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Proper flat genus-one fibrations have one-dimensional fibres
Statement
Assume full AC. Let be a connected integral base and let be a proper, flat, finitely presented morphism whose geometric generic fiber is a smooth geometrically integral genus-one curve. For , write for the scheme-theoretic fiber. Then every has Krull dimension one.
Statement references
Facts & Assumptions
Given: Full AC, connected , and a proper flat finitely presented morphism with geometric generic fiber a smooth geometrically integral genus-one curve.
Full AC is the choice-function principle of The Axiom of Choice; the local-constancy supplier below assumes it.
For a proper flat finitely presented morphism, the function is locally constant, with empty fibers assigned dimension (Fibre dimension of proper flat finitely presented families).
The generic fiber and genus-one condition have the meanings fixed in Flat genus-one fibration: after algebraic closure of it is a smooth proper geometrically integral curve with .
Proof
By [F2], the geometric generic fiber is a curve, hence has Krull dimension one. In particular the fiber-dimension function of [F1] takes value one at the generic point .
By [F1], the fiber-dimension function is locally constant on . A locally constant function on a connected topological space is constant: the inverse image of each value is open, and its complement is the union of the other open level sets, hence also open; a nonempty level set is therefore all of . Since the value at is one by step 1.1, every fiber has dimension one, including that no fiber is empty.