Cellular algebras #
This file defines cellular algebras, in the style of Graham and Lehrer. The definition is not exactly the same as in their paper, but it is close enough for our purposes. We also define cell modules and the resultant representation theory.
Preliminary and helper definitions #
To speak naturally about cellular algebras it will be useful to have some shorthands for tableaux sets and spans. Here, as in the definition that follows, $\Lambda$ is some type of "weights" and there is map that takes a weight to a set of "tableaux" for that weight.
The set of all triples ${}^\mu_{s_1, s_2}$ where $\mu$ is a weight in some set and $s_1, s_2$ are tableaux of shape $\mu$.
We express it here as the preimage of the set $S$ under the map from all triples ${}^\mu_{s_1, s_2}$ to their weight $\mu$.
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- double_tableaux_in S tableau = Sigma.fst ⁻¹' S
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Increasing the set of weights increases the set of all double-tableaux.
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- all_tableaux_range A S c = ⇑c '' double_tableaux_in S tableau
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The k-linear span of a subset of tableaux.
When this is a subset of weights closed under the partial order, we will show taht this is a two-sided ideal of the cellular algebra. However, we need a definition now so that we can talk about the multiplication rule for the basis elements.
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- (c over S) = Submodule.span k (all_tableaux_range A S c)
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- term_Over_ = Lean.ParserDescr.trailingNode `term_Over_ 50 0 (Lean.ParserDescr.binary `andthen (Lean.ParserDescr.symbol "over") (Lean.ParserDescr.cat `term 0))
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Main definition #
A definition of a cellular algebra, in the style of Graham and Lehrer.
In this definition, instead of asking for a linear form $r$ over all of $A$, we just ask
for an arbitrary function defined on the basis elements. This is extended to all of $A$
by linearity in CellularAlgebra.r.
Similarly, the multiplication condition is most often written as a condition on
$a \cdot c^\mu_{s, t}$, but we ask for a condition on $c^ν_{s', t'} \cdot c^\mu_{s, t}$ for
simplicity, and then extend it to all of $A$ in CellularAlgebra.multiplication_rule.
- Λ : Type
- Λ_order : PartialOrder (Λ k A)
- decidable_eq_tableau (μ : Λ k A) : DecidableEq (tableau μ)
- c : Module.Basis ((μ : Λ k A) × tableau μ × tableau μ) k A
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Instances and restatements #
For convenience, we restate some of the data of a cellular algebra as instances, so that we can use
them without having to refer to the cellular namespace.
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- instFintypeSigmaΛProdTableau A = inferInstanceAs (Fintype ((μ : CellularAlgebra.Λ k A) × CellularAlgebra.tableau μ × CellularAlgebra.tableau μ))
The subspace of $A$ spanned by the basis elements corresponding to tableaux of weights in a set $S$.
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Unfolding some linear maps #
Some of the conditions on cellular algebras in the definition are written in terms of the basis elements but have obvious extensions to the entire algebra.
The multiplication map r_basis extended to all of A by linearity.
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- CellularAlgebra.r A μ s t = (CellularAlgebra.c.constr k) fun (b : (μ : CellularAlgebra.Λ k A) × CellularAlgebra.tableau μ × CellularAlgebra.tableau μ) => CellularAlgebra.r_basis b μ s t
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The key requirement for cellular algebras.
This is the extension of CellularAlgebra.multiplication_rule_basis to all of A by linearity.
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The involution #
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Cellular algebras are equipped with an involution, which is the linear map that swaps the two tableaux in the basis elements.
The basis c modulo cellular ideals #
The key fact about the cellular basis, is that it respects the ordering of the cellular weights. That is, each cellular ideal $A^{≤\mu}$, taken modulo the smaller ideal $A^{<\mu}$, has a basis given by the elements $c^\mu_{s, t}$ for $s, t$ tableaux of shape $\mu$.
The set of basis vectors $\{c^\mu_{s,t} : s ∈ M(μ)\}$ is linearly independent modulo $A^{<\mu}$.
If $\sum_{s} f₁(s) c^\mu_{s,t} ≡ \sum_{s} f₂(s) c^\mu_{s,t}$ modulo $A^{<\mu}$, then $f₁ = f₂$.
The set of basis vectors $\{c^\mu_{s,t} : t ∈ M(μ)\}$ is linearly independent modulo $A^{<\mu}$.
If $\sum_{t} f₁(t) c^\mu_{s,t} ≡ \sum_{t} f₂(t) c^\mu_{s,t}$ modulo $A^{<\mu}$, then $f₁ = f₂$.
The set of basis vectors $\{c^\mu_{s,t} : s, t ∈ M(μ)\}$ is linearly independent modulo $A^{<\mu}$.
If $\sum_{s, t} f₁(s, t) c^\mu_{s,t} ≡ \sum_{s, t} f₂(s, t) c^\mu_{s,t}$ modulo $A^{<\mu}$, then $f₁ = f₂$.
If $\um_{u} f(u) c^\mu_{s,u} = \sum_{v} g(v) c^\mu_{v,t}$ modulo $A^{<\mu}$, then $f(u) = \begin{cases} g(s)& u=t\\ 0\text{else}\end{cases}$.
Cell modules #
A cell module can be thought of as being build on the basis of tableaux
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- CellularAlgebra.cell_module k A μ = (CellularAlgebra.tableau μ →₀ k)
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- CellularAlgebra.cell_module_basis k A μ = { repr := LinearEquiv.refl k (CellularAlgebra.tableau μ →₀ k) }
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- ⋯ = ⋯
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- CellularAlgebra.cell_module_module k A μ = { toSMul := CellularAlgebra.cellular_action k A, mul_smul := ⋯, one_smul := ⋯, smul_zero := ⋯, smul_add := ⋯, add_smul := ⋯, zero_smul := ⋯ }
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- ⋯ = ⋯
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- CellularAlgebra.simple_module k A μ = (CellularAlgebra.cell_module k A μ ⧸ CellularAlgebra.cell_module_radical k A μ)
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- CellularAlgebra.instModuleSimple_module k A = sorry