The primitive-idempotent multiplicity package #
For a primitive idempotent e : A, this file constructs the projective
right ideal eA, the injective right module D(Ae), and the common
coordinate Xe. Evaluation identifies Hom_A(eA, X) with Xe, while
duality identifies Hom_A(X, D(Ae)) with D(Xe). On a finite
indecomposable skeleton these literal modules give the source, sink, and
weight satisfying both factor mesh unit equations used in the frozen
manuscript.
The right regular module is linearly equivalent to the regular module over the opposite ring.
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Left multiplication by e, regarded as an endomorphism of the right
regular module.
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The literal right ideal eA.
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The canonical generator e of eA.
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Every element of eA is fixed by left multiplication by e.
The right ideal eA as a literal finitely generated right module.
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The regular right module is projective.
The idempotent right ideal eA is projective.
The idempotent right ideal is categorically projective in the literal finitely generated module category.
The e-coordinate Xe, realized as the range of right multiplication
by e on a right module X.
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An element of Xe is fixed by right multiplication by e.
Evaluation at the generator identifies right-ideal maps with the idempotent coordinate.
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The categorical Hom space from eA is linearly equivalent to Xe.
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The map eA → X associated to an element x ∈ X, namely
y ↦ x y.
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The idempotent coordinate has dimension zero exactly when e acts by
zero on the whole module.
The Hom dimension from eA is exactly the dimension of the
e-coordinate.
Right multiplication by e, regarded as an endomorphism of the left
regular module.
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The literal left ideal Ae.
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The canonical generator e of Ae.
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The left ideal Ae as a literal finitely generated left module.
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The idempotent left ideal Ae is projective.
The idempotent left ideal is categorically projective.
The manuscript's injective I(e)=D(Ae) as a literal finitely generated
right module.
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The principal right ideal generated by op e over the opposite algebra,
viewed as the original left ideal Ae.
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The inverse identification of Ae with the principal right ideal
generated by op e over the opposite algebra.
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Evaluation identifies the opposite principal projective (op e)Aᵒᵖ
with the contragredient dual of the original primitive injective D(Ae).
The scalar calculation is exactly the reversal of multiplication under
MulOpposite.
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The evaluation map from the opposite principal projective to the dual of the primitive injective is bijective.
The opposite principal projective is linearly equivalent to the contragredient dual of the original primitive injective.
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Categorical form of the identification
(op e)Aᵒᵖ ≅ D(D(Ae)).
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Contragredient duality sends the projective left ideal Ae to the
injective right module D(Ae).
Every element of Ae is fixed by right multiplication by e.
Multiplication of x : X by an element of Ae lands in Xe.
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The inherited k-action on the literal left ideal agrees with the
action obtained by restricting its A-module structure.
Evaluation of a left-ideal map at the generator, bundled in the common corner coordinate.
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A common corner coordinate acts by right multiplication to produce a
map Af → Ae.
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Evaluation at the generator of Af identifies maps Af → Ae with
the same corner coordinate that represents maps eA → fA.
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Categorical left-ideal Hom and right-ideal Hom have the same corner coordinate.
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Contragredient duality sends a map Af → Ae to the reversed map
D(Ae) → D(Af), linearly over the coefficient field.
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The Hom space between primitive injectives is the reversed Hom space between their defining left ideals.
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Maps between primitive injectives and maps between the corresponding primitive projectives have the same corner coordinate.
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The coordinate product is k-linear when the left ideal is equipped
with the restricted scalar action used by contragredient duality.
The concrete contragredient object has the ordinary vector-space dual
as its underlying k-module.
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The elementary dual-coordinate form of the standard isomorphism
Hom_A(X,D(Ae)) ≅ D(Xe).
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The categorical sink Hom space is linearly equivalent to the dual of the primitive coordinate.
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The Hom dimension into D(Ae) is the dimension of Xe.
Module-theoretic indecomposability implies categorical indecomposability in the finitely generated module category.
A primitive idempotent, expressed by the absence of nontrivial
idempotents in its corner eAe.
- idempotent : IsIdempotentElem e
- nonzero : e ≠ 0
- corner_idempotent (b : A) : IsIdempotentElem b → e * b = b → b * e = b → b = 0 ∨ b = e
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A primitive idempotent remains primitive in the opposite algebra.
The right ideal of a primitive idempotent is indecomposable in the module-theoretic sense.
The literal right ideal of a primitive idempotent is categorically indecomposable.
The left ideal of a primitive idempotent is indecomposable in the module-theoretic sense used by contragredient duality.
Contragredient duality sends the primitive left ideal to an indecomposable right injective.
The unique skeleton label representing the primitive projective eA.
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The chosen identification of eA with its skeleton representative.
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The chosen primitive-projective skeleton object is projective.
Transporting evaluation at e across the chosen skeleton isomorphism
identifies the distinguished source Hom space with Xe.
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The unique skeleton label representing the primitive injective
D(Ae).
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The chosen identification of D(Ae) with its skeleton
representative.
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The chosen primitive-injective skeleton object is injective.
Transport across the chosen skeleton isomorphism identifies the
distinguished sink Hom space with the dual of Xe.
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Labels killed by the primitive deletion are exactly those on which e
acts by zero.
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The paper's primitive coordinate, before identifying it with composition multiplicity.
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The zero set of the primitive coordinate is the killed subcategory.
The primitive coordinate is the source Hom dimension on every skeleton object.
The same primitive coordinate is the sink Hom dimension on every skeleton object.
The primitive projective itself survives deletion.
The primitive projective as a surviving factor label.
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The primitive injective itself survives deletion.
The primitive injective as a surviving factor label.
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A primitive idempotent supplies the complete ambient multiplicity package used to prove the two factor mesh unit equations.
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The primitive coordinate satisfies both mesh unit equations in the literal factor category.