This paper extends the (IR) property — rigidity of sets of independent mean-zero functions against low-dimensional linear approximation — from the L_p scale to general symmetric spaces. The main results are clean: Theorem 1 gives sufficient conditions (lower 2-estimate, X⊃L₂, X' has Kruglov property), Theorems 2 and 3 give obstructions (fundamental function growing faster than √t, or ℓ_q roughly lattice finitely representable with q>2), and these combine to give a complete classification for Lorentz L_{p,q} spaces: (IR) holds for 1
2. The Orlicz case is partially covered with explicit conditions on Φ. This is a genuine extension of prior work by Malykhin, and the results are natural and well-motivated. The proofs are careful and use established tools — Johnson-Schechtman disjointification inequalities, the Kruglov property, Kolmogorov width estimates — in a straightforward way. The duality argument in Theorem 1 (constructing a conjugate system via conditional expectations, then applying the ℓ₂-estimate on X') is clean and correct. Theorem 2's explicit construction of badly approximable independent functions is elementary but effective. Theorem 5's disjointification step checks out. I went through the potential weak points the reader flagged and the stress-test examined, and I agree: no load-bearing flaw. The Kruglov property of X' is a real hypothesis, not automatic, and the authors are honest about this — Remark 3.8 shows (IR) can hold without it (ExpL_p for 1
Referee Report
3 major / 8 minor
Summary. This paper introduces the (IR) property for symmetric function spaces on [0,1], which captures the rigidity of finite sets of mean-zero independent functions against approximation by low-dimensional linear subspaces. The authors establish broad sufficient conditions for (IR) (Theorem 1, Theorem 4) based on lower 2-estimates, the embedding X⊃L₂, and the Kruglov property of the associate space X'. They also prove failure of (IR) under two types of conditions: when the fundamental function grows faster than t^{1/2} (Theorem 2), and when ℓ_q for q>2 is roughly lattice finitely representable in a space with the Kruglov property (Theorem 5). These results yield a near-complete classification for Lorentz L_{p,q} spaces and partial results for Orlicz spaces. The proofs use Johnson–Schechtman disjointification inequalities, Kruglov property machinery, Kolmogorov width estimates for finite-dimensional bodies, and a duality argument via biorthogonal systems constructed from conditional expectations.
Significance. The paper extends the L_p rigidity theory to the broader setting of symmetric spaces, which is a natural and substantive generalization. The positive results (Theorems 1 and 4) provide a clean, checkable framework: lower 2-estimate plus X⊃L₂ plus X'∈K suffices for (IR), with the key inequality (16) serving as the operative condition. The negative results (Theorems 2 and 5) are complementary and use explicit constructions. The Lorentz classification (Corollaries 3.9, 3.12) is clean, and the Orlicz results (Corollaries 3.7, 3.15, 3.16) cover natural scales. The open problems (3.11, 3.14, 3.18) are well-posed and indicate genuine gaps rather than omissions. The paper builds on the authors' prior work [29, 30, 31] but the extension to symmetric spaces via the Kruglov property and lattice-geometric conditions is new. The proofs are detailed and verifiable; the key steps (separability argument, conditional-expectation biorthogonal construction, application of inequality (7), width bounds) are all carried out explicitly.
major comments (3)
Theorem 1 (stated in the Introduction) requires X⊃L₂, but the proof in §3.1 uses this assumption to conclude X'⊂L₂, which is then needed for inequality (7) via [9, Theorem II.2.4]. However, the more general Theorem 4 in §3.1 does not list X⊃L₂ among its hypotheses (condition (i) does, but the main statement only requires inequality (16)). The relationship between Theorem 1 and Theorem 4 should be clarified: is Theorem 1 a strict corollary of Theorem 4(i), or does the proof of Theorem 1 use X⊃L₂ in an essential way beyond what Theorem 4 captures? Remark 3.2 suggests that (16) itself implies X⊃L₂, which would close the loop, but this logical relationship is not stated explicitly in the theorems. Adding a sentence clarifying the precise logical dependencies would strengthen the presentation.
In the proof of Theorem 2 (§3.2), the construction fixes ε satisfying C₁(γ)ε/φ_X(ε)² ≤ 1/2 and then fixes M via (22). The parameter s* is then set by (24) as s* = ⌊Nγ²/(4C(γ)²M²)⌋. For the tail bound m(C) ≤ 2^{-s*} to yield Mφ_X(2^{-s*}) ≤ γ/2, one needs s* → ∞ as N → ∞, which requires M to be fixed (independent of N). Since M = 1/φ_X(ε) and ε is fixed, this is indeed the case. However, the argument would benefit from an explicit statement that M is a constant depending only on X and γ (not on N), as this is load-bearing for the limit s* → ∞.
