A Crystalline Criterion for Good Reduction on Semi-stable $K3$-Surfaces over a $p$-Adic Field
1. A Crystalline Criterion for Good Reduction on
Semi-stable K3-Surfaces over a p-Adic Field
Thesis Advisor: Prof. Adrian Iovita
J. Rogelio P´erez Buend´ıa
Concordia University
January 10 2014
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
2. Objective:
To give a criterion for the reduction of an algebraic K3-surface over
a p-adic field in terms of its p-adic ´etale cohomology.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
3. Objective:
To give a criterion for the reduction of an algebraic K3-surface over
a p-adic field in terms of its p-adic ´etale cohomology.
The desired Criterion
A K3-surface over a Local field with semistable reduction has good
reduction if and only if its second ´etale cohomology group is crystalline.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
4. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
5. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
6. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
7. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
3 W := W (k) the ring of Witt vectors with coefficients in k
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
8. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
3 W := W (k) the ring of Witt vectors with coefficients in k
4 K0 = Frac(W ) its field of fractions. It is an unramified extension of
Qp.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
9. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
3 W := W (k) the ring of Witt vectors with coefficients in k
4 K0 = Frac(W ) its field of fractions. It is an unramified extension of
Qp.
5 K = K0 if k is ifninite or K be a finite extension of K0 if
[k : Fp] < ∞.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
10. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
3 W := W (k) the ring of Witt vectors with coefficients in k
4 K0 = Frac(W ) its field of fractions. It is an unramified extension of
Qp.
5 K = K0 if k is ifninite or K be a finite extension of K0 if
[k : Fp] < ∞.
6 OK the ring of integers of K.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
11. Notations:
Fix a prime number p and let Qp the field of p-adic numbers. Consider:
1 k be a perfect field of characteristic p.
2 ¯k be a fix algebraic closure.
3 W := W (k) the ring of Witt vectors with coefficients in k
4 K0 = Frac(W ) its field of fractions. It is an unramified extension of
Qp.
5 K = K0 if k is ifninite or K be a finite extension of K0 if
[k : Fp] < ∞.
6 OK the ring of integers of K.
7 π be a (fixed) uniformizer. So mK = πOK and
k = OK /πOK = W /pW .
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
12. K3-surfaces
Definition
A K3-surface over K is a smooth proper surface XK −→ Spec(K) such
that
1 q := H1
(XK , OXK
) = 0 and
2 ωXK
OXK
. Equivalently KX = 0.
where ωXK
stands for the canonical sheaf and KX its canonical divisor.
This definition is independent of the field K, so we can consider this
definition for K = C and we get complex algebraic K3-surfaces.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
13. Examples:
K3-surfaces were named by Andr´e Weil in honour of three algebraic
geometers, Kummer, K¨ahler and Kodaira, and the mountain K2 in
Kashmir.
Figure: A quartic in P3
K given by x2
y2
+ y2
z2
+ z2
x2
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
14. Examples
Let S be a non-singular sixtic curve in P2
k where k is a field and consider
a double cover i.e., a finite generically ´etale morphism, π : X → P2
k which
is ramified along S. Then X is a K3 surface.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
15. Example
Complete intersections: Let X be a smooth surface which is a complete
intersection of n hypersurfaces of degree d1, . . . , dn in Pn+2
over a field k.
The adjunction formula shows that Ω2
X/k
∼= OX (d1, . . . , dn − n − 3). So a
necessary condition for X to be a K3 surface is d1 + . . . + dn = n + 3.
The first possibilities are:
n = 1 d1 = 4
n = 2 d1 = 2, d2 = 3
n = 3 d1 = d2 = d3 = 2.
