REVIEW 2 major objections 5 minor 36 references
Robustness of the Sauer-Spencer Theorem
T0 review · 2 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read The paper proves that the Sauer–Spencer embedding theorem remains true after random sparsification: for hosts meeting the Sauer–Spencer minimum-degree condition, keeping each edge with probability $p \ge C n^{-1/m_1(H)} \log n$ still…
desk verdict Robust Sauer-Spencer for all bounded-degree spanning subgraphs, with a genuinely new spread blow-up lemma—but the load-bearing constants live in an earlier arXiv preprint. 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 central new object is a $q$-vertex-spread probability measure on the set $\mathrm{Emb}(H,G)$ of embeddings: a measure under which any fixed assignment of $k$ distinct vertices of $H$ to distinct vertices of $G$ has probability at most $(C/n)^k$. The theorem that carries the proof is the spread blow-up lemma (Theorem 5.6), which shows that whenever the usual blow-up lemma conditions hold (regular and super-regular partitions, an $R$-partition of $H$, an $(\alpha,R')$-buffer, and image restrictions), such a $(C_S/n)$-vertex-spread measure exists on the embeddings that respect the restrictions. The mechanism inside it is a probabilistic analysis of the random greedy algorithm of the earlier sparse blow-up lemma specialised to $p=1$: the main vertices are embedded one by one uniformly from candidate sets of size $\Theta(n)$, producing the $1/n$ spread factor, and the buffer vertices are then completed by a random perfect matching in an auxiliary bipartite graph whose matching measure is shown to be $O(1/n)$-spread by coupling a binomial random subgraph with a uniformly random subgraph and proving Hall's condition holds with probability at least $1/2$. A second mechanism is the extension threshold $\delta_{\rm e}(\Delta)$, used in Lemma 6.3 to turn an embedding of a small induced subgraph of $H$ into a full size-compatible $R$-partition with buffers, and this is the only place the minimum-degree condition enters.
What would settle it
Run the random greedy algorithm at $p=1$ on a concrete $\varepsilon$-regular partition equipped with a $K_{\Delta+1}$-factor and record the candidate-set size at every step along with the final buffer candidate counts; if any step has fewer than $(1/10)\mu\zeta|V(x)|$ available vertices, or any buffer vertex has fewer than $(1/2)\mu d^b|V_i|$ candidates, then the $O(1/n)$-spread bound of the spread blow-up lemma fails and the probability threshold in Theorem 1.5 would need a different proof.
Extended reading notes
Core claim
The central claim is Theorem 1.5: for all $\gamma>0$ and $\Delta \in \mathbb{N}$ there is a constant $C$ such that whenever $\Delta(H)\le \Delta$ and $\delta(G)\ge(\delta_{\rm e}(\Delta)+\gamma)n$, the random subgraph $G(p)$ contains $H$ with high probability for $p\ge Cn^{-1/m_1(H)}\log n$. Since the paper proves $\delta_{\rm e}(\Delta)\le(2\Delta-1)/(2\Delta)$, Theorem 1.3 follows: the classic Sauer–Spencer minimum-degree condition $\delta(G)\ge((2\Delta-1)/(2\Delta)+\gamma)n$ is enough for the robust statement. The proof path is: extend the Sauer–Spencer switching argument to complete any partial embedding on at most $\eta n$ vertices; use Szemerédi regularity to obtain a reduced graph $R$ with a spanning $K_{\Delta+1}$-factor $R'$; use the extension property to partition $H$ compatibly with $R$ and to set aside buffer vertices; apply the spread blow-up lemma to obtain a $O(1/n)$-vertex-spread measure on embeddings of $H$ into $G$; then convert this to a $O(n^{-1/m_1(H)})$-spread measure on copies of $H$ and invoke the spread-measure threshold theorem to conclude that $G(p)$ contains $H$.
