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Local Shearer bound

T0 review · 1 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash

Pith's one-line read Every triangle-free graph admits a probability distribution on independent sets that contains each degree-$d$ vertex with probability at least $(1-o(1))\ln d/d$.

desk verdict A serious and likely correct line of attack on two 2018 conjectures, but the proof of the key weighted theorem has an unproven sign step that needs repair. read the letter →

arxiv 2501.00567 v1 pith:HZORBK4B submitted 2024-12-31 math.CO

classification math.CO MSC 05C1505C3505C6905C50
keywords triangle-freegraphsindependencenumberfractionalchromaticShearerboundlocaldemandsspectralradiusindependentsetsweightedgraphcoloring
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

The paper proves a local, degree-by-degree strengthening of Shearer's independence bound: in every triangle-free graph, one can choose a probability distribution over independent sets so that each vertex $v$ is included with probability at least $(1-o(1))\ln d(v)/d(v)$, where $d(v)$ is its degree. This is exactly the guarantee conjectured in 2018 for fractional coloring with local demands. As a consequence the paper confirms the predicted bound $\chi_f(G)\le(\sqrt{2}+o(1))\sqrt{n/\ln n}$ for $n$-vertex triangle-free graphs, proves an analogous bound in terms of the number of edges, and obtains a spectral bound $\chi_f(G)\le(1+o(1))\rho(G)/\ln\rho(G)$. A weighted version of the result is the engine behind all four applications.

What carries the argument

The carrying object is the function $f(x)$, the continuous extension of $(1-x+x\ln x)/(x-1)^2$ to $[0,\infty)$, with $f(0)=1$, $f(1)=1/2$, and $f(x)=(1-o(1))\ln x/x$ as $x\to\infty$; it satisfies the differential equation $x(x-1)f'(x)+(x+1)f(x)=1$. The proof of Theorem 2.1 inducts on the number of vertices, maintaining the smallest possible uniform slack $\delta_0$. At the contradiction step it perturbs weights by $w'(v)=w(v)\exp(\varepsilon w(N_G(v)))$ and mixes the distribution on $G$ with distributions on $G-N_G[u]$, using convexity of $f$ and the differential equation to make all lower-order terms cancel.

What would settle it

Take a triangle-free graph and positive weights with $w(N_G(v))>w(v)$ at some vertex $v$, and check the sign of $x f'(x) z-2\varepsilon^2$ with $x=w(N_G(v))/w(v)$ and $z=\varepsilon(w(v)-w(N_G(v)))$ for small $\varepsilon$; when it is positive, the inequality before "Rearranging yields" in the proof is invalid, so the stated weighted theorem would need a different argument.

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Extended reading notes

Core claim

The central claim is Theorem 2.1: for every triangle-free graph $G$ and every strictly positive vertex weight function $w$, there exists a probability distribution on the independent sets of $G$ with $\mathbb{P}_{I\sim D}[v\in I]\ge f(w(N_G(v))/w(v))$ for all $v$, where $f$ is Shearer's function, the continuous extension of $(1-x+x\ln x)/(x-1)^2$. The all-weights-one case gives the local Shearer bound of Theorem 1.2, and choosing weights from a Perron–Frobenius eigenvector gives the spectral bound. The authors view the weighted theorem as the real contribution, from which the $n$-vertex, edge, and spectral bounds follow by short arguments.

Load-bearing premise

The load-bearing premise is that a certain error term at the single vertex that violates the claimed probability is automatically nonpositive; the paper does not prove this sign condition, and it can fail when a vertex's neighborhood carries more total weight than the vertex itself.

Editorial extensions

If this is right

  • The local demand version of fractional coloring is confirmed: each vertex $v$ can be assigned a measurable subset of $[0,1]$ of measure $(1-o(1))\ln d(v)/d(v)$ so that adjacent vertices receive disjoint subsets.
  • The fractional chromatic number of every $n$-vertex triangle-free graph is at most $(\sqrt{2}+o(1))\sqrt{n/\ln n}$, matching the conjectured maximum.
  • For triangle-free graphs with $m$ edges, the fractional chromatic number is at most $(18^{1/3}+o(1))m^{1/3}/(\ln m)^{2/3}$, close to the conjectured $16^{1/3}$ version.
  • Every triangle-free graph satisfies $\chi_f(G)\le (1+o(1))\rho(G)/\ln\rho(G)$, saving a logarithmic factor over the classical spectral bound on the chromatic number.
  • The all-ones and Perron–Frobenius weight choices recover the original Shearer bound and the spectral bound from the same weighted theorem.

