The category algebra of a finite family of objects #
Construction 2.8 and the proof of Proposition 2.10
(graded-category.tex): "Put G = ⊕_{X ∈ ob C} X and
Λ = End_{add C}(G)^{op}. For X in add C, the right Λ-modules
R_X = C(G, X) are projective". For objects F x (x ∈ ι) of a k-linear
category we realize Λ as ⊕_{x,y} C(F x, F y) with the product in path order
((a b)_{xz} = ∑_y a_{xy} ≫ b_{yz}: first a, then b). The paper's right
Λ-modules are then left modules over this ring; R_X = (C(F y, X))_y with
(a · m)_x = ∑_y a_{xy} ≫ m_y. The algebra A₀ of Construction 2.8 is the
category algebra of the projective labels.
The category algebra ⊕_{x,y} C(F x, F y).
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The identity of F x, as a morphism F x ⟶ F y when x = y.
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The modules R_X #
R_X = (C(F y, X))_y.
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Idempotents and the Yoneda lemma #
The generator 1_x ∈ R_{F x}.
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The idempotent e_x.
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Yoneda: maps out of R_{F x} are determined by the image of 1_x.
The map R_{F x} → M with 1_x ↦ m.
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Every map out of R_{F x} is ofElem of the image of 1_x.