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arxiv: 2510.08947 · v4 · pith:2RE35YZ7new · submitted 2025-10-10 · 🧮 math.AP

On positive solutions of Lane-Emden equations on the integer lattice graphs

Pith reviewed 2026-05-21 20:52 UTC · model grok-4.3

classification 🧮 math.AP
keywords Lane-Emden equationinteger lattice graphpositive solutionsSobolev critical exponentSerrin critical exponentvariational methodsiterative decay estimatesHardy potential
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The pith

Positive solutions to the Lane-Emden equation exist on the integer lattice in Sobolev super-critical regions of the decay exponent and power, but not in Serrin sub-critical regions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines positive solutions of the semilinear equation minus the graph Laplacian equals Q times |u| to the p minus 2 times u on the d-dimensional integer lattice graph, and on its half-space and quadrant subdomains with zero Dirichlet boundary values. It proves existence of such solutions by variational methods whenever the pair of parameters consisting of the potential decay rate alpha and the power p lies in the Sobolev super-critical regime. Nonexistence is shown in the Serrin sub-critical regime by an iterative argument that forces the solution to decay so rapidly at infinity that it must be identically zero, yielding a contradiction. In the full space and half-space an open interval of parameters sits between the two critical lines and is left unresolved, while the quadrant has no such gap.

Core claim

For the Lane-Emden problem with positive Hardy-type potential Q asymptotically equivalent to (1 plus absolute value of x) to the minus alpha on the integer lattice and its half-space and quadrant, positive solutions exist by variational methods precisely when the parameter pair (alpha, p) lies above the Sobolev critical line; the same solutions are shown not to exist below the Serrin critical line by repeated application of decay estimates that produce a contradiction at infinity.

What carries the argument

The Sobolev critical line and the Serrin critical line in the (alpha, p) parameter plane, which bound the regions where the variational functional is well-defined and coercive versus the regions where iterative decay forces the only solution to be zero.

If this is right

  • Existence holds throughout the region above the Sobolev line for every dimension d greater than or equal to 2.
  • Nonexistence holds throughout the region below the Serrin line on the full space, half-space, and quadrant alike.
  • The quadrant domain admits a complete classification with no unresolved parameter interval between the two lines.
  • The proofs rely only on the asymptotic decay of Q and the discrete structure of the lattice, not on any continuous embedding.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same variational-plus-decay strategy may apply to other discrete graphs whose volume growth matches that of Z to the d.
  • Resolving the open intermediate region would likely require a new test function or a moving-plane argument adapted to the lattice.
  • The quadrant result suggests that corner boundaries can eliminate the gap by strengthening the decay control near the boundary.
  • The critical lines themselves are expected to coincide with the corresponding lines for the continuous Laplacian on R to the d when alpha is fixed.

Load-bearing premise

The potential Q must be positive and decay exactly like (1 plus absolute value of x) to the minus alpha at infinity, both to set up the functional space for the variational argument and to close the decay estimates that produce the nonexistence contradiction.

What would settle it

An explicit construction or numerical approximation of a positive solution for any pair (alpha, p) strictly below the Serrin critical line on the full lattice would falsify the nonexistence claim.

read the original abstract

In this paper, we investigate the existence and nonexistence of positive solutions to the Lane-Emden equations $$ -\Delta u = Q |u|^{p-2}u $$ on the $d$-dimensional integer lattice graph $\mathbb{Z}^d$, as well as in the half-space and quadrant domains, under the zero Dirichlet boundary condition in the latter two cases. Here, $d \geq 2$, $p > 0$, and $Q$ denotes a Hardy-type positive potential satisfying $Q(x) \sim (1+|x|)^{-\alpha}$ with $\alpha \in [0, +\infty]$. \smallskip We identify the Sobolev super-critical regions of the parameter pair $(\alpha, p)$ for which the existence of positive solutions is established via variational methods. In contrast, within the Serrin sub-critical regions of $(\alpha, p)$, we demonstrate nonexistence by iteratively analyzing the decay behavior at infinity, ultimately leading to a contradiction. Notably, in the full-space and half-space domains, there exists an intermediate regions between the Sobolev critical line and the Serrin critical line where the existence of positive solutions remains an open question. Such an intermediate region does not exist in the quadrant domain.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript investigates existence and nonexistence of positive solutions to the Lane-Emden equation −Δu = Q|u|^{p−2}u on the integer lattice ℤ^d (d≥2), as well as on the half-space and quadrant with zero Dirichlet boundary conditions. The potential Q is positive and satisfies Q(x)∼(1+|x|)^{-α} for α∈[0,∞]. Existence is established via variational methods (mountain-pass geometry and Palais-Smale condition) in the Sobolev supercritical regime of the parameter pair (α,p). Nonexistence is proved in the Serrin subcritical regime by iterative pointwise decay estimates leading to a contradiction at infinity. An intermediate region between the two critical lines remains open in the full space and half-space but is absent in the quadrant.