For a complete intersection M of dimension n one has that
Hi
(M, OM (m)) = 0 for all m ∈ Z and 1 ≤ i ≤ n − 1. Hence in those
three cases we have H1
(X, OX ) = 0 and therefore X is a K3 surface.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
16. Example
Let A be an abelian surface over a field k of characteristic different from
2. Let A[2] be the kernel of the multiplication by-2-map, let π : ˜A → A
be the blow up of A[2] and let ˜E be the exceptional divisor. The
automorphism [−1]A lifts to an involution [−1]˜A on ˜A. Let X be the
quotient variety of ˜A by the group of automorphisms {id˜A, [−1]˜A} and
denote by ι : ˜A → X the quotient morphism. It is a finite map of degree
2. We have the following diagram of morphisms over k. The variety X is
a K3 surface and it is called the Kummer surface associated to A.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
17. Semistable K3-surfaces
Definition
XK has semistable reduction if it has a semi-stable model:
XK
//
X
Spec(K) // Spec(OK)
that is a proper (flat) model X → Spec(OK) whose special fibre ¯X is
smooth over k or ´etale locally a normal crossing divisor.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
18. Good reduction
If the special fibre ¯X → Spec(k) of such a model X is smooth, then we
say that XK has good reduction.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
19. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
20. p-adic representations
Let GK := Gal(K, K) be the absolute Galois group of K.
Definition
A p-adic representation V of GK is a finite dimensional Qp-vector space
with a continuous action of GK .
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
21. Examples of p-adic representations
The main example:
Main example
The ´etale cohomology of a K3-surface. Indeed, in general we have that if
X is a proper and smooth variety over K, then
Hi
´et(X¯K , Qp)
is a p-adic representation of GK .
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
22. Examples of p-adic representations
The main example:
Main example
The ´etale cohomology of a K3-surface. Indeed, in general we have that if
X is a proper and smooth variety over K, then
Hi
´et(X¯K , Qp)
is a p-adic representation of GK .
Tate modules of abelian varieties.
The r-Tate twists of Qp, Qp(r).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
23. Ring of periods
In order to study the p-adic representations, Fontaine defines what we
know as ring of periods, which are topological Qp-algebras B (or B•),
with a continuous linear action of GK and some additional structures
which are compatible with the action of GK (for example the monodromy
operator N, Frobenius, filtrations).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
24. Poincar´e duality
For a smooth and projective variety X of dimension n over the complex
numbers C, we have the Betti cohomlogy Hi
(X(C), Z).
By Poincar´e duality we have that:
H2n−i (X, C) Hi
(X(C), C).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
25. Complex periods
Also we have a perfect pairing given by the periods:
Hi
dR(X(C)/C) × H2n−i (X(C), C) −→ C
(ω, λ) →
λ
ω.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
26. Comparison isomorphism
We have a natural comparison isomorphism:
Hi
dR(X(C)/C) Hi
(X(C), C).
We remark that in order to have this isomorphism it is very important to
have coefficients in C (for example, this is not an isomorphism over Q).
In this sense C is a ring of periods (it contains all the periods λ
ω).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
27. The p-adic case of Cp
We denote by
Cp = ˆ¯K
the p-adic completion of ¯K.
We want analogous comparison isomorphisms in the p-adic cases.
However the situation is not as easy as in the complex case, mainly
because Cp does not have enough periods.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
28. Fontaine’s Idea
The original idea of Fontaine was to construct these ring of periods, in
order to be able to have analogous comparison isomorphism between the
different cohomologies in p-adic settings.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
29. B•
Examples of this rings are
1 BHT := q∈Z Cp(q) B Hodge-Tate. is a graded C p-algebra with
GK -action rescts gradings and BGK
HT = K.
