Root subgroups of the symplectic group #
For i : Fin m, the elementary matrices
x_{2eᵢ}(c) = 1 + c E_{i,m+i},
x_{-2eᵢ}(c) = 1 + c E_{m+i,i}
preserve the standard alternating form. For distinct i,j, products of two commuting elementary
matrices similarly give the short roots eᵢ-eⱼ, eᵢ+eⱼ, and -eᵢ-eⱼ. This file promotes all
five families to affine group-scheme morphisms 𝔾ₐ → Sp₂ₘ over an arbitrary commutative base
ring.
The construction first selects the matrix homomorphism through
TauCeti.GLSymplecticFin.RootSubgroupIndex. One shared pipeline constructs its natural map on
algebra-valued points, recovers the coordinate Hopf-algebra morphism by full faithfulness of the
functor of points, and applies relative spectrum. The long-root composites with the closed immersion
Sp₂ₘ → GL₂ₘ are proved to be the corresponding general-linear root subgroups. Thus the
factorization through the symplectic equations is recorded scheme-theoretically, over every base.
Main definitions #
TauCeti.Symplectic.positiveLongRootSubgroupPointsandTauCeti.Symplectic.negativeLongRootSubgroupPoints: the homomorphisms on algebra-valued points.TauCeti.Symplectic.positiveLongRootSubgroupCoordinateMapandTauCeti.Symplectic.negativeLongRootSubgroupCoordinateMap: their coordinate morphisms.TauCeti.Symplectic.positiveLongRootSubgroupandTauCeti.Symplectic.negativeLongRootSubgroup: the affine group-scheme morphisms.TauCeti.Symplectic.shortRootSubgroup: the affine group-scheme morphism for any short root.TauCeti.Symplectic.schemePointsMulEquiv_rootSubgroup: the root subgroup's action on scheme-valued points.
References #
- J. S. Milne, Algebraic Groups (2017), §21 and §24.6.
- R. W. Carter, Simple Groups of Lie Type (1972), §11.3.
- J. E. Humphreys, Linear Algebraic Groups (1975), §26.3.
- The scheme-points root-subgroup construction follows the formal template in
TauCeti.Algebra.AlgebraicGroup.GeneralLinear.Root.Subgroup.
The homomorphism on A-valued points selected by a symplectic root index.
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Under the symplectic point equivalence, a root point is the selected matrix homomorphism.
Every symplectic root subgroup is injective on algebra-valued points.
Symplectic root point homomorphisms are natural in the value algebra.
The positive long-root homomorphism on A-points, sending c to
1 + c E_{i,m+i} in Sp₂ₘ(A).
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The negative long-root homomorphism on A-points, sending c to
1 + c E_{m+i,i} in Sp₂ₘ(A).
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The short-root homomorphism on algebra-valued points.
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Under the symplectic point equivalence, the positive long-root point is its elementary transvection.
Under the symplectic point equivalence, the negative long-root point is its elementary transvection.
Under the symplectic point equivalence, a short-root point is its paired elementary-matrix one-parameter subgroup.
A short-root subgroup is injective on points over every value algebra.
The positive long-root subgroup on points is natural in the value algebra.
The negative long-root subgroup on points is natural in the value algebra.
Short-root point homomorphisms are natural in the value algebra.
The natural transformation of group-valued points selected by a symplectic root index.
Equations
- TauCeti.Symplectic.rootSubgroupPointsMap root = { app := fun (A : CommAlgCat R) => GrpCat.ofHom (TauCeti.Symplectic.rootSubgroupPoints root), naturality := ⋯ }
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The natural transformation of group-valued points for the positive long root 2eᵢ.
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The natural transformation of group-valued points for the negative long root -2eᵢ.
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The natural transformation of group-valued points attached to a short root.
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A component of the generic root points map is the constructed point homomorphism.
A component of the positive long-root natural transformation is the corresponding point homomorphism.
A component of the negative long-root natural transformation is the corresponding point homomorphism.
A component of the natural short-root points map is the constructed point homomorphism.
The coordinate Hopf-algebra morphism selected by a symplectic root index.
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The coordinate morphism of the positive long-root subgroup, recovered from its natural action on points.
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The coordinate morphism of the negative long-root subgroup, recovered from its natural action on points.
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The coordinate Hopf-algebra morphism of a short-root subgroup.
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Precomposition by a root coordinate morphism is its natural map on points.
Precomposition by the positive long-root coordinate morphism gives its natural point map.
Precomposition by the negative long-root coordinate morphism gives its natural point map.
Precomposition by the short-root coordinate morphism is its natural map on points.
On a same-universe algebra, a root coordinate morphism induces the constructed point map.
On a same-universe algebra, the positive coordinate morphism induces the constructed point homomorphism.
On a same-universe algebra, the negative coordinate morphism induces the constructed point homomorphism.
On a same-universe algebra, a short-root coordinate morphism induces its point homomorphism.
A root coordinate morphism sends the generic symplectic matrix to the identity plus its normalized linear term. Entries are indexed in paired coordinates.
The positive long-root coordinate morphism factors the matching general-linear root coordinate morphism through the symplectic quotient.
The negative long-root coordinate morphism factors the matching general-linear root coordinate morphism through the symplectic quotient.
The affine group-scheme morphism selected by a symplectic root index.
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- One or more equations did not get rendered due to their size.
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A root subgroup is relative spectrum applied to its coordinate morphism.
A symplectic root subgroup on scheme-valued points is its standard root matrix.
The positive long-root subgroup of Sp₂ₘ attached to 2eᵢ, as an affine
group-scheme morphism.
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The positive long-root subgroup is the relative spectrum of its coordinate morphism, transported to the named source and target schemes.
The negative long-root subgroup of Sp₂ₘ attached to -2eᵢ, as an affine
group-scheme morphism.
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The negative long-root subgroup is the relative spectrum of its coordinate morphism, transported to the named source and target schemes.
The affine group-scheme morphism 𝔾ₐ → Sp₂ₘ attached to a short root.
Equations
- TauCeti.Symplectic.shortRootSubgroup family hij = TauCeti.Symplectic.rootSubgroup (TauCeti.GLSymplecticFin.RootSubgroupIndex.short family i j hij)
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The short-root subgroup is relative spectrum applied to its coordinate morphism.
The positive long-root subgroup followed by the symplectic inclusion is the corresponding general-linear root subgroup.
The negative long-root subgroup followed by the symplectic inclusion is the corresponding general-linear root subgroup.