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REVIEW 2 major objections 5 minor 28 references

Extremal functions for a singular Hardy-Moser-Trudinger inequality

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read This paper proves that a singular Hardy-Moser-Trudinger inequality on the unit disk is sharp and has an extremal function.

desk verdict The theorem is a genuine extension of the singular Hardy-Moser-Trudinger line, the proof strategy is sound, and the main objection in the stress test comes from misreading Eq. (26), not from the paper. read the letter →

arxiv 1908.03982 v1 pith:5C7ICRGH submitted 2019-08-12 math.FA

classification math.FA MSC 46E35
keywords Hardy-Moser-TrudingerinequalitysingularMoser-Trudingerextremalfunctionblow-upanalysisunitdiskradialrearrangementGreenEuler-Lagrangeequation
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

This paper proves a sharp exponential inequality with a singular weight on the unit disk: for $0\le\beta<1$ and $0\le\alpha<\lambda_1(B)$, the supremum of $\int_B e^{4\pi(1-\beta)u^2}|x|^{-2\beta}\,dx$ over the class $\|u\|_{H,\alpha}\le 1$ is finite, and the supremum is attained by a radial function. The paper matters because it combines two features that had been treated separately: the Hardy-type norm that improves the classical Moser-Trudinger bound, and the singularity $|x|^{-2\beta}$. It proves existence by showing that maximizing sequences for slightly subcritical exponents cannot concentrate: blow-up at the origin would force a limiting profile whose mass contradicts a carefully constructed test function. A sympathetic reading of the proof therefore establishes not only a best constant but a genuine extremal, which is the object needed for Euler-Lagrange and compactness arguments in geometric PDEs.

What carries the argument

The carrying object is the blow-up analysis of the subcritical maximizers $u_\varepsilon$. Two rescaled sequences are formed around the origin, $\psi_\varepsilon(x)=c_\varepsilon^{-1}u_\varepsilon(r_\varepsilon^{1/(1-\beta)}x)$ and $\phi_\varepsilon(x)=c_\varepsilon(u_\varepsilon(r_\varepsilon^{1/(1-\beta)}x)-c_\varepsilon)$, where $r_\varepsilon$ is the blow-up radius; the second sequence converges to a solution of the singular Liouville equation whose explicit profile (35) is fixed by a classification theorem. A second load-bearing object is the Green function of the operator $L_\alpha=-\Delta-(1-|x|^2)^{-2}-\alpha$ with pole at the origin, written $G=-(1/2\pi)\log r+A_0+\Phi$ with smooth $\Phi$; the constant $A_0$ controls the sharp upper bound and the test functions that exclude blow-up.

What would settle it

To check the central claim, substitute the printed definition (26) into the coefficient of the nonlinear term in equation (32): with $r_\varepsilon^2=\lambda_\varepsilon c_\varepsilon^{-1}e^{-2\pi(1-\beta-\varepsilon)c_\varepsilon^2}$, that coefficient is $\lambda_\varepsilon^{-1}c_\varepsilon^2 r_\varepsilon^2e^{4\pi(1-\beta-\varepsilon)c_\varepsilon^2}=c_\varepsilon e^{2\pi(1-\beta-\varepsilon)c_\varepsilon^2}$, which diverges rather than tending to 1. The corrected scale $r_\varepsilon^2=\lambda_\varepsilon c_\varepsilon^{-2}e^{-4\pi(1-\beta-\varepsilon)c_\varepsilon^2}$ makes the coefficient exactly 1, so a one-line limit calculation settles whether the claimed profile (34) and mass normalization (36) are valid.

