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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 of nonempty sets and all functions between them, under the Axiom of Choice.
Products represent set-indexed families of maps, including the empty family (Products and coproducts as limits and colimits of discrete diagrams, including their existence-and-uniqueness equations).
Equalizers represent equalizing maps (Equalizers and coequalizers as limits and colimits of a parallel pair).
General small limits require products together with equalizers (Every small limit can be constructed as an equalizer between products over the objects and arrows of the index category).
Choice is equivalent to nonemptiness of a product of an arbitrary family of nonempty sets (The Axiom of Choice).
Counterexample
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 has all small products.
Let be the constant maps with values and . If equalized them, then and would be the distinct constant functions on the nonempty set . Hence no equalizing cone exists in , so in particular no equalizer [F2] exists.
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].
Depends on
- Products and coproducts as limits and colimits of discrete diagrams, including their existence-and-uniqueness equations
- Equalizers and coequalizers as limits and colimits of a parallel pair
- Every small limit can be constructed as an equalizer between products over the objects and arrows of the index category
- The Axiom of Choice
Used by
Nothing in the library uses this result yet.
Dependency tree · next 3 levels
Direct dependencies and their dependencies through the next three levels: 17 results over 9 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.
Sources
- E. Riehl, Category Theory in Context, Theorem 3.5.11 (standard reference, not scraped)