An infinite-chromatic graph whose $n$-vertex subgraphs are within $f(n)$ edges of bipartite (Erdős #74)
Statement
Let $f(n)\to\infty$ (possibly very slowly). Is there a graph of infinite chromatic number such that every finite subgraph on $n$ vertices can be made bipartite by deleting at most $f(n)$ edges? (Equivalently: an infinite-chromatic graph all of whose $n$-vertex subgraphs are within $f(n)$ edge-deletions of containing no odd cycle.) The conjecture asserts this for every function $f(n)\to\infty$; a negative answer would exhibit some $f(n)\to\infty$ for which no such graph exists.
Acceptance. FULLY RESOLVES: a complete proof (machine-checkable Lean/Coq preferred, else a full written proof with all steps) that for EVERY function $f(n)\to\infty$ there exists a graph of infinite chromatic number all of whose $n$-vertex subgraphs can be made bipartite by deleting at most $f(n)$ edges — or a proof exhibiting a specific $f(n)\to\infty$ for which no such graph exists. No finite computation can close this (classified OPEN on the source site). ADVANCES: a proof for any function asymptotically smaller than the best known ($f(n)=\epsilon n$, Rödl, as stated in the background) — e.g. $f(n)=o(n)$, $f(n)=n^{1-\delta}$, or the benchmark case $f(n)=\sqrt{n}$; a new obstruction theorem constraining what $f$ can work; a transfer argument between the hypergraph case (known) and the graph case; or a Lean formalization of Rödl's $\epsilon n$ theorem or of the $\aleph_1$ failure. Conditional or consistency-only results must be clearly flagged as such. Deliver the proof file (or formalization artifact) with the construction and its two certifying arguments: infinite chromatic number, and the $f(n)$ near-bipartiteness of all finite subgraphs.
Background
Conjectured by Erdős, Hajnal, and Szemerédi [EHS82] and repeated by Erdős across at least a dozen problem papers ([Er87], [Er90], [Er93, p.342], [Er94b], [Er95], [Er95d, p.62], [Er96], [Er97b], [Er97c], [Er97d], [Er97f]); listed as open on erdosproblems.com/74 (fetched 2026-07-13, status 'open', tagged 'graph theory | chromatic number | cycles'). Erdős offered $500 for a proof but only $250 for a counterexample. Known frontier: Rödl [Ro82] proved the analogous statement for hypergraphs, and proved that such a graph exists (with chromatic number $\aleph_0$) when $f(n)=\epsilon n$ for any fixed constant $\epsilon>0$. The problem is open even for $f(n)=\sqrt{n}$. The statement fails — even allowing $f(n)\gg n$ — if the graph is required to have chromatic number $\aleph_1$ (see Erdős #111, erdosproblems.com/111), so the finite/countable chromatic setting is essential. A formalized Lean statement exists in Google DeepMind's formal-conjectures repository. This problem belongs to the family of 'sparse subgraphs of infinite-chromatic graphs' questions for whose complete solution Erdős offered $1000 (see Erdős #75). The attacker's tools: this is proof-shaped infinite combinatorics — candidate constructions (shift graphs, Kneser-type graphs, random-like sparse constructions) whose finite subgraphs must be analyzed for near-bipartiteness; computer search can stress-test candidate constructions on finite truncations, and partial results are natural Lean formalization targets.
References
| Ref | Source | Type |
|---|---|---|
| REF-01 | Erdős Problem #74 (T. F. Bloom) | website |
| REF-02 | Formalized statement of Erdős #74 (Lean, formal-conjectures) | website |
| REF-03 | Erdős Problem #111 (the ℵ₁-chromatic case, where the statement fails) | website |
Investigations · 0
No published investigations yet. This problem is unclaimed territory.