Chow's lemma, named after Wei-Liang Chow, is one of the foundational results in algebraic geometry. It roughly says that a proper morphism is fairly close to being a projective morphism. More precisely, a version of it states the following:
If
X
is a scheme that is proper over a noetherian base
S
, then there exists a projective
S
-scheme
X
′
and a surjective
S
-morphism
f
:
X
′
→
X
that induces an isomorphism
f
−
1
(
U
)
≃
U
for some dense open
U
⊆
X
.
The proof here is a standard one (cf. EGA II, 5.6.1).
It is easy to reduce to the case when
X
is irreducible, as follows.
X
is noetherian since it is of finite type over a noetherian base. Then it's also topologically noetherian, and consists of a finite number of irreducible components
X
i
, which are each proper over
S
(because they're closed immersions in the scheme
X
which is proper over
S
). If, within each of these irreducible components, there exists a dense open
U
i
⊂
X
i
, then we can take
U
:=
⨆
(
U
i
∖
⋃
i
≠
j
X
j
)
. It is not hard to see that each of the disjoint pieces are dense in their respective
X
i
, so the full set
U
is dense in
X
. In addition, it's clear that we can similarly find a morphism
g
which satisfies the density condition.
Having reduced the problem, we now assume
X
is irreducible. We recall that it must also be noetherian. Thus, we can find a finite open affine cover
X
=
⋃
i
=
1
n
U
i
.
U
i
are quasi-projective over
S
; there are open immersions over
S
,
ϕ
i
:
U
i
→
P
i
into some projective
S
-schemes
P
i
. Put
U
=
∩
U
i
.
U
is nonempty since
X
is irreducible. Let
ϕ
:
U
→
P
=
P
1
×
S
⋯
×
S
P
n
.
be given by
ϕ
i
's restricted to
U
over
S
. Let
ψ
:
U
→
X
×
S
P
.
be given by
U
↪
X
and
ϕ
over
S
.
ψ
is then an immersion; thus, it factors as an open immersion followed by a closed immersion
X
′
→
X
×
S
P
. Let
f
:
X
′
→
X
be the immersion followed by the projection. We claim
f
induces
f
−
1
(
U
)
≃
U
; for that, it is enough to show
f
−
1
(
U
)
=
ψ
(
U
)
. But this means that
ψ
(
U
)
is closed in
U
×
S
P
.
ψ
factorizes as
U
→
Γ
ϕ
U
×
S
P
→
X
×
S
P
.
P
is separated over
S
and so the graph morphism
Γ
ϕ
is a closed immersion. This proves our contention.
It remains to show
X
′
is projective over
S
. Let
g
:
X
′
→
P
be the closed immersion followed by the projection. Showing that
g
is a closed immersion shows
X
′
is projective over
S
. This can be checked locally. Identifying
U
i
with its image in
P
i
we suppress
ϕ
i
from our notation.
Let
V
i
=
p
i
−
1
(
U
i
)
where
p
i
:
P
→
P
i
. We claim
g
−
1
(
V
i
)
are an open cover of
X
′
. This would follow from
f
−
1
(
U
i
)
⊂
g
−
1
(
V
i
)
as sets. This in turn follows from
f
=
p
i
∘
g
on
U
i
as functions on the underlying topological space. Thus it is enough to show that for each
i
the map
g
:
g
−
1
(
V
i
)
→
V
i
, denoted by
h
, is a closed immersion (since the property of being a closed immersion is local on the base).
Fix
i
. Let
Z
be the graph of
u
:
V
i
→
p
i
U
i
↪
X
. It is a closed subscheme of
X
×
S
V
i
since
X
is separated over
S
. Let
q
1
:
X
×
S
P
→
X
,
q
2
:
X
×
S
P
→
P
be the projections. We claim that
h
factors through
Z
, which would imply
h
is a closed immersion. But for
w
:
U
′
→
V
i
we have:
v
=
Γ
u
∘
w
⇔
q
1
∘
v
=
u
∘
q
2
∘
v
⇔
q
1
∘
ψ
=
u
∘
q
2
∘
ψ
⇔
q
1
∘
ψ
=
u
∘
ϕ
.
The last equality holds and thus there is
w
that satisfies the first equality. This proves our claim.
◻
In the statement of Chow's lemma, if
X
is reduced, irreducible, or integral, we can assume that the same holds for
X
′
. If both
X
and
X
′
are irreducible, then
f
:
X
′
→
X
is a birational morphism. (cf. EGA II, 5.6).