REVIEW 1 major objections 5 minor 36 references
Second order regularity for elliptic and parabolic equations involving $p$-Laplacian via a fundamental inequality
T0 review · 1 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read A dimension-weighted Hessian inequality yields second-order Sobolev regularity for p-Laplacian equations and fully settles the planar parabolic normalized case.
desk verdict A solid, self-contained regularity paper built on a new algebraic inequality; the main theorems are new and the proofs are mostly careful, with one standard-but-unproven approximation step that a referee should ask about. 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 load-bearing object is the dimension-weighted pointwise inequality (2.1), which bounds the algebraic expression $|D^2vDv|^2-\Delta v\,\Delta_\infty v-\tfrac12(|D^2v|^2-(\Delta v)^2)|Dv|^2$ by the nonnegative quantity $|D^2v|^2|Dv|^2-|D^2vDv|^2$ times $(n-2)/2$. Its proof diagonalizes the Hessian at a point, reducing the inequality to an $n$-term eigenvalue statement whose direction is the unit vector $Dv/|Dv|$, and then invokes an elementary vector inequality. The argument applies this inequality to smooth regularized solutions of the elliptic $p$-Laplace equation and of the two parabolic equations under study; it yields uniform-in-$\varepsilon$ estimates on $|D^2u_\varepsilon|^2$, $|u_{\varepsilon,t}|^2$, and weighted products of Hessian and gradient. Those uniform estimates are what pass to the limiting solution through compactness and a higher-integrability step.
What would settle it
Take an explicit planar solution of the parabolic normalized $p$-Laplace equation, for instance a radial or self-similar profile, and compute whether $\int_{Q_r}(|D^2u|^q+|u_t|^q)\,dx\,dt$ is finite for some $q>2$; a single example where the integral diverges for every $q>2$ would refute Theorem 1.3. Similarly, a $p$-harmonic function for which $|Du|^{(p-\gamma)/2}Du$ fails to lie in $W^{1,2}_{\rm loc}$ for some $\gamma<\gamma_{n,p}$ would refute Theorem 1.1.
Extended reading notes
Core claim
The paper's central claim is that the planar identity $$ |$D^{2}$vDv|^2-\$\Delta$ v\,\Delta_\infty v=\tfrac12(|$D^{2}$v|^2-(\$\Delta$ v)^2)|Dv|^2 $$ admits a higher-dimensional replacement: for every smooth $v$, $$ \Bigl||$D^{2}$vDv|^2-\$\Delta$ v\,\Delta_\infty v-\tfrac12(|$D^{2}$v|^2-(\$\Delta$ v)^2)|Dv|^2\Bigr| \le \frac{n-2}{2}\bigl(|$D^{2}$v|^2|Dv|^2-|$D^{2}$vDv|^2\bigr), $$ where $\Delta_\infty v=D^2vDv\cdot Dv$. Using this inequality on regularized solutions, the paper proves that $p$-harmonic functions satisfy $|Du|^{(p-\gamma)/2}Du\in W^{1,2}_{\rm loc}$ for $\gamma<\min\{p+\frac{n}{n-1},\,3+\frac{p-1}{n-1}\}$; that viscosity solutions of the parabolic normalized $p$-Laplace equation have $D^2u,u_t\in L^q_{\rm loc}$ for some $q>2$ in the exponent range $(1,2)\cup(2,3+\frac{2}{n-2})$, which for $n=2$ means every $p\ne2$; and that for the degenerate parabolic $p$-Laplace equation, $D^2u\in L^2_{\rm loc}$ and $u_t\in L^2_{\rm loc}$ for $1<p<3$, with the upper endpoint sharp.
Load-bearing premise
The proof works first on smooth approximate solutions; if those approximations did not have uniformly bounded gradients and did not converge to the true solution, the second-order estimates would not carry over to the limit.
Editorial extensions
If this is right
- The exponent range for the weighted gradient quantity in Theorem 1.1 improves the earlier bound $\gamma\le2$ for every $p\ne2$ and every dimension $n\ge2$.
- For the parabolic normalized $p$-Laplace equation in the plane, the open question on second-order regularity is settled for all $p\ne2$, with the stronger conclusion that the integrability exponent is $q>2$ rather than merely $2$.
- For $n\ge3$, viscosity solutions of the parabolic normalized equation have $D^2u$ and $u_t$ in $L^q_{\rm loc}$ for some $q>2$ throughout $p\in(1,2)\cup(2,3+\frac{2}{n-2})$, a wider range than the coefficient-degeneracy approach could reach.
