REVIEW 4 minor 30 references
The paper proves that, for any r-vertex graph F and any host graph H above the critical chromatic degree threshold, a random induced subgraph H[p] contains an F-factor with asymptotic probability at least 1/(rq), where q is the order of the
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-07-31 23:57 UTC pith:V3VUK5IU
load-bearing objection Generalizes recent random-induced-subgraph factor results from Hamilton cycles and cliques to all F-factors, with a genuinely new lattice-coset mechanism; the proof holds up, though one exponent is garbled.
On the number of factorable induced subgraphs
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central assertion is Theorem 1.5: for every r-vertex k-chromatic graph F and every gamma > 0, if H is an n-vertex graph with minimum degree at least (1 - 1/chi_cr(F) + gamma)n, then for each fixed p in (0,1), the random induced subgraph H[p] contains an F-factor with probability at least 1/(rq) - o_n(1). Here q is the order of the coset group Q(P, L^mu_{P,F}(H)) for a suitable partition P, and q is bounded by (2r-1)^r. The factor 1/r reflects the necessary divisibility of the sampled vertex count; the new factor 1/q reflects deeper divisibility constraints of the lattice generated by the index vectors of robustly many copies of F in H. The probability is asymptotically best possible for
What carries the argument
The central object is the robust index-vector lattice. Given a partition P = {V0, V1, ..., Vd} of the host graph H, the index vector of a copy of F records how many of its vertices lie in each non-exceptional part. The lattice L^mu_{P,F}(H) is the additive subgroup of Z^d generated by all r-vectors that occur as index vectors of at least mu n^r copies of F. Its coset group Q(P, L) = L^d_max / L has order q and encodes the divisibility obstructions to packing F. The proof shows that this lattice is inherited by the random induced subgraph at a rescaled threshold (Lemma 4.1), and that the sampled index vector lands in the correct coset with probability asymptotically 1/(rq) via a lattice-point
Load-bearing premise
The proof relies on the assumption that the collection of copy index vectors of F that occur with more than a fixed density in H is essentially unchanged after random vertex deletion, once the density threshold is rescaled; if random sampling deletes too many copies of a borderline index-vector type, the lattice and the 1/(rq) probability can fail.
What would settle it
Take F = K_{2,4} and H = K_{n/2 - t, n/2 + t} with the natural bipartition; the paper's Example 2.2 gives q = 2, so the theorem predicts that a p-random induced subgraph is factorable with probability tending to 1/12 for every fixed p. A direct count of subsets inducing a K_{2,4}-factor for large n, or a simulation for moderately large n, should produce a limit of 1/12. Finding any host H satisfying the degree condition for which this fraction does not approach the predicted 1/(rq), or for which the robust index-vector set changes after sampling, would falsify the central claim.
If this is right
- At least a 1/(rq) - o(1) fraction of all vertex subsets of H induce F-factors, regardless of whether H itself has an F-factor.
- For fixed p, the probability 1/(rq) is asymptotically best possible for infinitely many host graphs, so no universal improvement beyond this value is possible without extra assumptions.
- In hypergraphs, the same lattice mechanism gives asymptotic probabilities 1/(kq) for perfect matchings under minimum degree conditions, improving to 1/(k(s-1)) under minimum codegree conditions; these are again asymptotically tight.
- When every possible copy index vector of F in H is mu0-robust, the probability is exactly 1/(rq) ± epsilon, not merely a lower bound.
Where Pith is reading between the lines
- Because the authors note their bound on p is likely not optimal, one natural extension is to find the true threshold on p below which the robust index-vector lattice is not faithfully inherited; if such a threshold exists, the 1/(rq) probability would hold for much sparser induced subgraphs.
- The coset-membership view suggests a randomized algorithmic corollary: sample a vertex subset, test whether its index vector lies in the correct coset, and only then run the deterministic factor algorithm; the success probability would be exactly the stated 1/(rq).
- The authors conjecture that q decreases to 1 when the minimum degree is raised to the ordinary chromatic threshold; a concrete test would be to compute q for balanced blow-ups of F and check monotonicity as the host graph becomes denser.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies F-factors in random induced subgraphs of dense graphs and hypergraphs. Its main structural result (Theorem 3.2) asserts that, under a minimum-degree condition and a lattice condition coming from robust index vectors, the probability that H[p] contains an F-factor is at least 1/(rq) - o(1), where q is the order of a coset group associated with H. Applications include graph F-factors under the critical-chromatic-number threshold (Theorem 1.5), hypergraph perfect matchings under minimum ℓ-degree conditions (Theorem 1.6), and perfect matchings under codegree conditions (Theorem 1.7). The proof combines concentration inequalities, a Gauss-type lattice-point counting argument, and prior deterministic structural theorems of Han–Treglown and others. Sharpness constructions show the winning probabilities are asymptotically best possible for many H.
Significance. If the result holds, it is a substantial contribution to the study of spanning structures in random induced subgraphs. It moves beyond clique factors and gives a general lattice-based explanation of the probability 1/(rq): the 1/r factor is the usual divisibility of the sampled order, and the 1/q factor reflects the coset structure of the robust-index lattice. The paper also provides matching upper-bound constructions. The proof is detailed and internally coherent; the inheritance step (Lemma 4.1(V4)) is justified by Claim 4.4 and the concentration bounds of Lemma 4.3, and the use of prior deterministic theorems is not circular. The main issue is that several displayed definitions and constants are garbled or too optimistic, but these are local and repairable.
minor comments (4)
- [Theorems 1.5–1.7] The displayed definitions of h are garbled and do not match the proofs. From the values t=2r^{k-1}-1, t=2s-1, and t=2s-2 used in the proofs, Theorem 3.2 gives h=4r^k-2r-2, h=max{2k, 2k(2s-1)-2}, and h=max{2k, 4k(s-1)-2}, respectively. The current Theorem 1.5 statement even appears to have a division-by-zero issue for r=2. These should be corrected.