2 BdR : B de Rham is a complete discrete valuation field over K with
residue field Cp. It contains K (but not Cp). It has an action of GK
and a filtration by its valuation, and its graded quotient
gri
BdR = Cp(i)
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
30. Bcris
Bcris: B crys is an algebra over K0 and a GK -stable subring of BdR . It
contains K0 but not K. We have a filtration coming from form BdR , a
σ-semilinear injective GK -equivariant endomorphism φ (Frobenius
endomorphism). BGK
cis = K0.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
31. Bst
Bst: B semistable is an algebra over K0 and has a GK -action. It contains
Bcris and KO but not K The Frobenius of Bcris extends to Bst and has a
Bcris-derivation
N : Bst −→ Bst. Nφ = pφN
and
BGK
st = K0, BN=0
st = Bcris.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
32. Dieudonn´e Modules
These rings are such that the BGK
-modules
DB (V ) := (B ⊗Qp
V )GK
give us (or expected to give us) good invariants for V . For example
comparison isomorphisms for the p-´adic ´etale cohomology and de Rham
cohomologie or crystalline cohomology or nice criterion for good
reduction of varieties.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
33. B-admissible
Let L = BGK
.
Definition
A p-adic representation V is B-admissible, if
dimL DB (V ) = dimQp
V .
Definition
A p-adic representation V , is crystalline (semistable, Hodge-Tate,
semistable) if V is Bcris-admissible (B•-admissible).
B-admissibility translates to isomorphisms which are analogous to the
comparison isomorphisms in the complex case.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
34. Fontaine has defined several subcategories of the category of all p-adic
representations, denoted by RepGK
.
This categories are formed by the property of being B-admissible objects.
So for any of the period rings B we have a subcategory of the category of
p-adic representations denoted by RepB . These categories satisfy proper
contention relations as follows:
RepBcris
⊂ RepBst
⊂ RepBdR
⊂ RepBHT
⊂ RepGK
.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
35. C•-conjectures
Let XK be a proper smooth variety over K.
CHT : The Hodge-Tate conjecture. There exists a canonical
iomorphism, which is compatible with the Galois action.
Cp ⊗Qp Hm
et (X¯K , Qp)
0≤i≤m
Cp(−i) ⊗K Hm−i
(X¯K , Ωi
XK /K ).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
36. C•-conjectures
Let XK be a proper smooth variety over K.
CHT : The Hodge-Tate conjecture. There exists a canonical
iomorphism, which is compatible with the Galois action.
Cp ⊗Qp Hm
et (X¯K , Qp)
0≤i≤m
Cp(−i) ⊗K Hm−i
(X¯K , Ωi
XK /K ).
CdR : The de Rham conjecture. There exist a conaonical
isomorphism, which is compatible with Galois action and filtrations.
BdR ⊗Qp
Hm
et (X¯K , Qp) BdR ⊗K Hm
dR (XK /K).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
37. C•-conjectures
Ccris: The Crystalline conjecture. Let X be a proper smooth model
of XK over OK . Let ¯X be the special fibre of X. There exist a
canonical isomorphism which is compatible with the Galois action,
and Frobenius endomorphism.
Bcris ⊗Qp
Hm
´et (X¯K , Qp) Bcris ⊗W Hm
crys(¯X/W )
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
38. C•-conjectures
Ccris: The Crystalline conjecture. Let X be a proper smooth model
of XK over OK . Let ¯X be the special fibre of X. There exist a
canonical isomorphism which is compatible with the Galois action,
and Frobenius endomorphism.
Bcris ⊗Qp
Hm
´et (X¯K , Qp) Bcris ⊗W Hm
crys(¯X/W )
Barthelo-Ogus isomorphism:
K ⊗W Hm
crys(¯X/W ) Hm
dR (XK /K).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
39. Cst conjecture
1 Cst: The semistable conjecture: Let X be a proper semistable model
of XK over OK . Let Y be the special fiber of X, and MY be a
naural log-structure on Y . There is a canonical isomorphsim,
compatible with Galois action, Frobenius and operator N.
Bst ⊗Qp
Hm
et (XK , Qp) Bst ⊗W Hm
log −crys((Y , MY ), (W , O∗
))
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
40. Cst conjecture
1 Cst: The semistable conjecture: Let X be a proper semistable model
of XK over OK . Let Y be the special fiber of X, and MY be a
naural log-structure on Y . There is a canonical isomorphsim,
compatible with Galois action, Frobenius and operator N.
