THC Science

In algebraic geometry, especially in scheme theory, a property is said to hold geometrically over a field if it also holds over the algebraic closure of the field. In other words, a property holds geometrically if it holds after a base change to a geometric point. For example, a smooth variety is a variety that is geometrically regular.

Geometrically irreducible and geometrically reduced[edit]

Given a scheme X that is of finite type over a field k, the following are equivalent:^[1]

X is geometrically irreducible; i.e., $X\times _{k}{\overline {k}}:=X\times _{\operatorname {Spec} k}{\operatorname {Spec} {\overline {k}}}$ is irreducible, where ${\overline {k}}$ denotes an algebraic closure of k.
$X\times _{k}k_{s}$ is irreducible for a separable closure $k_{s}$ of k.
$X\times _{k}F$ is irreducible for each field extension F of k.