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Extended reading notes

Core claim

The central claim is Theorem 1: for fixed $0\le\beta<1$ and $0\le\alpha<\lambda_1(B)$, the supremum $\sup_{\|u\|_{H,\alpha}\le1}\int_B |x|^{-2\beta}e^{4\pi(1-\beta)u^2}\,dx$ is finite, and there is a function $u_0\in H$ with $\|u_0\|_{H,\alpha}=1$ attaining it. The proof proceeds by solving subcritical problems: for each $\varepsilon>0$ there is a radial maximizer $u_\varepsilon$ with $\|u_\varepsilon\|_{H,\alpha}=1$, satisfying the Euler-Lagrange equation (22). If the maximum values $c_\varepsilon=u_\varepsilon(0)$ stay bounded, the weak limit is the desired $u_0$. The long part of the paper assumes $c_\varepsilon\to\infty$ and runs a blow-up analysis around the origin; the rescaled profiles converge to the explicit solution $\phi_0(x)=-\frac{1}{4\pi(1-\beta)}\log(1+\frac{\pi}{1-\beta}|x|^{2(1-\beta)})$ of the singular Liouville equation $-\Delta\phi_0=|x|^{-2\beta}e^{8\pi(1-\beta)\phi_0}$ on $\mathbb{R}^2\setminus\{0\}$, and the associated Green function constant $A_0$ enters an upper bound. A final test-function construction produces a value strictly larger than that upper bound, contradicting the assumption that blow-up occurs; hence $c_\varepsilon$ is bounded and the extremal exists.

Load-bearing premise

The argument stands on one choice of blow-up scale: the rescaled equations (30) and (32) only hold when $r_\varepsilon^2=\lambda_\varepsilon c_\varepsilon^{-2}e^{-4\pi(1-\beta-\varepsilon)c_\varepsilon^2}$, whereas equation (26) as printed uses $r_\varepsilon^2=\lambda_\varepsilon c_\varepsilon^{-1}e^{-2\pi(1-\beta-\varepsilon)c_\varepsilon^2}$; with the printed choice the limit equation and the mass identity do not follow.

Editorial extensions

If this is right

  • If the theorem is correct, the sharp constant in (13) is attained, so the variational problem has a true maximizer rather than a supremum that is only approached.
  • That maximizer is radial, belongs to $H$, has norm $\|u_0\|_{H,\alpha}=1$, and solves the Euler-Lagrange equation (22); it is smooth away from the origin and continuous on the closed disk.
  • Setting $\beta=0$ recovers the improved Hardy-Moser-Trudinger inequality with an extremal, and setting $\alpha=0$ recovers the singular Moser-Trudinger existence statement; the two earlier results become boundary cases of one theorem.
  • The proof yields the explicit upper bound $\frac{\pi}{1-\beta}(1+e^{1+4\pi(1-\beta)A_0})$ for the sharp constant, so the value of the best constant is tied to a single Green-function quantity.
  • The blow-up profile (35) describes the geometry of any concentrating maximizing sequence near the singularity, which quantifies why the critical exponent $4\pi(1-\beta)$ is the threshold.

Reading between the lines

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

  • One testable extension is the borderline case $\alpha=\lambda_1(B)$: the norm $\|u\|_{H,\alpha}$ degenerates there, and the paper's argument does not apply, so one can check whether the supremum stays finite or becomes infinite.
  • The proof is specific to the disk because of the hyperbolic rearrangement; on a general domain the Hardy term $(1-|x|^2)^{-2}$ would not be preserved by symmetrization, so a different argument would be needed.
  • The explicit limit profile suggests a quantization principle: any concentrating maximizing sequence carries exactly one unit of mass at the singularity, a statement the paper does not isolate but which follows from the mass identity (36).
  • A numerical experiment on the disk for small $\beta$ could test whether computed maximizers match the asymptotic picture—the bubble shape near the origin plus a Green-function tail away from it—giving a concrete check of the blow-up description.
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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

2 major / 5 minor

Summary. The paper proves a singular Hardy-Moser-Trudinger inequality on the unit disc with the Hardy weight (1-|x|^2)^{-2} built into the norm: for 0≤β<1 and 0≤α<λ1(B), the supremum of ∫_B |x|^{-2β} e^{4π(1-β)u^2} dx over the unit ball of the weighted norm ||u||_{H,α} is finite and attained. The proof follows the standard three-step scheme: reduction to radial functions and existence of subcritical maximizers; blow-up analysis of the maximizers; upper-bound estimates via the Iula-Mancini inequality and construction of competing test functions to rule out blow-up. The main novelty is the combination of the singular weight |x|^{-2β} with the Hardy norm, extending previous results of Wang-Ye, Yang-Zhu, and Csató-Roy.