Proposition 3.19 (§3.3) extends the (IR)-type lower bound to functions satisfying a support condition measured by R_{1-δ}(f). The proof involves a dilation argument with factor R (line after equation (34)) that uses the bound ‖σ̃_R‖ ≤ max{1, R} on symmetric spaces on the half-axis. The application of this dilation to the Z²_{X'} norm is correct, but the step from (34) to the final estimate involves several implicit constants (C_X from the lower 2-estimate, δ^{-1}, R, C(X)). It would help the reader to state explicitly that the final constant B = δ^{-1}C(X)R_{1-δ}(f)(C_X + 2) is the one appearing in the proposition's conclusion, and to confirm that all dependencies are as claimed (B depends on X, δ, and R_{1-δ}(f)).
minor comments (8)
Abstract: 'sufficienly' should read 'sufficiently'.
Proof of Theorem 2 (§3.2): 'Conbining' should read 'Combining'.
§2.1: 'detailes' should read 'details'.
§2.2: 'duscussed' should read 'discussed'.
Reference [16]: the page range contains a formatting artifact ('1048ı¿œ1052'); please correct to '1048–1052'.
Reference [21]: the author list formatting ('A. Kamińska A., L. Maligranda, L.-E. Persson') appears to have a stray 'A.' after the first author's name; please verify and correct.
In the proof of Theorem 1 (§3.1), the step verifying ∫f_j g_k = 0 for j≠k uses independence of f_j and E_{A_k}g'_k. It would improve readability to note explicitly that this independence follows from the independence of the σ-algebras A_j and A_k for j≠k, which is a standard but non-trivial fact.
In Proposition 3.19 (§3.3), the quantity R_{1-δ}(f) is defined as a sum over k of m{|f_k| > 1-δ}, but it is used in the proof as a single number R. A brief remark that R_{1-δ}(f) is finite by the assumption R_0(f) < ∞ (in Corollary 3.20) or by the finiteness of the collection would help the reader.
Simulated Author's Rebuttal
3 responses · 0
unresolved
We thank the referee for a careful reading and for three constructive comments, all of which concern clarification of logical dependencies and explicit tracking of constants. We agree with all three points and will incorporate the requested clarifications in the revised manuscript.
read point-by-point responses
Authors: The referee is correct that the logical dependencies between Theorem 1, Theorem 4, and Remark 3.2 are not stated as explicitly as they should be. To clarify: Theorem 1 is indeed a corollary of Theorem 4(i). The proof of Theorem 1 in §3.1 proceeds by verifying the hypotheses of Theorem 4(i): the lower 2-estimate and X⊃L₂ together imply (by duality) that X' satisfies an upper 2-estimate and X'⊂L₂; combined with X'∈K, these yield inequality (16) via [9, Theorem II.2.4]. Thus X⊃L₂ is used in the proof of Theorem 1 only to establish (16), not in any additional way. Furthermore, as the referee notes, Remark 3.2 (citing [7, Theorem 38(b)]) shows that (16) itself implies both a lower 2-estimate and X⊃L₂, so condition (16) is in fact equivalent to the conjunction of these properties (given X'∈K). We will add an explicit sentence after the statement of Theorem 4 clarifying that Theorem 1 is a direct corollary of Theorem 4(i), and we will cross-reference Remark 3.2 to note that (16) implies X⊃L₂, closing the logical loop.
revision: yes
Authors: The referee's observation is correct. The parameter ε is fixed depending only on X and γ (via the condition C₁(γ)ε/φ_X(ε)² ≤ 1/2), and M = 1/φ_X(ε) is then also fixed depending only on X and γ. Consequently s* = ⌊Nγ²/(4C(γ)²M²)⌋ → ∞ as N → ∞, which is what drives the tail estimate Mφ_X(2^{-s*}) → 0. We will add an explicit sentence after the point where ε and M are fixed, stating that both ε and M are constants depending only on X and γ (and not on N), and that this independence from N is what ensures s* → ∞ as N → ∞.
revision: yes
Authors: We agree that the tracking of constants in the proof of Proposition 3.19 could be made more transparent. The final constant is indeed B = δ^{-1}C(X)R_{1-δ}(f)(C_X + 2), where C(X) is the constant from the disjointification inequality (6)/(33), C_X is the constant from the lower 2-estimate of X (equivalently, the upper 2-estimate of X'), δ is the parameter from the support condition, and R = R_{1-δ}(f) = Σ_k m{|f_k| > 1-δ}. All dependencies are as claimed: B depends on X (through C(X) and C_X), on δ, and on R_{1-δ}(f). We will add an explicit display of the constant B at the end of the proof, immediately before the concluding sentence, and confirm the dependency structure.
