Motivation
The cyclotomic Birman–Wenzl–Murakami algebra is an algebra of braids and contractions equipped with an additional cyclotomic relation. It connects diagram algebras, representation theory, and knot theoretic constructions. For calculations over a coefficient ring, one needs to know whether the diagrammatic spanning elements remain independent after parameters are specialized. The freeness theorem of Goodman and Hauschild Mosley answers that question for admissible integral ground rings: the algebra has a basis of the size predicted by labeled Brauer diagrams. The paper also identifies the algebra with a cyclotomic Kauffman tangle algebra. This mission targets the algebra's freeness and rank assertion.
The cited paper builds on admissibility relations developed with Wilcox and Yu. Its earlier definitions and preliminary statements already appear in Prove2Me mission 92e1197e-907f-4ab8-92be-01ea44f6cfef. The present mission reuses that published layer and poses the deferred freeness theorem and the intermediate results that support it. The distinction matters because two algebras with similar names but different generators or parameter conventions need not have the same rank statement.
Setting
A ground ring S is a commutative unital ring with a positive integer r and parameters ρ,q,δ0,δ1,…,u1,…,ur. The parameters ρ,q,δ0,u1,…,ur are units, and they satisfy
ρ−1−ρ=(q−1−q)(δ0−1).
The algebra Wn,S,r is generated over S by invertible braids gi, contractions ei, and an invertible affine generator y1. It obeys the affine BMW relations of Definition 2.1 and the cyclotomic relation ∏j=1r(y1−uj)=0 of Definition 2.8. The shared published presentation uses indices i=0,…,n−2 for gi and ei, corresponding to the paper's gi+1 and ei+1. At n=0 there are no generators.
The parameters are admissible when q−q−1 is nonzero and not a zero divisor, weak admissibility holds for every integer shift of the δ relations, and e1,y1e1,…,y1r−1e1 are S-linearly independent in W2,S,r. These are the conditions of Definition 3.3, expressed using condition (1) of Theorem 3.2. A morphism of ground rings preserves all δj and ui and permits exactly the four paired sign actions on ρ and q in Definition 2.13. Consequently, base change includes the sign twists allowed by the paper.
For the generic case, F=Q(q,u1,…,ur) is the rational function field in algebraically independent variables. Put p=∏juj, set ρ=p for odd r and ρ=q−1p for even r, and define δa=∑jγjuja, where γj is the rational expression in equation (3.6). A shape in Γn is an r-tuple of Young diagrams whose total number of boxes is at most n and congruent to n modulo two. An up–down tableau of shape λ is a path from the empty tuple to λ, adding or removing one box at each step. Its set is T(n,λ).
Formalization targets
The central target is the freeness half of Theorem 6.1. For every admissible integral ground ring S and every n≥0,
Wn,S,r≅SSrn(2n−1)!!.
Here (−1)!!=1, so the asserted rank at n=0 is one. A basis indexed by a finite type of that cardinality states both freeness and rank.
The milestone path includes the paper's admissibility equivalences, generic parameter construction, universal admissible integral domain, base change, the spanning family of rn(2n−1)!! elements, the relevant Jones ideal and Hecke quotient results, and the generic structure results. At the generic field, Theorem 4.8 supplies split matrix blocks indexed by Γn, with block size ∣T(n,λ)∣. The count in the proof of Theorem 5.2 is
λ∈Γn∑∣T(n,λ)∣2=rn(2n−1)!!,
which agrees with the generic dimension asserted by Theorem 5.2. The final theorem applies over an admissible integral domain, rather than only over F.
Significance
The result supplies an exact finite basis for Wn,S,r throughout the admissible integral-domain regime. It fixes the module rank independently of a choice of specialization and gives a numerical benchmark for presentations and representations of these algebras. The generic matrix decomposition also records the simple-block labels and their sizes; the tableau identity explains how those dimensions total the same rank.
Goodman and Hauschild Mosley proved these results. This mission asks for machine-checked proofs of the statements already known in the paper. Its reusable output includes ground-ring morphisms with sign twists, the generic parameter model, and finite tableau types. The published preliminary layer in mission 92e1197e-907f-4ab8-92be-01ea44f6cfef contains two proved and four open results among the six referenced milestones; none of the eight new theorem statements is presented here as already machine proved.
Difficulty
A spanning family alone does not show a basis: specializations can introduce linear dependence, and an equality of dimensions over one field does not itself establish independence over every integral ground ring. The argument also has to maintain the exact admissibility conditions when moving between coefficient rings. Over the generic field, identifying the right semisimple blocks requires the branching shapes and tableau sizes, while the final statement concerns arbitrary admissible integral domains. These are distinct algebraic regimes, and the same presentation and parameter conventions must be kept through each one.
Formalization scope
Lean uses the published GroundParameters, BMW, BMWAdmissible, UAdmissible, and equation (3.6) gamma expression from QAlgorithms.Papers.Independent.Support21.P14. The algebra is the quotient of a free algebra by the source's relations, with zero-based generator indices. The new definition layer records all four sign cases of Definition 2.13, the generic field as a fraction ring of a multivariate polynomial ring over Q, and shapes and tableaux as finite-indexed Young diagrams. A generic ground-parameter value is asserted to exist as a theorem because the required unit and compatibility proofs are substantive.
The universal-ring milestone states all five parts of Theorem 3.19 for R0 and R+; part (5) is read inside F, where the universal u-admissible ring is the subring generated by q±1, (q−1−q)−1, δ0−1 and ui±1. The base-change statement uses S′⊗SW for the source's W⊗SS′. Theorem 4.8 is expressed as a product of full matrix algebras with tableau-sized blocks, combining its split and labeling clauses with the block dimensions stated elsewhere in §4. Theorem 6.1's Kauffman-tangle isomorphism is not posed because a corresponding tangle algebra has not been provided here. The cited spanning set is expressed as an indexed family, which remains meaningful when a degenerate coefficient ring identifies elements.
The goal requires an actual basis of cardinality rn(2n−1)!! over an admissible integral domain. A mere dimension equality or spanning claim does not satisfy it. Contributions toward the missing generic decomposition, tableau count, base-change isomorphism, and passage to arbitrary admissible domains are all within scope. The ground-parameter and quotient-presentation definitions are reusable independently of this mission.
Selected references
- F. M. Goodman and H. Hauschild Mosley, Cyclotomic Birman–Wenzl–Murakami algebras, II: Admissibility Relations and Freeness, 2008, arXiv:math/0612065v5.