- For the degenerate parabolic $p$-Laplace equation, the range $p\in(1,3)$ is sharp for spatial $W^{2,2}$-regularity: an explicit solution has $|D^2w|$ comparable to $|x_1|^{(2-p)/(p-1)}$, which is in $L^2_{\rm loc}$ exactly when $p<3$.
- All estimates come with constants that do not blow up as $p\to2$, so the results connect continuously to the classical theory at $p=2$.
Reading between the lines
- Because the fundamental inequality is purely algebraic and needs no convexity or monotonicity of the $p$-Laplacian, it may transfer to other equations whose operators interpolate between Laplacian and infinity-Laplacian, such as game-theoretic or image-processing parabolic models; the paper does not explore these applications.
- One way to probe sharpness is to compute the weighted gradient quantity for explicit power-type $p$-harmonic functions and check whether $W^{1,2}$ integrability fails as $\gamma$ approaches the paper's upper bound; the paper does not carry out this endpoint test.
- The authors conjecture that for $n\ge3$ and $p\ge3+\frac{2}{n-2}$, the expression $|D^2u|^2-(\Delta u)^2$ can change sign for some $p$-harmonic function; if verified, the exponent ranges in the second-order results would be sharp, and the method's limitation would be intrinsic rather than technical.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves a pointwise algebraic inequality (Lemma 2.1, Eq. (1.2)) controlling the structure of ΔvΔ∞v in terms of |D^2v| and Dv, and then uses this inequality as the central tool to establish second-order Sobolev regularity for p-harmonic functions (Theorem 1.1, Corollary 1.2), for viscosity solutions of the parabolic normalized p-Laplace equation (Theorem 1.3), and for weak/viscosity solutions of the parabolic p-Laplace equation (Theorem 1.5). The results include improved ranges of γ and p, a complete answer for n=2 to an open question of Høeg and Lindqvist, and a sharpness example for the range p<3 in Theorem 1.5. The proofs pass through ε-regularized smooth solutions and then take limits using known convergence and uniform gradient bounds.
Significance. If the claims hold, this is a significant contribution to the regularity theory of degenerate elliptic and parabolic equations. The fundamental inequality (1.2) is new, is proved from scratch via an elementary spectral decomposition, and is likely to be a useful tool beyond the applications considered here. The paper also gives explicit quantitative estimates and a clean sharpness calculation for the parabolic p-Laplace equation. The central analytic arguments are self-contained apart from standard approximation facts, and the main results improve previously known ranges in the elliptic case and are the first higher-integrability results in the parabolic normalized case for n=2.
major comments (1)
- [Sections 4 and 5, Eqs. (1.12) and (1.13)] The passage from the regularized solutions u_ε to the limit solution u relies on the assertions that u_ε ∈ C^∞(U_T) ∩ C^0(overline{U_T}), Du_ε ∈ L^∞(U_T) uniformly in ε, and u_ε → u in C^0(U_T). In Section 4 this is attributed to [22], and in Section 5 to [11,36]. However, the cited papers concern the unregularized degenerate equations, not the ε-regularized equations (1.12) and (1.13) with boundary data inherited from a viscosity solution. These approximation properties are load-bearing: without them the weak-limit identification of D^2u_ε and u_{ε,t} in Theorems 1.3 and 1.5 collapses. The authors should either provide precise statements or proofs from the standard theory of uniformly parabolic quasilinear equations, or give correct references that cover the regularized problems uniformly in ε. This gap is likely fillable but must be addressed in the manuscript.
minor comments (5)
- [Theorem 1.3 and Corollary 1.2] The range p ∈ (1,2) ∪ (2, 3+2/(n−2)) is stated without comment for n=2, where the expression 3+2/(n−2) is undefined; it should be interpreted as the whole interval (1,∞), and this convention should be stated explicitly.
- [Theorem 1.3, Eq. (1.7)] The statement 'u_t, D^2u ∈ L^q_loc(Ω)' should read 'u_t, D^2u ∈ L^q_loc(Ω_T)', since the estimates are on space-time cylinders Q_r ⊂ Ω_T.
- [Section 4, Lemma 4.1 and proof of Theorem 1.3] The notation Q_r and Q(0,r) is used inconsistently: the definition Q_r(z,s) = (s−r^2,s) × B(z,r) gives a cylinder ending at time s, but the proof of Lemma 4.1 appears to translate time so that the bottom of the cylinder is t=0. Please clarify the time normalization and the appearance of integrals over B_{2r} at t=0 in Lemmas 4.7 and 4.8.