Significance. If correct, the theorem is a natural and nontrivial extension of the Hardy-Moser-Trudinger theory: it establishes sharp finiteness and existence of extremals under a norm that is weaker than the Dirichlet norm but stronger than L^2 due to the Hardy term. The strategy is standard for this literature, and the paper makes appropriate use of external classification and compactness results (Chen-Li, Iula-Mancini, Wang-Ye). The result would be a useful addition to the extremal-function literature for Trudinger-Moser inequalities. However, the printed proof contains a load-bearing scaling inconsistency in the blow-up analysis that must be repaired before the claims are checkable.

major comments (2)
  1. [Section 2.2, Eq. (26) vs Eqs. (30), (32), (34)] Equation (26) defines r_ε^2 = λ_ε c_ε^{-1} e^{-2π(1-β-ε)c_ε^2}. Substituting the blow-up coordinates (29) into the Euler-Lagrange equation (22) gives, for ψ_ε, a nonlinear coefficient λ_ε^{-1} r_ε^2 e^{4π(1-β-ε)c_ε^2 ψ_ε^2}; with (26) this equals c_ε^{-1} e^{4π(1-β-ε)c_ε^2(ψ_ε^2 - 1/2)}, not the c_ε^{-2} e^{4π(1-β-ε)(1+ψ_ε)φ_ε} that appears in (30). Similarly, the coefficient in (32) is λ_ε^{-1} c_ε^2 r_ε^2 e^{4π(1-β-ε)c_ε^2}, which equals 1 only if r_ε^2 = λ_ε c_ε^{-2} e^{-4π(1-β-ε)c_ε^2}. With the printed (26), the limit equation would not be (34), and the classification (35), the mass normalization (36), and all subsequent estimates in Lemmas 4-9 and Section 2.4 would not follow. The manuscript should correct (26) to the latter scaling and verify that (27)-(28) and the blow-up limits remain valid under that definition.
  2. [Section 2.2, proof of Lemma 6] The sentence 'Lemma 5 yields λ_ε/c_ε → +∞, hence c_ε/λ_ε → 0' is not a consequence of Lemma 5, which only bounds limsup ∫ |x|^{-2β} e^{4π(1-β-ε)u_ε^2} dx by π/(1-β) + limsup λ_ε/c_ε^2. This estimate is used to control the term I_2, and Lemma 6 (convergence of f_ε to δ_0) is later applied in Lemma 9 to identify the constant term in E_2(ρ). The proof therefore needs a correct estimate for I_2 (for example, using the explicit bubble scaling and τ<1) or an alternative justification of f_ε ⇀ δ_0.
minor comments (5)
  1. [Abstract and Introduction] There are several typos: 'Morser' should be 'Moser' in the abstract and in the first line of the introduction; 'u/nequivalence0' should read 'u≠0'; and reference [6] has 'Hardys inequality' instead of 'Hardy's inequality'.
  2. [Section 2.1, Eq. (27)] The use of (16) to bound λ_ε requires normalizing by ||u_ε||_H, which is only known to tend to 1 after the proof that u_0≡0. The manuscript should state this normalization explicitly; as written the line 'by the Hölder inequality and (16)' skips a step.
  3. [Section 2.2, Lemma 6] The notation 'o_ε(R)' in the estimates for I_1 and I_3 is ambiguous: it should mean a quantity that tends to 0 as ε→0 for fixed R, and then one lets R→∞. Please clarify the order of the limits.
  4. [Section 2.4] The test-function construction reuses ε for the new small parameter after ε was used for the subcritical approximation; this is potentially confusing and should be renamed, for instance δ or η.
  5. [Introduction, reference [13]] The text says 'We derive an upper bound ... by Onofri's inequality ([13], Theorem 1.1)', but reference [13] is Iula-Mancini's paper. If Onofri's inequality is being used, a separate citation should be given; if Iula-Mancini's result is meant, the wording should be corrected.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the proof is built on independent external results, not on its own target inequality.

