problems / caccetta-haggkvist-k3

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Research log — Caccetta–Häggkvist for directed triangles

2026-08-17

Fetched and read tonight, in the order used.

The conjecture

Caccetta–Häggkvist (1978): every simple n-vertex digraph of minimum out-degree at least r has a directed cycle of length at most ⌈n/r⌉. The first open case is r = n/3: every n-vertex oriented graph with δ⁺ ≥ n/3 has a directed triangle. (2-cycles already have length 2, so one reduces to oriented graphs.)

AIM workshop summary: Blair D. Sullivan, A Summary of Results and Problems Related to the Caccetta-Häggkvist Conjecture (14 Apr 2006). Local: compute/refs/sullivan-aim-caccetta.pdf. The Egres page timed out.

Published out-degree thresholds for a triangle

c source notes
(3−√5)/2 ≈ 0.3820 Caccetta–Häggkvist 1978 original paper
(2√6−3)/5 ≈ 0.3798 Bondy, Discrete Math. 1997 subgraph counting
3−√7 ≈ 0.3542 Shen, J. Combin. Theory Ser. B 74 (1998) one page
0.3532 Hamburger–Haxell–Kostochka, Electron. J. Combin. 14 (2007) uses Chudnovsky–Seymour–Sullivan; local compute/refs/hamburger-haxell-kostochka-eljc.pdf
0.3465 Hladký–Král'–Norin, Combinatorica 37 (2017); arXiv:0908.2791v4 flag algebras on F₄ + induction + CSS. Ancillary CH.mw. Local compute/refs/hladky-kral-norin-0908.2791.pdf
0.3388 de Joannis de Verclos–Sereni–Volec, March 2014 personal communication. HKN v4 footnote; Grzesik–Volec IMRN 2023 [18]. F₆, no public file
0.343545 (two-sided) Lichiardopol, Discrete Math. 2010 both δ⁺ and δ⁻; different theorem

Grzesik–Volec, Degree conditions forcing directed cycles, IMRN 2023 / arXiv:2102.12830v2, still quote HKN 0.3465 as the published unrestricted bound and mention 0.3388 only as a personal communication. Local compute/refs/grzesik-volec-2102.12830.pdf.

Exact r, additive error, classes

Razborov, arXiv:math/0604317: the conjecture holds if three specific 4-vertex digraphs are forbidden. Local compute/refs/razborov-math0604317.pdf. Not used tonight.

Behzad–Chartrand–Wall (regular girth-g cages) is implied by CH. Not attacked separately.

The cyclic construction

Out-neighbours {1,…,⌊(n−1)/3⌋} on ℤ/nℤ. C₃-free because three steps sum to at most 3⌊(n−1)/3⌋ < n. This meets the conjectured max min-outdegree, so the exact finite statement is δ⁺ ≥ ⌈n/3⌉ ⇒ C₃.

What HKN actually certify

Theorem 3.3: every homomorphism of the flag algebra of triangle-free oriented graphs has δα < 0.3465. The proof is emptiness of a set R(c) ⊂ ℝ³² of 4-vertex type densities, via a linear combination of

Tonight independently rebuilt every matrix and linear form (flags4.py, ind_fork.py). They match. The printed two-decimal aᵢ do not reproduce the printed expansion (4.22). Re-optimising the combination on the rebuilt system, the F₄ threshold is ≈ 0.346439. A stored certificate at 0.34645 (worst F = −0.12343) is certs/f4_certificate.json.

What we compare against

False or unused full-proof claims

None used. The 0.3388 communication is not taken as a theorem for the purpose of claiming a new threshold.

2026-08-27

Opened tonight, in order.

Failed lookup: no arXiv hit after 2024 that states a published unrestricted c < 0.3465 with a public certificate. The 0.3388 figure remains a personal communication.

Replay tonight: the stored F₄ certificate at c=0.34645 still has min eig(Q)=9.53e-9 and worst F=−0.12343. The same ray is negative at 0.34644447 and positive at 0.34644, so this system does not give a clean step below 0.34645.

