Alphabeta Math
False statementConstruction: AI-adaptedVerification: AI-generatedprecheck passaudited 2026-08-28
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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.

[L1]
[L3]

The rationals are dense in R, and the irrationals are nonempty (Both Q and R∖Q are dense in R, and every nonempty open subset of R is uncountable).

Refutation

1.1L1L2L3

Let i:QR↪R be the inclusion of the rationals into the real line, regarded as Hausdorff spaces. By [L3], QR is dense in R, so [L2] says that i is epic: any two continuous maps out of R into a Hausdorff space that agree on QR are equal.

2.1L1L2L3step 1.1∎

Choose an irrational point x∈R using [L3], and let j:{x}↪R be the inclusion. The pullback of i along j is the map ∅→{x}, because {x}∩QR=∅. Let D={0,1} with the discrete topology, which is Hausdorff by [L1]. The two constant maps {x}→D are distinct, but their composites with ∅→{x} are equal. So the pullback map is not epic, and the universal statement is false.

Depends on

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