imaging and neural network topology. By systematically developing the theoretical foundations and providing concrete implementations, this work establishes Bredon's trick as a unifying framework for modern local-to-global arguments in geometric analysis and applied topology.
Reference graph
Works this paper leans on
- [1]
-
[2]
The index formula for families of Dirac type operators on pseudomanifolds
Pierre Albin, Jesse Gell-Redman, and S lawomir Radosz. “The Index Formula for Families of Dirac Operators on Pseudomanifolds”. In: Journal of Geometric Analysis 29.2 (2019), pp. 1773–1825. doi: 10.1007/s12220-018-0054-y . eprint: 1712.08513 (math.DG)
work page Pith review arXiv 2019
-
[3]
Una aplicaci´ on del Truco de Bredon a la Cohomolog ´ ıa de De Rham
M. Angel. “Una aplicaci´ on del Truco de Bredon a la Cohomolog ´ ıa de De Rham”. Tesis de Grado. Universidad Central de Venezuela, 2002
work page 2002
-
[4]
Ricci Flow and Diffeomorphism Grouups of 3- Manifolds
Richard H. Bamler and Bruce Kleiner. “Ricci Flow and Diffeomorphism Grouups of 3- Manifolds”. English (US). In: Journal of the American Mathematical Society 36.2 (Aug. 2023). Publisher Copyright: © 2022 American Mathematical Society., pp. 563–589. issn: 0894-0347. doi: 10.1090/jams/1003. EXTENSIONS AND APPLICATIONS OF BREDON’S TRICK Page 15 of 16
-
[5]
Raoul Bott and Loring W. Tu. Differential Forms in Algebraic Topology . New York: Springer-Verlag, 1982
work page 1982
-
[6]
Glen E. Bredon. Topology and Geometry . Vol. 139. Graduate Texts in Mathematics. Springer-Verlag, 1993
work page 1993
-
[7]
Stability and Interpretability of Mapper Reconstruc- tions
Mathieu Carri` ere and Steve Oudot. “Stability and Interpretability of Mapper Reconstruc- tions”. In: Proceedings of the Twenty-Eighth Annual ACM-SIAM Symposium on Discrete Algorithms. Philadelphia, PA, USA: Society for Industrial and Applied Mathematics, 2017, pp. 1353–1367
work page 2017
-
[8]
Cohomolog ´ ıa Invariante por acciones libres de S1
E. Cede˜ no. “Cohomolog ´ ıa Invariante por acciones libres de S1”. Tesis de Grado. Universidad Central de Venezuela, 2010
work page 2010
Show all 29 references
-
[9]
Small curvature concentration and Ricci flow smoothing
Pak-Yeung Chan, Eric Chen, and Man-Chun Lee. “Small curvature concentration and Ricci flow smoothing”. In: Journal of Functional Analysis 282.10 (2022), p. 109420. issn: 0022-1236. doi: https : / / doi . org / 10 . 1016 / j . jfa . 2022 . 109420. url: https : //www.sciencedire...
2022
-
[10]
The structure and stability of persistence modules
Fr´ ed´ eric Chazal, William Crawley-Boevey, and Vin de Silva. “The structure and stability of persistence modules”. In: Foundations of Computational Mathematics (2016)
2016
-
[11]
Dey and Yusu Wang
Tamal K. Dey and Yusu Wang. Computational Topology for Data Analysis . Manuscript completed 2020, published 2022. Cambridge University Press, 2022. isbn: 978- 1009098168
2020
-
[12]
Trudinger
David Gilbarg and Neil S. Trudinger. Elliptic partial differential equations of second order. Springer, 2001
2001
-
[13]
The formation of singularities in the Ricci flow
Richard S. Hamilton. “The formation of singularities in the Ricci flow”. In: Surveys in Differential Geometry 2 (1993), pp. 7–136
1993
-
[14]
Deconstructing the Mapper algorithm to extract richer topological and temporal features from functional neuroimaging data
Daniel Ha¸ segan et al. “Deconstructing the Mapper algorithm to extract richer topological and temporal features from functional neuroimaging data”. In: Network Neuroscience 8.4 (Dec. 2024), pp. 1355–1382
2024
-
[15]
Topological Analysis of Neural Network Dynamics