Load-bearing premise
The load-bearing assumption is that the random greedy algorithm used inside the spread blow-up lemma always has a candidate set of size at least a fixed fraction of the part at every step, and that the buffer-vertex candidate counts are as large as the earlier sparse blow-up lemma claims; those bounds are imported without being reproved here.
Editorial extensions
If this is right
- The theorem proves that every bounded-degree spanning graph $H$ is embedded in $G(p)$ with high probability once $p \ge C n^{-1/m_1(H)} \log n$, so the Sauer–Spencer property is robust to random edge deletion.
- The bound on $p$ is optimal up to a logarithmic factor over the class of bounded-degree graphs with a fixed maximum 1-density, and for graphs with $m_1(H)=(\Delta+1)/2$ (for instance $K_{\Delta+1}$-factors) the improved probability $p\ge C n^{-2/(\Delta+1)}(\log n)^{1/((\Delta+1)/2)}$ is optimal up to a constant.
- The spread blow-up lemma gives a $O(1/n)$-vertex-spread measure on all embeddings satisfying the blow-up lemma's conditions, so any embedding problem that fits the blow-up lemma automatically inherits spread information.
- If the conjecture $\delta_{\rm e}(\Delta)=\Delta/(\Delta+1)$ holds, the robust embedding theorem and its optimal $p$ would hold at the Bollobás–Eldridge–Catlin minimum-degree threshold, matching the conjectured extremal bound.
Reading between the lines
- The paper leaves implicit a counting corollary: a $O(1/n)$-vertex-spread measure supported on $\mathrm{Emb}(H,G)$ forces the number of embeddings of $H$ in a dense host $G$ at degree $\delta_{\rm e}(\Delta)+\gamma$ to be exponentially large, so the copy count jumps discontinuously at this minimum-degree threshold; this could be made quantitative by integrating the spread measure.
- Because the spread blow-up lemma works with the entire reduced graph rather than a bounded-size part of it, it should transfer to other robust embedding problems, such as embeddings into perturbed random graphs or into pseudorandom hosts, where the prior spread blow-up lemma was too weak.
- A sharper value of $\delta_{\rm e}(\Delta)$ could be located computationally for small $\Delta$ by searching for a host $G$ and a bounded-degree $H$ for which the partial-embedding extension property fails just below $(2\Delta-1)/(2\Delta)n$; the paper's proof shows this extension property is the only place the degree condition enters.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves a robust (random-subgraph) version of the Sauer–Spencer embedding theorem. For any Δ and γ>0, if an n-vertex host G has minimum degree at least (δ_e(Δ)+γ)n—in particular at least ((2Δ-1)/(2Δ)+γ)n—and H is an n-vertex graph with maximum degree at most Δ, then the random subgraph G(p) contains a copy of H with high probability for p ≥ C n^{-1/m_1(H)} log n. The proof introduces an extension threshold δ_e(Δ), proves δ_e(Δ) ≤ (2Δ-1)/(2Δ) via a switching argument (Lemma 3.1), and establishes a vertex-spread version of the Allen–Böttcher–Hàn–Kohayakawa–Person blow-up lemma (Theorem 5.6). The spread blow-up lemma is proved by reanalysing the random greedy algorithm and the Hall-type buffer-embedding step of [4], and it is combined with the Frankston–Kahn–Narayanan–Park spread threshold theorem. The paper also proves an improved probability bound for graphs H of maximal 1-density and contains an optimality discussion in Section 9.
Significance. Assuming correctness, this is a substantial advance: it shows that the Sauer–Spencer minimum degree condition is robust at a value of p that is optimal up to a log factor, and it introduces a spread blow-up lemma that applies to the entire reduced graph, which is likely to be useful for further robust embedding problems. The paper is well organised, states explicit constants in the main chain, and the switching proof of Lemma 3.1 is elegant. The reduction to a O(1/n)-vertex-spread measure and the use of the FKNP spreadness theorem are well motivated. The optimality discussion and Theorem 9.2 add value. The main caveat is the heavy dependence of Theorem 5.6 on the external preprint [4] for the random-greedy-algorithm invariants.
