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Unibranch points of a classical variety

Definition

Assume the Axiom of Choice. Let X be an irreducible classical variety over an algebraically closed field with normalization ν ⁣:Xν→X (Normalization of a classical variety by gluing affine normalizations). A point x∈X is unibranch when the fibre ν−1(x) is a single point (Images and fibres of a regular map).

The fibre is finite because ν 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 ν−1(x) correspond to the local branches of X at x, 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 X 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 x. A point on two distinct components therefore cannot be unibranch.

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