The endomorphism algebra of the primitive boundary #
The root-plus-projective boundary is indexed by the finite poset obtained by
adjoining a bottom element to OrderDual T. Its opposite endomorphism ring
is therefore the ordinary incidence algebra of that augmented poset. This is
the ring-level form of the incidence-category calculation.
The boundary family reindexed by its actual augmented incidence poset.
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The biproduct of the incidence-indexed boundary family.
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Taking a (q,r) component is additive in the boundary endomorphism.
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The (q,r) component of an endomorphism of the boundary generator.
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Assemble a universe-polymorphic matrix of boundary morphisms. Mathlib's
finite biproduct.matrix is intentionally small-universe, while the
representation-theoretic index type here lives in the algebra's universe.
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The biproduct resolution of the identity, without Mathlib's small-index restriction on the corresponding convenience theorem.
Matrix multiplication for boundary endomorphisms, stated with a universe-polymorphic finite sum.
The scalar coordinate of one component of a boundary endomorphism.
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The coordinate of a composable pair of arbitrary matrix components. If an intermediate index lies outside the interval, the corresponding component vanishes by acyclicity.
An endomorphism of the boundary biproduct gives its incidence matrix.
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Reassemble an incidence matrix as an endomorphism of the boundary biproduct.
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The opposite endomorphism ring of the primitive boundary generator is
the incidence algebra of the augmented boundary poset. The opposite is
essential: multiplication in End is reverse categorical composition.