Reading between the lines

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

  • The sign condition that appears unproved in Theorem 2.1 may be repairable by letting $\varepsilon$ shrink with $x-1$ or by using a higher-order Taylor expansion; whether the weighted theorem survives this repair is independent of the truth of its corollaries.
  • The spectral proof only uses triangle-freeness to keep neighborhoods disjoint; the same eigenvector-weighting trick may extend to $K_{r+1}$-free graphs with the corresponding Shearer-type function.
  • The paper's existential compactness argument suggests an LP formulation; an efficient rounding algorithm for the independent-set distribution would make the local bound algorithmic, which the current proof does not provide.
  • Because the theorem's applications rely only on two specific weight choices, a counterexample to the full weighted statement would not necessarily refute any of Theorems 1.2, 1.4, 1.5, or 1.6.
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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

1 major / 4 minor

Summary. The paper proves a local strengthening of Shearer's independence bound for triangle-free graphs: for every triangle-free graph there is a probability distribution on its independent sets such that each vertex v is included with probability (1-o(1)) ln d(v)/d(v). The proof is organized around a weighted technical theorem, Theorem 2.1, proved by induction together with a compactness/linear-programming argument. From Theorem 2.1 the authors derive the conjectured fractional chromatic number bound of Cames van Batenburg et al. for n-vertex triangle-free graphs, an edge-count analogue, and the spectral bound chi_f(G) <= (1+o(1)) rho(G)/ln rho(G).

Significance. If the main theorem is correct, it resolves a 2018 conjecture of Kelly and Postle and confirms the subsequent conjectures of Cames van Batenburg et al.; the spectral application is also a natural and notable strengthening of the fractional version of Molloy's bound. The deduction of Theorems 1.2, 1.4, 1.5, and 1.6 from Theorem 2.1 is clean and mostly self-contained, with no fitted parameters. The key proof is, however, invalid at a load-bearing sign step, as detailed below.

major comments (1)
  1. [Section 2, proof of Theorem 2.1] The displayed chain immediately after 'Plugging this estimate into the above lower bound' replaces (1 - epsilon a)(f(x) + T) by f(x) + T - epsilon a f(x), where a = w(v) + w(N_G(v)) and T = x z f'(x) - 2 epsilon^2. This is a valid lower bound only if T <= 0, as the manuscript's underbrace '<0' asserts. No proof of T <= 0 is given, and the assertion is not generally true for the admissible positive weight assignments: for example, with w(v) = 0.2, w(N_G(v)) = 0.6 (so x = 3), and epsilon = 0.01, one has z = -0.004, f'(3) approximately -0.0493, and T approximately 3.9 x 10^-4 > 0. Such weights are compatible with the proof because the weights are arbitrary strictly positive functions normalized to total weight 1. The subsequent 'Rearranging yields' step and the final contradiction delta_0^2/8 > delta_0^2/8 rely exactly on this sign: if T > 0, the same computation gives delta_0^2/8 > delta_0^2/8 - epsilon a T, which is true rather than absurd. Since the proof of Theorem 2.1 neither derives T <= 0 from the choice of the violating vertex nor offers an alternative argument, Theorem 2.1 is not established as written.
minor comments (4)
  1. [Section 2 and Theorem 2.1] The notation N_G is overloaded: in the statement of Theorem 2.1 it denotes the open neighborhood, while in the proof N_G(u) is explicitly redefined as the closed neighborhood. This makes later expressions such as x in V(G) \ N_G(v) ambiguous and should be clarified.
  2. [Section 2, proof of the Claim] In the Claim, the inequality replacing 1/(1 - epsilon w(N_G(v))) by 1 uses that w(N_G(y)) - w(v) >= 0 for y in N_G(v); this follows from v in N_G(y) and positivity of w, but it is not stated and should be made explicit.
  3. [Proof of Theorem 1.6] There is a typographical error in the line 'chi_f(G) <= 1/f(rho(G)))': an extra closing parenthesis appears after rho(G).
  4. [Abstract] The abstract contains several typographical artifacts, including 'W e' and 'cl assic'; these should be corrected in the final version.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the main proof is a self-contained induction and linear-programming argument, and prior results are used only as downstream applications, not as inputs to the key theorem.