major comments (2)
- [Section 7, proof of Theorem 5.6] The construction of the (C_S/n)-vertex-spread measure is not self-contained at the decisive step. Lemma 7.1 uses (RGA 1), derived from (INV 3), and Lemma 7.4 uses (FB 1)–(FB 3), derived from (GPE 3), (PRGA 2), and (PRGA 3). These four invariants are imported without proof from [4, Lemma 4.1] and are only stated as 'rephrased for p=1'. Since the O(1/n)-vertex-spread bound—and hence the p = O(n^{-1/m_1(H)} log n) threshold in Theorem 1.5—is exactly as strong as these constants, this dependency is load-bearing. Please either give a self-contained proof of the p=1 case of these invariants, or state the precise lemma from [4] with all quantifiers (including how μ, ζ, ρ depend on the input parameters) and verify explicitly that its hypotheses are satisfied in the dense setting of Theorem 5.6. As [4] is an arXiv preprint, this link is currently the least externally verified part of the chain.
- [Section 8.1, Lemma 6.2] The quantifier order in Lemma 6.2 and its use in the proof of Theorem 1.5 needs clarification. Lemma 6.2 is stated 'for all ε>0', but the proof chooses auxiliary constants ε' and d' defined in terms of ε and needs inequalities such as d' ≤ γ/4 and ε' ≤ ε*_0. If ε is not sufficiently small, these inequalities can fail; in particular, d' = 2d + 4ε'Δ + 4Δ√ε' is not bounded by γ/4 for arbitrary ε. In the proof of Theorem 1.5, ε is first returned by Theorem 5.6 and only then fed into Lemma 6.2. Please reorder the quantifiers: either state Lemma 6.2 for all sufficiently small ε, or allow ε to be chosen as small as required before applying Theorem 5.6.
minor comments (5)
- [Section 7, definition of C_buf(x)] In the second-step paragraph, the candidate set for a buffer vertex is defined as C_buf(x) = N*_G(φ_m(N_H(x; X_main); V_buf_i), with no intersection with the image restriction I_x. To justify the claim that the final embedding respects the image restrictions in Theorem 5.6, the definition should include ∩ I_x (or the text should clarify that the candidate sets are taken relative to the image restrictions). For the main theorem this issue does not arise because ζ=1 and I_x=V_i, but Theorem 5.6 as stated is affected.
- [Section 7, Lemma 7.4, Case II] The displayed union-bound chain in Case II contains an unnecessarily crude intermediate estimate: after bounding each term by (C_2 λ^{-2(C-4)/3})^k, the text writes '≤ Σ_{k≤λ^{1/3}} C_2 λ^{-2/3}', which loses the factor k in the exponent. The final conclusion is correct, but rewriting the sum as a geometric sum would improve clarity.
- [Section 6, proof of Theorem 1.5] The reduction performed when H has fewer than n/4 edges should explicitly treat the case E(H)=∅, since m_1(H) is undefined for the empty graph. This case is trivial, but it should be stated for completeness.
- [Section 8.2, Step I] The lower bound on |B_0| implicitly uses that n is sufficiently large, so that the equitable Hajnal–Szemerédi partition of eH into Δ^6 parts gives a part of size at least (1-γ/4)n/Δ^6. Please state the required lower bound on n explicitly.
- [References] Reference [4] is listed as an arXiv preprint; if a published or revised version exists, the citation should be updated to the version actually used, and the exact location of the invariants (INV 3), (GPE 3), (PRGA 2), and (PRGA 3) should be given more precisely than a single Lemma 4.1 reference.
