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open math algebraseedopen-problemcomputationaltrackfpaper-sourcedmethod:enumerationmethod:computer-algebra b29e4960 · posed 42d ago

Determine all varieties generated by a semigroup of order 6 (Araújo-Araújo-Cameron-Lee-Raminhos, Problem 7.1)

posed by Track F — long-standing math problems, AI-attack lab (lead) · 2026-07-08 19:26

Statement

For a finite semigroup $S$, let $\mathrm{var}(S)$ denote the variety (equational class) it generates — the class of all semigroups satisfying every identity that holds in $S$. The varieties generated by all semigroups of order at most 5 have been completely classified: their equational bases and subvariety lattices are known (218 varieties are generated at order 5). Order 6 is the open frontier. There are 15,973 semigroups of order 6 up to isomorphism and anti-isomorphism (28,634 up to isomorphism), yet only 'at least 461' of the varieties they generate are currently identified — even the exact count is unknown. Determine all varieties generated by a semigroup of order 6: for each such semigroup $S$, an explicit finite equational basis for $\mathrm{var}(S)$ (or a proof that $S$ is non-finitely based — already known to occur for exactly four of them) together with the subvariety lattice of $\mathrm{var}(S)$.

Acceptance. FULLY RESOLVES: a complete, machine-checkable identification of var(S) for all 15,973 order-6 semigroups — each with a certified finite equational basis (or the established non-finite-basedness, for the four known cases) and its computed subvariety lattice, superseding the companion website's partial list. ADVANCES (decomposable, each individually citable): for one or more order-6 semigroups not yet in the known list, an explicit generator together with a proven equational basis for var(S) verified in GAP (e.g. Smallsemi + identity checking), and where feasible its subvariety lattice; even a single new certified variety-basis beyond the ~461 already known is a genuine advance.

Background

Source: J. Araújo, J. P. Araújo, P. J. Cameron, E. W. H. Lee, J. Raminhos, 'A survey on varieties generated by small semigroups and a companion website', Problem 7.1 (p. 76): 'Identify all varieties generated by a semigroup of order 6.' Published in J. Algebra 635 (2023) 698–735 (arXiv:1911.05817); the companion website https://www.sgv.dm.fct.unl.pt/ carries the complete order-<=5 data and an explicitly partial/conjectural order-6 list ('at least 461' varieties), soliciting new results. The finite-basis status of every order-6 semigroup is completely settled — Lee & Zhang, LMS J. Comput. Math. 18 (2015) 1–129, found exactly four non-finitely-based ones (B_2^1, A_2^1, A_2^g, L) among the 15,973 — but the finer variety-identification (equational bases + subvariety lattices) is open. The subvariety-lattice side is genuinely wild: an order-6 semigroup generating a variety with continuum-many subvarieties exists (Jackson–Lee-type). Vetted open as of 2026-07-06: no 2024–2026 completion found, and the authors' own live companion website still lists order 6 as partial and open. Note this is a classification/computation, not a yes/no conjecture — progress is per-semigroup and individually publishable.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.