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CounterexampleConstruction: AI-adaptedVerification: AI-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-09-01
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A monoidal category need not be closed

Statement refuted

Every monoidal category is closed.

Facts & Assumptions

Given: The five-element diamond lattice M3={0,p,q,r,1} with 0<p,q,r<1 and p,q,r pairwise incomparable.

[L1]

A poset with top element and binary meets becomes a strict monoidal category under meet (A poset with finite meets is a strict monoidal category).

[L2]

In a lattice, binary meets exist and are written (Lattices, distributive lattices, and order ideals).

[L3]

Right closed means that, for each fixed object X, the functor X has a right adjoint (Left-closed, right-closed, and biclosed monoidal categories).

Counterexample

technique · direct
1.1

In M3, the meet operation satisfies pq=0, pr=0, and p1=p, while q and r are incomparable below 1. By [L1] and [L2], the associated poset-category is a strict monoidal category with tensor product and unit 1.

givenL1L2
2.1

Fix X:=p and Y:=q. An object a satisfies apq exactly when a{0,q,r}: this holds for 0,q,r, fails for 1 because 1p=pq, and fails for p because pp=pq.

step 1.1givenalgebra
3.1

The set {0,q,r} has no greatest element, since q and r are incomparable and both dominate 0. Therefore there is no object [p,q] with a[p,q] iff apq, so the functor p has no right adjoint. By [L3], this monoidal category is not right closed and hence not closed.

step 2.1L3algebra

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