- [Lemma 4.6 and Lemma 4.7] The boundary terms involving ln[|Du_ε(x,0)|^2+ε] are controlled only after invoking uniform boundedness of Du_ε and the prefactor ε; this should be stated explicitly, since ln[|Du_ε|^2+ε] is not bounded below uniformly as ε→0.
- [Abstract and Section 1.1] In the abstract, the formula for γn,p appears as min{p + (n−1)/n, 3+(p−1)/(n−1)}, while Theorem 1.1 states min{p + n/(n−1), 3+(p−1)/(n−1)}. The theorem version is the correct one; the abstract should be corrected.
Circularity Check
No significant circularity; the fundamental inequality is proved from scratch and the self-citation [24] is only motivational.
full rationale
The central derivation is self-contained. Lemma 2.1, the fundamental inequality, is proved directly from linear algebra (Lemma 2.2) via spectral decomposition, with no use of the planar identity (1.1) except as background motivation attributed to [24]. When n=2, inequality (2.1) reproduces (1.1), but the proof does not rely on [24]; the identity is a special case of the paper's own Lemma 2.1. The main theorems follow from (2.1), Lemma 2.3, elementary integration by parts, and standard analytic tools such as Gehring's lemma, Sobolev-Poincare inequalities, and compact embedding. The approximation and convergence facts cited from [33, 26, 10, 22, 11, 36] are external regularity results, not results of this paper, and no fitted parameter is used to produce the predicted second-order estimates. The only self-citation, [24] by Koch-Zhang-Zhou, is not load-bearing: it is mentioned to motivate the planar identity but is not needed to prove Theorem 1.1, Theorem 1.3, or Theorem 1.5. Concerns about whether the cited approximation theory applies verbatim to the epsilon-regularized problems are correctness or technical-gap concerns, not circularity, because the paper does not define its outputs in terms of those citations or rename fitted quantities as predictions.
Assumptions & free parameters
assumptions (5)
- domain assumption Known C^{1,α} regularity and convergence of the elliptic regularization (1.9): Du_ε ∈ L∞ uniformly and Du_ε → Du in C^{0,α}.
- domain assumption Parabolic regularization theory for (1.12) and (1.13): existence of smooth viscosity/weak solutions u_ε with u_ε → u in C^0 and Du_ε ∈ L∞ uniformly.
- standard math Gehring's lemma (self-improvement of reverse Hölder inequalities).
- standard math Parabolic Sobolev-Poincaré inequality and compact embedding theorems.
- standard math Equivalence of weak and viscosity solutions for the p-Laplace equations.
Cite this review
Pith. "Pith review of Second order regularity for elliptic and parabolic equations involving $p$-Laplacian via a fundamental inequality." pith.science (2026). https://pith.science/paper/J34AEHOI
@misc{pith2026190801547,
author = {Pith},
title = {Pith review of: Second order regularity for elliptic and parabolic equations involving $p$-Laplacian via a fundamental inequality},
year = {2026},
howpublished = {\url{https://pith.science/paper/J34AEHOI}},
note = {Machine review of arXiv:1908.01547}
}
abstract
Denote by $\Delta$ the Laplacian and by $\Delta_\infty $ the $\infty$-Laplacian. A fundamental inequality is proved for the algebraic structure of $\Delta v\Delta_\infty v$: for every $v\in C^\infty$, $$\ | { |D^2vDv|^2} - {\Delta v \Delta_\infty v } -\frac12[|D^2v|^2-(\Delta v)^2]|Dv|^2\ | \le \frac{n-2}2 [|D^2v|^2{|Dv|^2}- |D^2vDv|^2 ]. $$ Based on this, we prove the following results: 1. For any $p$-harmonic functions $u$, $p\in(1,2)\cup(2,\infty)$, we have $$|Du|^{\frac{p-\gamma}2}Du\in W^{1,2}_{\rm loc},$$ with $\gamma<\min\{p+\frac{n-1}{n},3+\frac{p-1}{n-1}\}$. As a by-product, when $p\in(1,2)\cup(2,3+\frac2{n-2})$, we reprove the known $W^{2,q}_{\rm loc}$-regularity of $p$-harmonic functions for some $q>2$. 2. When $n\ge 2$ and $p\in(1,2)\cup(2,3+\frac2{n-2})$, the viscosity solutions to parabolic normalized $p $-Laplace equation have the $W_{\rm loc}^{2,q}$-regularity in the spatial variable and the $W_{\rm loc}^{1,q}$-regularity in the time variable for some $q>2$. Especially, when $n=2$ an open question in [17] is completely answered. 3. When $n\ge 1 $ and $p\in(1,2)\cup(2,3)$, the weak/viscosity solutions to parabolic $p $-Laplace equation have the $W_{\rm loc}^{2,2}$-regularity in the spatial variable and the $W_{\rm loc}^{1,2}$-regularity in the time variable. The range of $p$ (including $p=2$ from the classical result) here is sharp for the $W_{\rm loc}^{2,2}$-regularity.