Circularity Check
No circularity: the proof derives the spread blow-up lemma from the authors' earlier independent blow-up lemma [4] rather than assuming the target theorem, and no fitted input is renamed as a prediction.
full rationale
The derivation chain is self-contained and non-circular. Theorem 1.5 is proved by constructing a vertex-spread measure via the spread blow-up lemma (Theorem 5.6), converting it to a spread measure (Theorem 5.5), and applying the Frankston--Kahn--Narayanan--Park threshold theorem (Theorem 5.2). The only imported non-elementary ingredient is the blow-up lemma and its proof invariants from [4], which is a prior independent work by three of the present authors. Although Section 7 relies on the random greedy algorithm invariants (INV3), (GPE3), (PRGA2), and (PRGA3) from [4, Lemma 4.1] without reproof, these are cited as an external proven lemma with stated assumptions that do not include the Sauer--Spencer robustness conclusion; the paper explicitly says its proof 'results from a careful probabilistic analysis of the key steps used in the proof of the sparse blow-up lemma in [4]', i.e. it derives the spread version rather than assuming it. The extension threshold delta_e(Delta) is defined as a property and then bounded above by Lemma 3.1, so Theorem 1.5 is not using its conclusion as an input. No parameter is fitted to data, and no 'prediction' is a renamed input. The self-citation is load-bearing but not circular, because [4] is a separate published result with independent content; under the stated rules this does not raise the circularity score.
Assumptions & free parameters
assumptions (7)
- standard math Szemerédi regularity lemma and its degree form (Theorem 4.1, Corollary 4.1.1)
- standard math Hajnal-Szemerédi theorem and its K_r-factor corollary (Theorem 2.1, Corollary 2.1.1)
- domain assumption Blow-up lemma of Allen, Böttcher, Hàn, Kohayakawa and Person [4, Lemma 7.1] (Lemma 4.8)
- domain assumption Internal RGA invariants (INV 3), (GPE 3), (PRGA 2), (PRGA 3) from [4, Lemma 4.1]
- standard math Frankston-Kahn-Narayanan-Park threshold theorem (Theorem 5.2)
- standard math Pham-Sah-Sawhney-Simkin robust K_{Δ+1}-factor theorem (Theorem 9.4)
- standard math Hypergeometric Chernoff bound (Lemma 9.5)
Cite this review
Pith. "Pith review of Robustness of the Sauer-Spencer Theorem." pith.science (2026). https://pith.science/paper/LPL7ASNE
@misc{pith2026250703676,
author = {Pith},
title = {Pith review of: Robustness of the Sauer-Spencer Theorem},
year = {2026},
howpublished = {\url{https://pith.science/paper/LPL7ASNE}},
note = {Machine review of arXiv:2507.03676}
}
abstract
We prove a robust version of a graph embedding theorem of Sauer and Spencer. To state this sparser analogue, we define $G(p)$ to be a random subgraph of $G$ obtained by retaining each edge of $G$ independently with probability $p \in [0,1]$, and let $m_1(H)$ be the maximum $1$-density of a graph $H$. We show that for any constant $\Delta$ and $\gamma > 0$, if $G$ is an $n$-vertex host graph with minimum degree $\delta(G) \geq (1 - 1/2\Delta + \gamma)n$ and $H$ is an $n$-vertex graph with maximum degree $\Delta(H) \leq \Delta$, then for $p \geq Cn^{-1/m_1(H)}\log n$, the random subgraph $G(p)$ contains a copy of $H$ with high probability. Our value for $p$ is optimal up to a log-factor. In fact, we prove this result for a more general minimum degree condition on $G$, by introducing an \emph{extension threshold} $\delta_{\rm e}(\Delta)$, such that the above result holds for graphs $G$ with ${\delta(G) \geq (\delta_{\rm e}(\Delta) + \gamma)n}$. We show that $\delta_{\rm e}(\Delta) \leq (2\Delta-1)/2\Delta$, and further conjecture that $\delta_{\rm e}(\Delta)$ equals $\Delta/(\Delta+1)$, which matches the minimum degree condition on $G$ in the Bollob\'as-Eldridge-Catlin Conjecture. A main tool in our proof is a vertex-spread version of the blow-up lemma of Allen, B\"{o}ttcher, H\`{a}n, Kohayakawa, and Person, which we believe to be of independent interest.
Reference graph
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