full rationale

The paper's central result, Theorem 2.1, is proved by induction on the number of vertices, using the previously established Shearer function f and its elementary properties. The proof sets up a compact set K of slack parameters and derives a contradiction from the assumption that its minimum δ0 is positive. No parameter is fitted to the target data, and no 'prediction' is renamed from an input. The distribution D′ is constructed explicitly from the induction hypothesis and the assumed distribution D, and the contradiction is algebraic. The use of Kelly and Postle's Proposition 3.1 occurs only in the proof of Theorem 1.4, after Theorem 1.2 has already been established via Theorem 2.1, and it is not a self-citation by the present authors. The various deductions in Section 3 are straightforward applications of Theorem 2.1 or Theorem 1.2. There is no self-definitional reduction, no fitted input called a prediction, and no load-bearing self-citation. A possible gap in the proof of Theorem 2.1 concerning a dropped term of uncertain sign is a correctness concern, not a circularity concern: even if the step were unjustified, it would not make the theorem an input to itself. The paper is self-contained against external benchmarks, so the appropriate circularity score is 0.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

No fitted constants appear in the results. The main theorem rests on standard analytic properties of Shearer's function and linear-programming compactness. The proof also implicitly assumes a sign condition in its final algebraic step; this is an ad hoc and unproved assumption.

assumptions (4)
  • standard math Shearer's function f is convex, decreasing, differentiable on (0, infinity), satisfies x(x-1)f'(x)+(x+1)f(x)=1, and |x f'(x)|<1.
    Stated in Section 2 as 'It is not hard to check'; elementary but a nontrivial input to the proof.
  • standard math The linear program defining K_w is feasible and bounded, and K is compact so the minimum delta_0 is attained.
    Used in Section 2 to define delta_0 and to apply the induction hypothesis.
  • standard math Perron-Frobenius theorem for nonnegative matrices yields a positive eigenvector for a connected graph.
    Used to prove Theorem 1.6 from Theorem 2.1 by taking the Perron vector as the weight function.
  • ad hoc to paper Unstated sign condition x f'(x) z - 2 epsilon^2 <= 0 at the violated vertex.
    The displayed inequality before 'Rearranging yields' appears to require this sign; it is not proved and is false for some positive weight assignments, so the central proof depends on an unjustified assumption.

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Cite this review

Pith. "Pith review of Local Shearer bound." pith.science (2026). https://pith.science/paper/HZORBK4B

@misc{pith2026250100567,
  author       = {Pith},
  title        = {Pith review of: Local Shearer bound},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HZORBK4B}},
  note         = {Machine review of arXiv:2501.00567}
}
abstract

We prove the following local strengthening of Shearer's classic bound on the independence number of triangle-free graphs: For every triangle-free graph $G$ there exists a probability distribution on its independent sets such that every vertex $v$ of $G$ is contained in a random independent set drawn from the distribution with probability $(1-o(1))\frac{\ln d(v)}{d(v)}$. This resolves the main conjecture raised by Kelly and Postle (2018) about fractional coloring with local demands, which in turn confirms a conjecture by Cames van Batenburg et al. (2018) stating that every $n$-vertex triangle-free graph has fractional chromatic number at most $(\sqrt{2}+o(1))\sqrt{\frac{n}{\ln(n)}}$. Addressing another conjecture posed by Cames van Batenburg et al., we also establish an analogous upper bound in terms of the number of edges. To prove these results we establish a more general technical theorem that works in a weighted setting. As a further application of this more general result, we obtain a new spectral upper bound on the fractional chromatic number of triangle-free graphs: We show that every triangle-free graph $G$ satisfies $\chi_f(G)\le (1+o(1))\frac{\rho(G)}{\ln \rho(G)}$ where $\rho(G)$ denotes the spectral radius. This improves the bound implied by Wilf's classic spectral estimate for the chromatic number by a $\ln \rho(G)$ factor and makes progress towards a conjecture of Harris on fractional coloring of degenerate graphs.

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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. Full citation record

  1. Triangle-free $d$-degenerate graphs have small fractional chromatic number

    math.CO 2025-01 accept novelty 8.0 of 10

    Every triangle-free d-degenerate graph has fractional chromatic number at most (4+o(1))d/ln d, confirming Harris's conjecture.

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