Reference graph
Works this paper leans on
- [22]
-
[1]
A. Attouchi and M. Parviainen, H¨ older regularity for the gradient of the inhomogeneous parabolic normalized p-Laplacian. Commun. Contemp. Math. 20 (2018), 1750035
work page 2018
-
[2]
A. Attouchi and E. Ruosteenoja, Remarks on regularity for p-laplacian type equations in non-divergence form. J. Diff. Equ. 265 (2018), 1922-1961
work page 2018
-
[3]
L. Bers and L. Nirenberg, On linear and non-linear elliptic boundary value problems i n the plane. Convegno Internazionale sulle Equazioni Lineari alle Deri vate Parziali, Trieste, 1954, pp. 141-167, Edizioni Cremonese, Roma, 1955
work page 1954
-
[4]
B. Bojarski and T. Iwaniec, p-harmonic equation and quasiregular mappings. Partial differ- ential equations (Warsaw, 1984), Banach Center Publ, 19, PW N, Warsaw (1987) 25-38
work page 1987
-
[5]
A. Banerjee and N. Garofalo, Gradient bounds and monotonicity of the energy for some nonlinear singular diffusion equations . Indiana Univ. Math. J. 62 (2013), 699-736
work page 2013
-
[6]
A. Banerjee and N. Garofalo, On the Dirichlet boundary value problem for the normalized p-Laplacian evolution. Commun. Pure Appl. Anal., 14 (2015), 1-21
work page 2015
-
[7]
L. A. Caffarelli and X. Cabr´ e, Fully nonlinear elliptic eq uations. volume 43 of American Mathematical Society Colloquium Publications. American M athematical Society, Provi- dence, RI, 1995
work page 1995
Show all 36 references
-
[8]
Campanato, Un risultato relativo ad equazioni ellittiche del secondo o rdine di tipo non variazionale
S. Campanato, Un risultato relativo ad equazioni ellittiche del secondo o rdine di tipo non variazionale. (Italian) Ann. Scuola Norm. Sup. Pisa (3) 21 (1967), 701-707
1967
-
[9]
H. O. Cordes, Zero order a priori estimates for solutions of elliptic diffe rential equations. Proc. Sympos. Pure Math. 4 (1961), 157-166
1961
-
[10]
DiBenedetto, C 1+α local regularity of weak solutions of degenerate elliptic e quations
E. DiBenedetto, C 1+α local regularity of weak solutions of degenerate elliptic e quations. Nonlinear Anal. 7 (1983), 827-850
1983
-
[11]
DiBenedetto and A
E. DiBenedetto and A. Friedman, H¨ older estimates for nonlinear degenerate parabolic sys- tems. J. Reine Angew. Math., 357 (1985), 1-22
1985
-
[12]
Does, An evolution equation involving the normalized p-Laplacia n
K. Does, An evolution equation involving the normalized p-Laplacia n. Commun. Pure Appl. Anal., 10 (2011), 361-396
2011
-
[13]
Evans, A new proof of local C 1,α-regularity for solutions of certain degenerated elliptic P.D.E
L. Evans, A new proof of local C 1,α-regularity for solutions of certain degenerated elliptic P.D.E. J. Differential Equations 45 (1982), 356-373
1982
-
[14]
Elmoataz, M
A. Elmoataz, M. Toutain, and D. Tenbrinck. On the p-Laplacian and ∞-Laplacian on graphs with applications in image and data processing. SIAM J. Imaging Sci., 8 (2015), 2412-2451
2015
-
[15]
F. W. Gehring, TheLp-integrability of partial derivatives of a quasiconformal mapping. Acta Math. 130 (1973), 265-277
1973
-
[16]
Giaquinta, Multiple integrals in the calculus of var iations and nonlinear elliptic systems, Princeton University Press, Princeton, New Jersey 1983
M. Giaquinta, Multiple integrals in the calculus of var iations and nonlinear elliptic systems, Princeton University Press, Princeton, New Jersey 1983. 32
1983
-
[17]
F. A. Høeg and P. Lindqvist, Regularity of solutions of the parabolic normalized p-Laplace equation. Adv. Nonlinear Anal. 9 (2020), 7-15