2026-08-27 — q2 literature

Opened tonight, in order, before the n=21 SAT.

Failed lookup: no later Combinatorica / IMRN replacement of HKN 0.3465 with a public certificate. The 0.3388 figure remains a personal communication.

2026-08-27 — q3 literature

Opened tonight, in order, before the n=38 SAT.

Failed lookup: no later Combinatorica / IMRN replacement of HKN 0.3465 with a public certificate. The 0.3388 figure remains a personal communication. Do not treat it as published.

2026-08-27 — q4 literature

Opened tonight, in order, before the n=73 SAT and the CKLS-fork replay.

Failed lookup: no later Combinatorica / IMRN replacement of HKN 0.3465 with a public certificate. The 0.3388 figure remains a personal communication. Do not treat it as published.

2026-08-27 — F-coordinate dump, lemma chain

Re-opened for the independent 4Ψ(κ) / F₄ dump (no SAT).

2026-08-27 — wrap re-open (leftover through n=108)

Opened again before wrapping the leftover-SAT claim against the published record.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=113)

Opened again before wrapping leftover SAT past n=108.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=115)

Opened again before wrapping leftover SAT past n=114.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=116)

Opened again before wrapping leftover SAT past n=115.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=117)

Opened again before wrapping leftover SAT past n=116.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=118)

Opened again before wrapping leftover SAT past n=117.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=119)

Opened again before wrapping leftover SAT past n=118.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=120)

Opened again before wrapping leftover SAT past n=119.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=121)

Opened again before wrapping leftover SAT past n=120.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=122)

Opened again before wrapping leftover SAT past n=121.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=123)

Opened again before wrapping leftover SAT past n=122.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=124)

Opened again before wrapping leftover SAT past n=123.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=125)

Opened again before wrapping leftover SAT past n=124.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=126)

Opened again before wrapping leftover SAT past n=125.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=127)

Opened again before wrapping leftover SAT past n=126.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=128)

Opened again before wrapping leftover SAT past n=127.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=129)

Opened again before wrapping leftover SAT past n=128.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=130)

Opened again before wrapping leftover SAT past n=129.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=131)

Opened again before wrapping leftover SAT past n=130.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-27 — wrap re-open (leftover through n=132)

Opened again before wrapping leftover SAT past n=131.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — wrap re-open (leftover through n=133)

Opened again before wrapping leftover SAT past n=132.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — wrap re-open (leftover through n=134)

Opened again before wrapping leftover SAT past n=133.

Failed lookup: export.arxiv.org Caccetta+Haggkvist search returned HTTP 500 this session. No later Combinatorica replacement of HKN 0.3465 found on the abs pages opened above. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=135 (q26 start)

Opened before hunting leftover SAT past n=134.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=136 (q27 start)

Opened before hunting leftover SAT past n=135.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=137 (q28 start)

Opened before hunting leftover SAT past n=136.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=138 (q29 start)

Opened before hunting leftover SAT past n=137.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=139 (q30 start)

Opened before hunting leftover SAT past n=138.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=140 (q31 start)

Opened before hunting leftover SAT past n=139.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=141 (q32 start)

Opened before hunting leftover SAT past n=140.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=142 (q33 start)

Opened before hunting leftover SAT past n=141.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=143 (q34 start)

Opened before hunting leftover SAT past n=142.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-28 — leftover from n=145 (q35 start)

Opened before hunting leftover SAT past n=144.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=146 (q36 start)

Opened before hunting leftover SAT past n=145.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=147 (q37 start)

Opened before hunting leftover SAT past n=146.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=148 (q38 start)

Opened before hunting leftover SAT past n=147.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=149 (q39 start)

Opened before hunting leftover SAT past n=148.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=150 (q40 start)

Opened before hunting leftover SAT past n=149.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=151 (q41 start)

Opened before hunting leftover SAT past n=150.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.

2026-08-29 — leftover from n=152 (q42 start)

Opened before hunting leftover SAT past n=151.

Failed lookup: no later Combinatorica replacement of HKN 0.3465. Do not treat 0.3388 as published.