I am currently studying varieties over $\mathbb{C}$, i know some scheme theory.
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Let $f: X \rightarrow Y$ be a morphism of varieties. If we want to show flatness, is it enough to check the condition only at the closed points of $Y$? If yes, could you give an argument, if not, could you give an example where $f$ is flat over all closed points of $Y$, but not over some non-closed point?
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Same question with $f$ smooth of relative dimension $n$, using the following characterization of smooth (Hartshorne III.10.5):
$f$ is called smooth if it is flat and for all $y \in Y$ the "algebraic closure of the fibre" $X_{\overline{y}} := X_y \times_{k(y)} \overline{k(y)}$ is regular (and equidimensional of dimension $n$). Now it must be clear what is meant by smooth "at a point" $y \in Y$.
The question originally continued after this, but i decided to split it since it became too long. Second part: (Continued:) finiteness of étale morphisms
Thanks a lot!
PS tag "complex-geometry" is included since i'm happy to assume $k=\mathbb{C}$.
Best Answer
The answer is yes.
For the flatness: a module $M$ over a ring $A$ is flat if and only if for all maximal ideal $\mathfrak m$ of $A$, $M_\mathfrak m$ is flat over $A_\mathfrak m$.
The smooth locus is open in $X$. If it contains all closed points of $X$, then it is equal to $X$.