2020
-
[18]
Imbert, T
C. Imbert, T. Jin and L. Silvestre, H¨ older gradient estimates for a class of singular or degenerate parabolic equations. Adv. Nonlinear Anal., 8 (2019), 845-867
2019
-
[19]
Iwaniec and J
T. Iwaniec and J. J. Manfredi, Regularity of p-harmonic functions on the plane. Rev. Mat. Ibero. 5(1989),1-19
1989
-
[20]
Juutinen, P
P. Juutinen, P. Lindqvist and J. J. Manfredi, On the equivalence of viscosity solutions and weak solutions for a quasi-linear equation. SIAM J. Math. Anal. 33(3):699-717, 2001
2001
-
[21]
Julin and P
V. Julin and P. Juutinen, A new proof for the equivalence of weak and viscosity solutio ns for the p-Laplace equation. Comm. Part. Diff. Equ. 37 (2012), 934-946
2012
-
[23]
F. H. Lin, Second derivative Lp-estimates for elliptic equations of nondivergent type. Proc. Amer. Math. Soc. 96 (1986), 447-451
1986
-
[24]
H. Koch, Y. Zhang and Y. Zhou, An asymptotic sharp Sobolev regularity for planar infinity harmonic functions . J. Math. Pures Appl. 2019, pp. 25 (to appear)
2019
-
[25]
N. V. Krylov, Parabolic equations with VMO coefficients in Sobolev spaces w ith mixed norms. J. Funct. Anal. 250 (2007), 521-558
2007
-
[26]
Lewis, Regularity of the derivatives of solutions to certain ellip tic equations
J. Lewis, Regularity of the derivatives of solutions to certain ellip tic equations. Indiana Univ. Math. J. 32 (1983), 849-858
1983
-
[27]
J. J. Manfredi and A. Weitsman, On the Fatou theorem for p-harmonic functions, Commun. Part. Diff. Equ. 13 (1988), 651-668
1988
-
[28]
J. J. Manfredi, M. Parviainen and J. Rossi, An asymptotic mean value characterization for a class of nonlinear parabolic equations related to tug-of- war games . SIAM J. Math. Anal. 42 (2010), 2058-2081
2010
-
[29]
Maugeri, D
A. Maugeri, D. K. Palagachev and L. G. Softova, Elliptic and Parabolic Equations with Dis- continuous Coefficients. Mathematical Research vol. 109,Wi ley-VCH Verlag Berlin GmbH, Berlin (Federal Republic of Germany), 2000
2000
-
[30]
Nystr¨ om and M
K. Nystr¨ om and M. Parviainen, Tug-of-war, market manipulation and option pricing. J. Math. Finan. 0 (2014), 1-34
2014
-
[31]
Peres and S
Y. Peres and S. Sheffield, Tug-of-war with noise: a game-theoretic view of the p-Lapla cian, Duke Math. J. 145 (2008), 91-120
2008
-
[32]
Peres, O
Y. Peres, O. Schramm, S. Sheffield and D. B. Wilson, Tug-of-war and the infinity Laplacian. J. Amer. Math. Soc. 22 (2009), 167-210
2009
-
[33]
Ura’tselva, Degenerate quasilinear elliptic systems, Zap. Nauch. Sem. Otdel 7 (1968), 184- 222, Mat. Inst., Steklov, Leningrad. (In Russian.)
1968
-
[34]
Talenti, Sopra una classe di equazioni ellittiche a coefficienti misur abili
G. Talenti, Sopra una classe di equazioni ellittiche a coefficienti misur abili. Ann. Mat. Pura Appl. (4) 69 (1965) 285-304
1965
-
[35]
Uhlenbeck, Regularity for a class of non-linear elliptic systems
K. Uhlenbeck, Regularity for a class of non-linear elliptic systems. Acta Math. 138 (1977), 219–240
1977
-
[36]
Wiegner, On C α-regularity of the gradient of solutions of degenerate para bolic systems
M. Wiegner, On C α-regularity of the gradient of solutions of degenerate para bolic systems. Ann. Mat. Pura Appl. (4), 145 (1986), 385–405. 33 Hongjie Dong Division of Applied Mathematics, Brown University, 182 Geo rge Street, Providence, RI 02912, USA E-mail : hongjie dong@br...
1986
Reviewed August 14, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.