Alphabeta Math
CounterexampleConstruction: Literature-sourcedVerification: AI-adaptedprecheck passaudited 2026-08-13
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Assuming Choice, nonempty sets have all small products but a parallel pair with no equalizer and hence a diagram with no limit

Statement refuted

If a category has all small products, then it has all small limits.

Facts & Assumptions

Given: The full category Set≠∅ of nonempty sets and all functions between them, under the Axiom of Choice.

[F1]
[F3]

Choice is equivalent to nonemptiness of a product of an arbitrary family of nonempty sets (The Axiom of Choice).

Counterexample

technique · missing equalizer
1.1

The ordinary Cartesian product of any set-indexed family of nonempty sets is nonempty by [F3] and has the product property [F1] inside the full subcategory. For the empty family, the singleton is a nonempty terminal object. Thus Set≠∅ has all small products.

F1F3
1.2

Let f,g:{∗}⇉{0,1} be the constant maps with values 0 and 1. If h:X→{∗} equalized them, then fh and gh would be the distinct constant functions on the nonempty set X. Hence no equalizing cone exists in Set≠∅, so in particular no equalizer [F2] exists.

F2given
2.1

The parallel-pair diagram is finite and small but has no limit, refuting the statement. This is exactly the missing equalizer data isolated by [L1].

L1step 1.1step 1.2∎

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · two levels

8 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.

Sources