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arxiv: 1907.09950 · v1 · pith:FLL7KWTVnew · submitted 2019-07-23 · 🧮 math.CO

A rainbow blow-up lemma for almost optimally bounded edge-colourings

Pith reviewed 2026-05-24 17:26 UTC · model grok-4.3

classification 🧮 math.CO
keywords rainbow subgraphblow-up lemmaedge-coloringquasirandom graphspanning subgraphgraph decompositionbounded degree
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The pith

A rainbow blow-up lemma holds for edge-colorings that are almost optimally bounded.

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

The paper establishes a rainbow analogue of the Komlós–Sárközy–Szemerédi blow-up lemma that remains valid when the host graph's edge-coloring obeys a boundedness condition that is asymptotically tight. Under this condition, any quasirandom graph contains a rainbow copy of every spanning subgraph whose maximum degree is bounded by a constant. Sympathetic readers would care because the result transfers classical embedding power into the rainbow setting and immediately yields consequences for decompositions, orthogonal double covers, and labellings without requiring substantially stronger coloring hypotheses.

Core claim

We prove a rainbow version of the blow-up lemma of Komlós, Sárközy and Szemerédi that applies to almost optimally bounded colourings. A corollary of this is that there exists a rainbow copy of any bounded-degree spanning subgraph H in a quasirandom host graph G, assuming that the edge-colouring of G fulfills a boundedness condition that is asymptotically best possible. This has many applications beyond rainbow colourings, for example to graph decompositions, orthogonal double covers and graph labellings.

What carries the argument

The rainbow blow-up lemma, which embeds rainbow copies of bounded-degree graphs into dense quasirandom hosts whose edges satisfy an asymptotically tight per-color bound.

If this is right

  • Any quasirandom graph whose edges are colored under the stated boundedness condition contains a rainbow copy of every bounded-degree spanning subgraph.
  • The lemma supplies rainbow versions of many classical embedding results used in graph decompositions.
  • It yields new existence statements for orthogonal double covers via the same embedding mechanism.
  • It produces rainbow embeddings that can be repurposed for certain graph-labelling problems.

Where Pith is reading between the lines

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

  • The same boundedness condition may suffice for rainbow versions of other embedding lemmas that currently require stricter color restrictions.
  • One could check the result numerically on small random regular graphs to see how close the boundedness threshold must be in practice.
  • The technique might extend to hypergraph embeddings or to settings where only a fraction of the edges need to be rainbow.

Load-bearing premise

The edge-colouring must obey a per-color boundedness condition that is asymptotically the strongest possible for the embedding statement to remain true.

What would settle it

An explicit quasirandom graph together with an edge-coloring in which some color appears on more than the allowed number of edges (by a constant factor) yet no rainbow copy of a fixed bounded-degree spanning subgraph exists would refute the claim.

Figures

Figures reproduced from arXiv: 1907.09950 by Felix Joos, Stefan Ehard, Stefan Glock.

Figure 1
Figure 1. Figure 1: If x0 is mapped to v0 by σ, then only those candidates of xi remain that are neighbours of v0. Moreover, colour α of the edge v0vj is added to the the candidate edge xj vj , which captures the information that if xj is later embedded at vj , then this embedding uses α. We say that (H, G,(Ai)i∈[r]0 , c) is an (ε,(d G i )i∈[r] ,(di)i∈[r]0 , t,Λ)-embedding-instance if in ad￾dition, we have that • G[V0, Vi ] i… view at source ↗
read the original abstract

A subgraph of an edge-coloured graph is called rainbow if all its edges have different colours. We prove a rainbow version of the blow-up lemma of Koml\'os, S\'ark\"ozy and Szemer\'edi that applies to almost optimally bounded colourings. A corollary of this is that there exists a rainbow copy of any bounded-degree spanning subgraph $H$ in a quasirandom host graph $G$, assuming that the edge-colouring of $G$ fulfills a boundedness condition that is asymptotically best possible. This has many applications beyond rainbow colourings, for example to graph decompositions, orthogonal double covers and graph labellings.

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

0 major / 2 minor

Summary. The manuscript proves a rainbow version of the Komlós–Sárközy–Szemerédi blow-up lemma that holds for edge-colourings satisfying an asymptotically optimal boundedness condition. As a corollary, any bounded-degree spanning subgraph H admits a rainbow copy in a quasirandom host G under this colouring condition. The result is stated to have further applications to graph decompositions, orthogonal double covers and graph labellings.

Significance. If the central lemma holds, the work supplies a flexible embedding tool for rainbow problems at the natural density threshold. The optimality claim on the boundedness condition strengthens the result relative to earlier rainbow blow-up statements and directly supports the corollary on spanning subgraphs in quasirandom graphs.

minor comments (2)
  1. [Abstract] The abstract refers to an 'almost optimally bounded' colouring without stating the precise quantitative condition; the introduction or statement of the main lemma should give the exact bound (e.g., maximum colour degree o(n) or similar) so that the optimality claim can be verified immediately.
  2. [Section 2] Notation for the host graph G, the colouring, and the blow-up parameters should be introduced uniformly in §2 before the statement of the main lemma; occasional reuse of symbols from the classical blow-up lemma risks confusion.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive evaluation of the manuscript, the supportive summary of its contributions, and the recommendation for minor revision. No major comments were raised in the report.

Circularity Check

0 steps flagged

No significant circularity; direct proof of extended lemma

full rationale

The paper states a direct proof of a rainbow blow-up lemma extending the Komlós-Sárközy-Szemerédi result to almost optimally bounded edge-colourings, with a corollary for rainbow spanning subgraphs in quasirandom hosts. No self-definitional relations, fitted inputs renamed as predictions, load-bearing self-citations, uniqueness theorems imported from the authors' prior work, smuggled ansatzes, or renamings of known results appear in the abstract or claim structure. The boundedness condition is presented as an external asymptotic optimality assumption rather than an internal fit. The derivation chain is therefore self-contained as a mathematical existence proof.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Abstract-only review supplies no explicit free parameters, invented entities, or ad-hoc axioms; the work rests on the classical blow-up lemma and standard quasirandomness assumptions.

axioms (2)
  • standard math The blow-up lemma of Komlós, Sárközy and Szemerédi
    The new rainbow version is stated as an extension of this prior result.
  • domain assumption Quasirandomness of the host graph G
    Invoked for the corollary on spanning rainbow copies.

pith-pipeline@v0.9.0 · 5632 in / 1274 out tokens · 103643 ms · 2026-05-24T17:26:24.911340+00:00 · methodology

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Reference graph

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