How statement and proof provenance work
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Unibranch points of a classical variety
Definition
Assume the Axiom of Choice. Let be an irreducible classical variety over an algebraically closed field with normalization (Normalization of a classical variety by gluing affine normalizations). A point is unibranch when the fibre is a single point (Images and fibres of a regular map).
The fibre is finite because is a finite morphism (Normalization is finite, surjective and birational), so unibranchness is the statement that this finite fibre has exactly one element. In the curve case the points of correspond to the local branches of at , so a unibranch point is one with a single branch in this normalization sense; over an arbitrary algebraically closed field this wording makes no reference to complex analytic topology.
Recorded properties. Every normal point is unibranch: over the normal locus the normalization restricts to an isomorphism (The normalization is an isomorphism over the normal locus), so the fibre over a normal point is a single point, and a point of is normal exactly when its local ring is an integrally closed domain (Normal points and normal varieties). The converse fails: the cusp on the companion page is unibranch but not normal, and the node is not unibranch. Unibranchness is a property of the point alone, read in the normalization; for a reducible reduced variety one uses the full disjoint-union normalization of Normalization of a classical variety by gluing affine normalizations, counting preimages on every component through . A point on two distinct components therefore cannot be unibranch.
Depends on
- The normalization of an irreducible affine variety
- Normalization of a classical variety by gluing affine normalizations
- Normalization is finite, surjective and birational
- The normalization is an isomorphism over the normal locus
- Images and fibres of a regular map
- Normal points and normal varieties
- The Axiom of Choice
Used by
Dependency tree · two levels
43 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
- J. S. Milne, Algebraic Geometry (2025 version), Ch. 8 §b: branches of a curve and the normalization (standard reference, not scraped)
- Ravi Vakil, The Rising Sea: Foundations of Algebraic Geometry (November 18, 2017 public draft), §29.6 (standard reference, not scraped)