full rationale

The paper proves Theorem 1 by a standard three-step strategy: subcritical maximization, blow-up analysis, and a test-function contradiction. The load-bearing ingredients are imported from independent external works: Wang-Ye [22] supplies the radial compact embedding and the Hardy-Moser-Trudinger inequality; Adimurthi-Sandeep supplies the singular Moser-Trudinger inequality used in the subcritical boundedness estimate; Chen-Li [9] supplies the classification of the blow-up limit equation; and Iula-Mancini [13] supplies the Dirichlet-norm upper bound used in Lemma 9. None of these is a restatement of the target H-norm inequality, and none is fitted to the present theorem. The test-function construction in Section 2.4 uses the Green function representation and direct computation; it does not insert the desired supremum as an input and then recover it as a prediction. There is also no load-bearing self-citation: the author cites prior works by Yang-Zhu and others, but the cited results stand independently of this paper. The only conspicuous defect is internal rather than circular: as printed, Eq. (26) defines r_epsilon^2 = lambda_epsilon c_epsilon^{-1} e^{-2pi(1-beta-epsilon)c_epsilon^2}, whereas Eqs. (30) and (32) are consistent with the corrected scaling r_epsilon^2 = lambda_epsilon c_epsilon^{-2} e^{-4pi(1-beta-epsilon)c_epsilon^2}. Taking Eq. (26) literally would break the derivation of the limit profile (34), but this is a scaling/typo-level proof defect, not a case of the theorem being used to prove itself. Since no circular step can be exhibited with a quote and a specific reduction, the correct finding is no significant circularity.

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

The central claim rests on a chain of published external results: hyperbolic rearrangement, Wang-Ye embedding lemmas, Chen-Li classification, Iula-Mancini's upper bound, and the Green function expansion. No constants are fitted to data and no new entities are postulated. The only effective free choice in the proof is the blow-up scale, which is misstated in Eq. (26) and must be corrected.

assumptions (5)
  • standard math Hyperbolic rearrangement maps each u in C0∞(B) to a radial nonincreasing u* preserving ∫u²dμ and ∫u²/(1−|x|²)² dx, and not increasing ∫|∇u|².
    Invoked in Lemma 3 before Eq. (15) to reduce the variational problem to the radial subspace S; cited to Baernstein [5].
  • standard math The Chen-Li classification theorem determines all solutions of −Δφ0=|x|^{−2β}e^{8π(1−β)φ0} in R²\{0} with finite mass; the solution is φ0(x)=−(1/(4π(1−β)))log(1+π/(1−β)|x|^{2(1−β)}).
    Used in §2.2 after Eq. (34) to get Eqs. (35)-(36), which give the mass of the blow-up profile.
  • standard math Iula-Mancini's Lemma 8: for u_n in W0^{1,2}(B) with ||∇u_n||≤1 and u_n⇀0, limsup_n ∫ e^{4π(1−β)u_n²}/|x|^{2β} dx ≤ π(1+e)/(1−β).
    Used in Lemma 9 to bound the rescaled positive part of u_ε near the origin; cited to [13].
  • standard math The Green function G of Lα=−Δ−(1−|x|²)^{-2}−α with pole at 0 satisfies LαG=δ0 and has expansion G=−(1/(2π))log r + A0 + Φ with Φ∈C1loc(B).
    Assumed in Lemma 7 and used for the upper bound constant A0 and for the test functions in §2.4; the proof is sketched, not fully given.
  • standard math Yang-Zhu's Lemma 4 in [26]: for every γ>0 and every u in the radial space S, ∫_B e^{γu²} dx < ∞.
    Used in Lemma 3 and Lemma 6 to bound the second factors in the Hölder estimates.

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Pith. "Pith review of Extremal functions for a singular Hardy-Moser-Trudinger inequality." pith.science (2026). https://pith.science/paper/5C7ICRGH

@misc{pith2026190803982,
  author       = {Pith},
  title        = {Pith review of: Extremal functions for a singular Hardy-Moser-Trudinger inequality},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5C7ICRGH}},
  note         = {Machine review of arXiv:1908.03982}
}
read the original abstract

In this paper, using blow-up analysis, we prove a singular Hardy-Morser-Trudinger inequality, and find its extremal functions. Our results extend those of Wang-Ye (Adv. Math. 2012), Yang-Zhu ( Ann. Glob. Anal. Geom. 2016), Csat\'{o}- Roy (Calc. Var. 2015), and Yang-Zhu (J. Funct. Anal. 2017).

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