Christoph Hofer et al. “Topological Analysis of Neural Network Dynamics”. In: Nature Machine Intelligence 3.5 (2021), pp. 403–411. doi: 10.1038/s42256-021-00325-y
2021 doi
-
[16]
La cohomologie bElKa- cimiSergiescuHectorasique d’un feuilletage Riemannien est de dimension finie
Aziz El Kacimi-Alaoui, Vlad Sergiescu, and Gilbert Hector. “La cohomologie bElKa- cimiSergiescuHectorasique d’un feuilletage Riemannien est de dimension finie”. fr. In: Math. Z. 188.4 (Dec. 1985), pp. 593–599
1985
-
[17]
Comparing Riemannian foliations with transversally symmetric foliations
Franz W Kamber, Ernst A Ruh, and Philippe Tondeur. “Comparing Riemannian foliations with transversally symmetric foliations”. In: J. Differential Geom. 27.3 (Jan. 1988), pp. 461–475
1988
-
[18]
From “Local
Toshiyuki Kobayashi. “From “Local” to “Global” – Beyond the Riemannian Geometry”. In: Kavli IPMU News 25 (Mar. 2014), pp. 4–11
2014
-
[19]
A local to global argument on low-dimensional manifolds
Sam Nariman. “A local to global argument on low-dimensional manifolds”. In: Transac- tions of the American Mathematical Society 373.2 (2020), pp. 1307–1342
2020
-
[20]
Topology based data analysis identifies a subgroup of breast cancers with a unique mutational profile and excellent survival
Monica Nicolau, Arnold J. Levine, and Gunnar Carlsson. “Topology based data analysis identifies a subgroup of breast cancers with a unique mutational profile and excellent survival”. In: Proceedings of the National Academy of Sciences 108.17 (2011), pp. 7265–
2011
-
[21]
Proofs of the Compactness Theorem
Alexander Paseau. “Proofs of the Compactness Theorem”. In: History and Philosophy of Logic 31.1 (2010), pp. 73–98. doi: 10.1080/01445340903495340
2010 doi
-
[22]
The entropy formula for the Ricci flow and its geometric applications
Grisha Perelman. “The entropy formula for the Ricci flow and its geometric applications”. In: arXiv preprint math/0211159 (2002)
2002 arXiv
-
[23]
Cohomolog ´ ıa de los Espacios Proyectivos Complejos
M. A. P´ erez. “Cohomolog ´ ıa de los Espacios Proyectivos Complejos”. Tesis de Grado. Universidad Central de Venezuela, 2007. Page 16 of 16 EXTENSIONS AND APPLICATIONS OF BREDON’S TRICK
2007
-
[24]
Estudio Cohomol´ ogico de Flujos Riemannianos
J. I. Royo. “Estudio Cohomol´ ogico de Flujos Riemannianos”. Tesis Doctoral. Universidad del Pa ´ ıs Vasco-Euskal Herriko Unibertsitatea, 2004
2004
-
[25]
La sucesi´ on de Gysin
J. I. Royo. “La sucesi´ on de Gysin”. Tesis de Grado. Universidad del Pa ´ ıs Vasco-Euskal Herriko Unibertsitatea, 1999
1999
-
[26]
De Rham Intersection Cohomology for General Perversities
Martin Saralegui. “De Rham Intersection Cohomology for General Perversities”. In: Illinois Journal of Mathematics 49.3 (2005), pp. 737–758
2005
-
[27]
Deforming the metric on complete Riemannian manifolds
Wan-Xiong Shi. “Deforming the metric on complete Riemannian manifolds”. In: Journal of Differential Geometry 30.1 (1989), pp. 223–301. doi: 10.4310/jdg/1214443292. url: https://doi.org/10.4310/jdg/1214443292
1989
-
[28]
Local and global mappings of topology representing networks
´Agnes Vathy-Fogarassy and J´ anos Abonyi. “Local and global mappings of topology representing networks”. In: Information Sciences 179.21 (2009), pp. 3791–3803. M. Angel Grupo de ´Algebra y L´ ogica, Universidad Central de Venezuela Av.Los Ilustres, Caracas 1010, Venezuela. ma...
2009
-
[7270]
1073 / pnas
doi: 10 . 1073 / pnas . 1102826108. eprint: https : / / www . pnas . org / doi / pdf / 10.1073/pnas.1102826108 . url: https://www.pnas.org/doi/abs/10.1073/pnas. 1102826108
Reviewed August 6, 2026 · model on record in the stance chip above.
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