- [Proof of Theorem 3.2, (U1)–(U3)] After randomly splitting U' into two almost equal parts, the constants in the displayed events are too large. For a family of s-sets, the expected fraction that falls into one half is 2^{-s}, so (U1), (U2), and (U3) should have constants divided by 2^{r-1}, 2^{tr-1}, and 2^r, respectively. In particular, (U2) as written cannot follow from (V3) and Lemma 2.5 when |U^-|≈|U'|/2. The hierarchy can absorb the smaller constants, so the proof is repairable, but the current text is inaccurate.
- [Proof of Theorem 3.2, Step 1] In the greedy covering of V'_0, a previously chosen copy F_j has r vertices in U^+, so it eliminates at most r m^{r-2} candidate (r-1)-sets, not (r-1)m^{r-2}. The constant is immaterial because ρ is tiny, but the displayed bound should be corrected.
- [Theorem 3.2, 'In particular' part] The proof of the equality statement explicitly sets V0=∅. This condition is not stated in the 'In particular' sentence. If the robustness condition is interpreted as applying to every copy of F, then property (i) may force V0 to be empty, but the statement should make this explicit for clarity.
Circularity Check
No significant circularity found; the probability bound is derived from a new lattice-point-counting argument and external deterministic theorems, not from assuming the target result.
full rationale
The central claim in Theorem 1.5 is not obtained by fitting or by renaming an input. For each H in the stated class, q is defined as the order of the coset group Q(P, L^mu_{P,F}(H)) built from robust index vectors of F-copies in H. Lemma 4.2 then proves, by Gauss lattice-point counting, that the sampled index vector falls in any fixed coset of L^mu_{P,F}(H) with probability 1/(rq) +/- epsilon. This is a genuine counting result, not a consequence of the definition of q. The lower bound P[H[p] contains an F-factor] >= 1/(rq) - epsilon follows by combining this uniform coset distribution with Lemma 4.1 (inheritance of the structural hypotheses) and the external deterministic criterion Theorem 3.1. Lemma 4.1(V4), flagged in the reader's summary, is not an unproven inheritance assumption: it is established by Claim 4.4, which produces a stable plateau mu* with I^{mu*}_{P,F}(H)=I^{mu*/4}_{P,F}(H), and by the concentration bounds of Lemma 4.3, giving I^{mu*}_{P,F}(H)=I^{mu*/2}_{P',F}(H') with high probability. The sharpness examples in Section 5 use the same Lemma 4.2 to exhibit graphs for which the factor event is contained in the coset event of probability 1/(rq), so the upper bound is derived, not assumed. The cited results from Han-Treglown, Han, and Han-Zhao are external published theorems whose assumptions do not include the random-induced-subgraph conclusion; the fact that some authors overlap with the present paper does not make the derivation circular. No equation in the paper reduces to its inputs by construction, and no fitted parameter is renamed as a prediction. Therefore the appropriate circularity score is 0.
Axiom & Free-Parameter Ledger
axioms (6)
- domain assumption Han–Treglown deterministic lattice criterion (Theorem 3.1) is correct and applicable.
- domain assumption Good-partition and coset-bound lemmas from [13,12,14,9,3] (Lemmas 3.3, 3.4, 3.6–3.9, Proposition 3.5) hold.
- standard math Gauss lattice-point counting estimate (Lemma 2.6, from Tao–Vu) holds for all translates of a full-rank lattice.
- standard math Concentration inequalities (Hoeffding, McDiarmid, Liebenau–Wormald) are valid in the stated forms.
- standard math The hierarchy of constants can be ordered as 1/n0 ≪ 1/C′ ≪ β, μ0 ≪ ε, ρ, γ, c, η, 1/r, 1/D, 1/q, 1/d, 1/t.
- domain assumption The sharpness constructions (Construction 1, Construction 2, and the modular hypergraph construction in Section 5) satisfy the stated degree conditions.
read the original abstract
Let $F$ be an $r$-vertex graph. In this paper, we study the $F$-factor problem in random induced subgraphs of dense graphs. We show that for any $r$-vertex graph $F$ and $\gamma>0$, if $H$ is an $n$-vertex graph with minimum degree at least $(1-1/\chi_{cr}(F)+\gamma)n$, then for every fixed $p \in (0,1)$, the random induced subgraph $H[p]$ contains an $F$-factor with probability at least $1/(rq)-o_n(1)$, where $q\in \mathbb{N}$ is the order of certain coset group defined from $H$. The probability is asymptotically best possible for infinitely many $F$ and $H$ and yields that a $1/(rq)-o_n(1)$ proportion of the subsets of $H$ induce $F$-factors, interestingly, regardless of whether $H$ itself admits an $F$-factor. Similar results are obtained for perfect matchings in hypergraphs under minimum degree conditions. Our proof combines concentration inequalities, lattice point counting in $\mathbb{Z}^d$ and structural theorems for $F$-factors in dense (hyper)graphs.
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