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Greg Stevenson
Greg Stevenson
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An example (Hartshorne Ch II Ex 3.7) where the condition at the generic point is not vacuous is:

Suppose that f:X -> Y is a dominant finite type morphism of integral schemes such that Y is irreducible and the fibre over the generic point of Y is finite. Then there exists an open dense subscheme U of Y such that f: f^{-1}(U) -> U is finite.

One can also check flatness over a curve by checking whether certain points get mapped to the generic point.

"Generic vanishing" also holds for coherent sheaves in the sense that a torsion sheaf is defined as one which is not supported at the generic point.

I guess one point of view on the philosophy is that with appropriate finiteness conditions good properties at a point should extend to a neighbourhood. In particular, good properties at the generic point should extend to a dense neighbourhood.

An example (Hartshorne Ch II Ex 3.7) where the condition at the generic point is not vacuous is:

Suppose that f:X -> Y is a dominant finite type morphism of integral schemes such that Y is irreducible and the fibre over the generic point of Y is finite. Then there exists an open dense subscheme U of Y such that f: f^{-1}(U) -> U is finite.

One can also check flatness over a curve by checking whether certain points get mapped to the generic point.

"Generic vanishing" also holds for coherent sheaves in the sense that a torsion sheaf is defined as one which is not supported at the generic point.

I guess one point of view on the philosophy is that with appropriate finiteness conditions good properties at a point should extend to a neighbourhood. In particular, good properties at the generic point should extend to a dense neighbourhood.

An example (Hartshorne Ch II Ex 3.7) where the condition at the generic point is not vacuous is:

Suppose that f:X -> Y is a dominant finite type morphism of integral schemes such that Y is irreducible and the fibre over the generic point of Y is finite. Then there exists an open dense subscheme U of Y such that f: f^{-1}(U) -> U is finite.

One can also check flatness over a curve by checking whether certain points get mapped to the generic point.

"Generic vanishing" also holds for coherent sheaves in the sense that a torsion sheaf is defined as one which is not supported at the generic point.

Source Link
Greg Stevenson
Greg Stevenson
  • 8.7k
  • 1
  • 40
  • 38

An example (Hartshorne Ch II Ex 3.7) where the condition at the generic point is not vacuous is:

Suppose that f:X -> Y is a dominant finite type morphism of integral schemes such that Y is irreducible and the fibre over the generic point of Y is finite. Then there exists an open dense subscheme U of Y such that f: f^{-1}(U) -> U is finite.

One can also check flatness over a curve by checking whether certain points get mapped to the generic point.

"Generic vanishing" also holds for coherent sheaves in the sense that a torsion sheaf is defined as one which is not supported at the generic point.

I guess one point of view on the philosophy is that with appropriate finiteness conditions good properties at a point should extend to a neighbourhood. In particular, good properties at the generic point should extend to a dense neighbourhood.