Smooth morphism

In algebraic geometry, a morphism $f:X\to S$ between schemes is said to be smooth if

(i) it is locally of finite presentation
(ii) it is flat, and
(iii) for every geometric point $\overline {s}\to S$ the fiber $X_{{\overline {s}}}=X\times _{S}{\overline {s}}$ is regular.

(iii) means that each geometric fiber of f is a nonsingular variety (if it is separated). Thus, intuitively speaking, a smooth morphism gives a flat family of nonsingular varieties.

If S is the spectrum of an algebraically closed field and f is of finite type, then one recovers the definition of a nonsingular variety.

Equivalent definitions

There are many equivalent definitions of a smooth morphism. Let $f:X\to S$ be locally of finite presentation. Then the following are equivalent.

f is smooth.
f is formally smooth (see below).
f is flat and the sheaf of relative differentials $\Omega _{{X/S}}$ is locally free of rank equal to the relative dimension of $X/S$ .
For any $x\in X$ , there exists a neighborhood $\operatorname {Spec}B$ of x and a neighborhood $\operatorname {Spec} A$ of $f(x)$ such that $B=A[t_{1},\dots ,t_{n}]/(P_{1},\dots ,P_{m})$ and the ideal generated by the m-by-m minors of $(\partial P_{i}/\partial t_{j})$ is B.
Locally, f factors into $X{\overset {g}\to }{\mathbb {A}}_{S}^{n}\to S$ where g is étale.
Locally, f factors into $X{\overset {g}\to }{\mathbb {A}}_{S}^{n}\to {\mathbb {A}}_{S}^{{n-1}}\to \cdots \to {\mathbb {A}}_{S}^{1}\to S$ where g is étale.

A morphism of finite type is étale if and only if it is smooth and quasi-finite.

A smooth morphism is stable under base change and composition. A smooth morphism is locally of finite presentation.

A smooth morphism is universally locally acyclic.

Examples

Smooth morphisms are supposed to geometrically correspond to smooth submersions in differential geometry; that is, they are smooth locally trivial fibrations over some base space (by Ehresmann's theorem).

Trivial Fibrations

Given a smooth scheme $Y$ the projection morphism

Y\times X\to X

is smooth.

Vector Bundles

Every vector bundle $E \to X$ over a scheme is a smooth morphism. For example, it can be shown that the associated vector bundle of ${\mathcal {O}}(k)$ over $\mathbb {P} ^{n}$ is the weighted projective space minus a point

O(k)=\mathbb {P} (1,\ldots ,1,k)-\{[0:\cdots :0:1]\}\to \mathbb {P} ^{n}

sending

[x_{0}:\cdots :x_{n}:x_{n+1}]\to [x_{0}:\cdots :x_{n}]

Notice that the direct sum bundles $O(k)\oplus O(l)$ can be constructed using the fiber product

O(k)\oplus O(l)=O(k)\times _{X}O(l)

Separable Field Extensions

Recall that a field extension $K\to L$ is called separable iff given a presentation

L={\frac {K[x]}{(f(x)}}

we have that $gcd(f(x),f'(x))=1$ . We can reinterpret this definition in terms of kahler differentials as follows: the field extension is separable iff

\Omega _{L/K}=0

Notice that this includes every perfect field: finite fields and fields of characteristic 0.

Non-Examples

Singular Varieties

If we consider ${\text{Spec}}$ of the underlying algebra $R$ for a projective variety $X$ , called the affine cone of $X$ , then the point at the origin is always singular. For example, consider the cone of a quintic $3$ -fold given by

x_{0}^{5}+x_{1}^{5}+x_{2}^{5}+x_{3}^{5}+x_{4}^{5}

Then the Jacobian matrix is given by

{\begin{bmatrix}5x_{0}^{4}&5x_{1}^{4}&5x_{2}^{4}&5x_{3}^{4}&5x_{4}^{4}\end{bmatrix}}

which vanishes at the origin, hence the cone is non-singular. Affine hypersurfaces like these are popular in singularity theory because of their relatively simple algebra but rich underlying structures.

Degenerating Families

Consider the flat family

{\text{Spec}}\left({\frac {\mathbb {C} [t,x,y]}{(xy-t)}}\right)\to \mathbb {A} _{t}^{1}

Then the fibers are all smooth except for the point at the origin. Since smoothness is stable under base-change, this family is not smooth.

Non-Separable Field Extensions

For example, the field $\mathbb {F} _{p}(t^{p})\to \mathbb {F} _{p}(t)$ is non-separable, hence the associated morphism of schemes is not smooth. If we look at the minimal polynomial of the field extension,

f(x)=x^{p}-t^{p}

then $df=0$ , hence the kahler differentials will be non-zero.

Formally smooth morphism

One can define smoothness without reference to geometry. We say that an S-scheme X is formally smooth if for any affine S-scheme T and a subscheme $T_{0}$ of T given by a nilpotent ideal, $X(T)\to X(T_{0})$ is surjective where we wrote $X(T)=\operatorname {Hom}_{S}(T,X)$ . Then a morphism locally of finite type is smooth if and only if it is formally smooth.

In the definition of "formally smooth", if we replace surjective by "bijective" (resp. "injective"), then we get the definition of formally étale (resp. formally unramified).

Smooth base change

Let S be a scheme and $\operatorname {char}(S)$ denote the image of the structure map $S\to \operatorname {Spec}{\mathbb {Z}}$ . The smooth base change theorem states the following: let $f:X\to S$ be a quasi-compact morphism, $g:S'\to S$ a smooth morphism and ${\mathcal {F}}$ a torsion sheaf on $X_{{\text{et}}}$ . If for every $0\neq p$ in $\operatorname {char}(S)$ , $p:{\mathcal {F}}\to {\mathcal {F}}$ is injective, then the base change morphism $g^{*}(R^{i}f_{*}{\mathcal {F}})\to R^{i}f'_{*}(g'^{*}{\mathcal {F}})$ is an isomorphism.

References

J. S. Milne (2012). "Lectures on Étale Cohomology"
J. S. Milne. Étale cohomology, volume 33 of Princeton Mathematical Series . Princeton University Press, Princeton, N.J., 1980.

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