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The Lumenary

Practice · First-year graduate

Enriched categories

Enriched categories

Warm-upHuman

In a category enriched in ([0,∞],≥,+,0)([0, \infty], \ge, +, 0), the hom-object from xx to yy is a number d(x,y)d(x, y). What does the composition morphism X(y,z)⊗X(x,y)→X(x,z)X(y, z) \otimes X(x, y) \to X(x, z) say?

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Enriched categories

CoreComputational

Directed footpaths between four places take these times (minutes): A→BA \to B: 4, B→CB \to C: 3, C→DC \to D: 5, A→CA \to C: 9, D→AD \to A: 2, B→DB \to D: 10. In the free Lawvere metric space on this weighted graph (shortest-path distance), what is d(A,D)d(A, D)?

Enriched categories

CoreHuman

Replace ++ by max⁡\max: let V=([0,∞],≥,max⁡,0)\mathcal{V} = ([0, \infty], \ge, \max, 0). What is a V\mathcal{V}-category?

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Enriched categories

CoreHuman

Which statement about one-object enriched categories is correct?

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Enriched categories

CoreHuman

The category with abelian groups as objects and all functions as morphisms has an abelian group BAB^A of morphisms from AA to BB (pointwise addition). Why is it not an Ab\mathbf{Ab}-enriched category?

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Enriched categories

StretchHuman

Let f,g:X→Yf, g : X \to Y be non-expansive maps between Lawvere metric spaces, viewed as [0,∞][0, \infty]-functors. When is there a [0,∞][0, \infty]-natural transformation f⇒gf \Rightarrow g?

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Enriched categories

CoreFormal

In the Lean file, Dist is the order dual of ℝ≥0∞, so a ⟶ b holds when a ≥ b as distances, and Mathlib writes the composition of an enriched category as comp x y z : Hom x y ⊗ Hom y z ⟶ Hom x z. In lawvereEquivEnriched, what does homOfLE (M.triangle x y z) provide?

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Enriched categories

StretchHuman

Theorem 4 says d(x,y)=sup⁡z(d(z,y)−˙d(z,x))d(x, y) = \sup_z \big(d(z, y) \mathbin{\dot-} d(z, x)\big) in any Lawvere metric space. Which zz attains the supremum, and why is no term larger?

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Enriched categories

StretchComputational

Write free_metric(w): given a square matrix w (list of lists) of direct path lengths, with 0 on the diagonal and float('inf') where there is no path, return the shortest-path distance matrix — the free Lawvere metric. Use repeated min-plus squaring D↦D⊗DD \mapsto D \otimes D, (D⊗D)ik=min⁡j(Dij+Djk)(D \otimes D)_{ik} = \min_j (D_{ij} + D_{jk}), until the matrix stops changing.

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