revision: yes
Circularity Check
0 steps flagged · score 1.0 of 10
No significant circularity found; the derivation is self-contained against external benchmarks with only minor self-citation.
full rationale
The paper's central theorems (Theorems 1-5) are derived from established external results: the Johnson-Schechtman inequality (6) from [19], the Kruglov property framework from Braverman [9], Kolmogorov width estimates (9)-(12) from approximation theory, and lattice duality from Lindenstrauss-Tzafriri [27]. Self-citations [29, 30, 31] provide the (IR) definition and Lemma 2.1, but Lemma 2.1 is proved in full within the paper (Section 2.5), and its proof uses only the standard orthonormal width bound (2). The key derivation steps in Theorem 1 (duality via conditional expectations, application of inequality (7), invocation of Lemma 2.1) do not reduce to the (IR) definition by construction. Theorem 2 constructs an explicit counterexample using width estimate (10) for the Euclidean ball. Theorem 5 uses disjointification via inequality (6) and width estimate (11). None of these steps are self-definitional or fitted-input-as-prediction. The self-citations are to prior work defining the problem and providing a lemma that is re-proved here, not to an unverified uniqueness theorem that would force the conclusion. The derivation chain is genuinely independent of its outputs.
Assumptions & free parameters
3 free parameters ·
5 assumptions ·
0 invented entities
The paper introduces no new mathematical entities or postulated objects. The (IR) property is a definition, not an axiom. All background results are standard in the literature on symmetric spaces and approximation theory. The constants γ, B, δ are existence constants whose values are not fitted to data.
free parameters (3)
γ = γ(X) > 0 = not specified; existence proved
The (IR) property asserts existence of γ>0 depending only on X. The proofs show γ exists via constants B, C(X), etc., but do not compute it explicitly.
B = B(X) > 0 = not specified; existence proved
The constant in the ℓ_2-estimate (16) and the width lower bound in Theorem 4. Depends on X through the Kruglov property constant and lattice constants.
δ = δ(X) > 0 = not specified; existence proved
The decay exponent in Theorem 5's width upper bound d_n ≤ CN^{-δ}. Determined by q via the octahedron width estimate (11).
assumptions (5)
standard math Inequality (6): if X∈K, then ‖Σf_k‖_X ≤ C(X)‖Σf̄_k‖_{Z²_X} for independent mean-zero f_k Johnson-Schechtman [19], also in [3, 7]. Central to all positive results.
standard math Inequality (7): ℓ_2-estimate for sums of independent functions holds iff X satisfies upper 2-estimate and X⊂L_2 Braverman [9, Theorem II.2.4], extended in [7, Corollary 40]. Used in proof of Theorem 1.
standard math Width estimates (9)-(11) for Euclidean ball and octahedron Classical results in approximation theory, cited from [28, 30]. Used in Theorems 2 and 5.
standard math 1/α_X ∈ LFR(X) for symmetric spaces Lindenstrauss-Tzafriri [27, Theorem 2.b.6], full proof in [5, Theorem 4]. Used in Theorem 5 corollary.
standard math Conditional expectation has norm 1 in maximal symmetric spaces Krein-Petunin-Semenov [25, Theorem II.4.9]. Used in proof of Theorem 1 to bound ‖g_k‖_{X'}.
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@misc{pith2026260706530,
author = {Pith},
title = {Pith review of: Rigidity of sets of independent functions in symmetric spaces},
year = {2026},
howpublished = {\url{https://pith.science/paper/KQDNKK2E}},
note = {Machine review of arXiv:2607.06530}
}
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abstract
We say that a symmetric function space $X$ has the $(IR)$ property whenever all sets of $N$ independent mean zero functions $f_1,\ldots,f_N\in X$, $\|f_k\|_X\ge 1$, are poorly approximated by any linear combinations of arbitrary $n$ functions, if $n$ is sufficienly smaller that $N$; namely, for some $\gamma=\gamma(X)>0$ we have $d_n(\{f_1,\ldots,f_N\},X)\ge \gamma$, $n\le \gamma N$, where $d_n(K,X)$ is the Kolmogorov $n$-width of the set $K\subset X$. The spaces $X=L_p$ satisfy this property if and only if $1\le p\le2$ or $p=\infty$. The goal of this paper is to move from $L_p$ scale to a larger class of symmetric spaces. We obtain rather broad conditions, under which such a space $X$ has the $(IR)$ property and prove precise statements for particular scales of Lorentz $L_{p,q}$ spaces and Orlicz spaces.
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