Significance. If the central claims hold, the work supplies a discrete counterpart to classical weighted Lane-Emden theory, clarifying how the decay rate α of the potential and the geometry of the domain (full space vs. half-space vs. quadrant) determine the critical thresholds. The adaptation of variational methods and comparison lemmas to the lattice setting, together with the explicit disappearance of the intermediate region under Dirichlet conditions in the quadrant, constitutes a concrete advance that could guide further discrete or graph-based studies.

major comments (2)
  1. [§3.2] §3.2, the statement of the weighted Sobolev embedding used for the mountain-pass geometry: the constant in the embedding appears to depend on α through the weight, yet the super-criticality claim is phrased as if the critical line is independent of this dependence; a brief verification that the geometry persists uniformly for α in the claimed range would strengthen the argument.
  2. [§5.1] §5.1, the base step of the iterative decay argument: the comparison function chosen to initiate the decay iteration is constructed from the fundamental solution of the discrete Laplacian; it is not immediately clear whether the positivity and asymptotic equivalence of Q are sufficient to absorb the error terms when α=0, which is the boundary case of the Serrin regime.
minor comments (3)
  1. [§2.1] The definition of the graph Laplacian Δ on ℤ^d is introduced without an explicit formula; adding the standard expression Δu(x) = ∑_{y∼x} (u(y)−u(x)) would remove ambiguity for readers unfamiliar with graph Laplacians.
  2. [Theorem 1.1 and Theorem 1.2] In the statement of the main theorems, the precise location of the Sobolev and Serrin critical lines in the (α,p)-plane is described only qualitatively; including the explicit formulas for the critical exponents (even if standard) would improve readability.
  3. [Figure 1] Figure 1 (the phase diagram) uses shading that is difficult to distinguish in black-and-white print; a hatching pattern or clearer labels for the open intermediate region would help.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the positive recommendation for minor revision. The two major comments identify points where additional clarification would improve the presentation. We address each comment below and will incorporate the suggested details into the revised version.

read point-by-point responses
  1. Referee: [§3.2] §3.2, the statement of the weighted Sobolev embedding used for the mountain-pass geometry: the constant in the embedding appears to depend on α through the weight, yet the super-criticality claim is phrased as if the critical line is independent of this dependence; a brief verification that the geometry persists uniformly for α in the claimed range would strengthen the argument.

    Authors: We thank the referee for this observation. The constant in the weighted Sobolev embedding does depend on α. However, the Sobolev critical line itself is independent of the specific value of the embedding constant, and the mountain-pass geometry can be verified uniformly over α ∈ [0, ∞] by absorbing the α-dependence into the choice of radius and test functions. We will insert a short paragraph immediately after the embedding statement in §3.2 that explicitly checks the geometry for the full range of α, confirming that the required inequalities hold with constants that may depend on α but do not affect the location of the critical line. revision: yes

  2. Referee: [§5.1] §5.1, the base step of the iterative decay argument: the comparison function chosen to initiate the decay iteration is constructed from the fundamental solution of the discrete Laplacian; it is not immediately clear whether the positivity and asymptotic equivalence of Q are sufficient to absorb the error terms when α=0, which is the boundary case of the Serrin regime.

    Authors: We agree that the base step merits a more explicit treatment when α=0. In this case the assumption Q(x) ∼ (1+|x|)^0 yields positive constants m and M such that m ≤ Q(x) ≤ M everywhere. The comparison function εΦ, where Φ is the fundamental solution of the discrete Laplacian, satisfies -Δ(εΦ) = ε δ_0. For large |x| the lower bound m on Q allows us to choose ε small enough that the nonlinear term Q(εΦ)^{p-1} is dominated by the linear term, absorbing the error arising from the asymptotic equivalence. We will expand the argument in §5.1 with the explicit choice of ε and the resulting inequality that initiates the iteration for α=0. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper establishes existence of positive solutions in Sobolev super-critical regions of (α, p) via standard variational methods (mountain-pass geometry and Palais-Smale condition) on the lattice, with the energy functional defined directly from the given positive potential Q satisfying the stated asymptotic. Nonexistence in Serrin sub-critical regions follows from iterative decay estimates at infinity that produce a contradiction, using comparison lemmas and cut-off arguments adapted to the discrete graph. The critical lines are obtained from standard weighted Sobolev embeddings and integral identities; the intermediate region and its domain dependence arise directly from boundary restrictions on admissible functions and decay rates. No step reduces by construction to a fitted input, self-defined quantity, or load-bearing self-citation chain; the derivation remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The paper rests on standard functional-analytic assumptions for graphs and the given asymptotic form of Q; no new entities are postulated and no parameters are fitted to data.

axioms (2)
  • domain assumption The d-dimensional integer lattice graph Z^d is equipped with the standard combinatorial Laplacian and the usual graph distance.
    This defines the setting in which the equation and the potential Q are posed.
  • domain assumption Variational methods apply once the energy functional satisfies appropriate coercivity or Palais-Smale conditions in the Sobolev super-critical regime.
    Invoked to obtain existence; location implicit in the statement that existence is established via variational methods.

pith-pipeline@v0.9.0 · 5747 in / 1487 out tokens · 48162 ms · 2026-05-21T20:52:13.644116+00:00 · methodology

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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.

  1. Sharp Criteria for the existence of positive solutions to Lane-Emden-type inequalities on weighted graphs

    math.AP 2026-04 unverdicted novelty 7.0

    A divergent volume-growth sum on weighted graphs forces every nonnegative solution of -Δu ≥ u^q to be zero.

Reference graph

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