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The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.
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FALSE: pullbacks preserve epimorphisms in every category with pullbacks
Statement
In every category with pullbacks, the pullback of an epimorphism is again an epimorphism.
Facts & Assumptions
Given: The full subcategory of Hausdorff spaces and continuous maps.
A Hausdorff space is a topological space in which points are separated by disjoint neighbourhoods (Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not).
A continuous map into a Hausdorff space is determined by its restriction to any dense subset (Dense, nowhere dense and codense subsets of a topological space, and the criterion by basic open sets, Two continuous maps into a Hausdorff space that agree on a dense subset are equal).
The rationals are dense in , and the irrationals are nonempty (Both and are dense in , and every nonempty open subset of is uncountable).
Refutation
Let be the inclusion of the rationals into the real line, regarded as Hausdorff spaces. By [L3], is dense in , so [L2] says that is epic: any two continuous maps out of into a Hausdorff space that agree on are equal.
Choose an irrational point using [L3], and let be the inclusion. The pullback of along is the map , because . Let with the discrete topology, which is Hausdorff by [L1]. The two constant maps are distinct, but their composites with are equal. So the pullback map is not epic, and the universal statement is false.
Depends on
- The pullback of an epimorphism is an epimorphism
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- Dense, nowhere dense and codense subsets of a topological space, and the criterion by basic open sets
- Two continuous maps into a Hausdorff space that agree on a dense subset are equal
- Both $\mathbb{Q}$ and $\mathbb{R} \setminus \mathbb{Q}$ are dense in $\mathbb{R}$, and every nonempty open subset of $\mathbb{R}$ is uncountable
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
33 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
- General Topology Notes (UC Riverside) (standard reference, not scraped)
- The Stacks Project, Section 12.5, Lemma 12.5.13 (standard reference, not scraped)