Bst ⊗Qp
Hm
et (XK , Qp) Bst ⊗W Hm
log −crys((Y , MY ), (W , O∗
))
2 Hydo-Kato isomorphism
K ⊗W Hm
log −crys((Y , MY ), (W , O∗
)) HdR (XK /K)
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
41. History
For X/K a proper smooth variety over K.
Fontaine: Proved that Tate modules V = TpA ⊗Zp Qp of abelian varieties A are de Rham
and that DdR(V ) (HdR(X/K))v
.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
42. History
For X/K a proper smooth variety over K.
Fontaine: Proved that Tate modules V = TpA ⊗Zp Qp of abelian varieties A are de Rham
and that DdR(V ) (HdR(X/K))v
.
Fontaine and Messing: Proved the comparison theorem for Hi
´et(X¯K , Qp) for i ≤ p − 1 and
K/Qp finite unramified.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
43. History
For X/K a proper smooth variety over K.
Fontaine: Proved that Tate modules V = TpA ⊗Zp Qp of abelian varieties A are de Rham
and that DdR(V ) (HdR(X/K))v
.
Fontaine and Messing: Proved the comparison theorem for Hi
´et(X¯K , Qp) for i ≤ p − 1 and
K/Qp finite unramified.
Kato, Hydo Tsuji: Extended these results to finally prove that one can recover Hi
dR(X/K)
from the data of V = Hi
´et(X¯K , Qp) as a p-adic representation.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
44. History
For X/K a proper smooth variety over K.
Fontaine: Proved that Tate modules V = TpA ⊗Zp Qp of abelian varieties A are de Rham
and that DdR(V ) (HdR(X/K))v
.
Fontaine and Messing: Proved the comparison theorem for Hi
´et(X¯K , Qp) for i ≤ p − 1 and
K/Qp finite unramified.
Kato, Hydo Tsuji: Extended these results to finally prove that one can recover Hi
dR(X/K)
from the data of V = Hi
´et(X¯K , Qp) as a p-adic representation.
Tsuji, Niziol, Faltings: Proved that if X is semistable, then V is Bst -admissible and that V
is crystalline if V has good reduction and BdR otherwise.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
45. For abelian varieties
For Abelian varieties, Bcris and Bst are exactly what it takes to decide:
whether A has good reduction or semistable reduction.
Crystalline criterion for abelian varieties:
Coleman-Iovita Breuil: A has good reduction if and only if V is
crystalline. A has semistable reduction if and only if V is semistable.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
46. My Thesis problem
Crystalline criterion for K3 surfacess:
Let X be a K3 surface over a p-adic field K with semistable reduction. X
has good reduction (¯X → Spec(k) is smooth) if and only if
V := H2
´et(X¯K , Qp)
is Crystalline (Bcris-admissible).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
47. One side is Falting’s result:
Remember that
RepBcris
⊂ RepBst
Since X has semistable reduction, then V is Bst-admissible. If X has
good reduction, then by Falting’s result, V is Bcris-admissible.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
48. Results of Y. Matsumoto
Theorem
Let K be a local field with residue characteristic p = 2 and X a Kummer
surface over K. Assume that X has at least one K-rational point. If
H2
et(X¯K , Qp) is crystalline, then XK has good reduction for some finite
unramified extension K /K.
Theorem
Let K be a local field with residue characteristic p = 2, 3, and Y a K3
surface over K with Shioda-Inose structure of product type. If
H2
et(YK , Qp) is crystalline, then YK has good reduction for some finite
extension K /K of ramification index 1, 2, 3, 4 or 6.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
49. Main tool
The main tooll is what we call p-adic logarithmic degenerations of a
K3-surface. These will be p-adic analogous of degeneration of K3
surfaces over the complex numbers constructed via degenerations.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
50. Complex degenerations of K3-surfaces
Definition
Over the complex numbers C, a semistable degeneration of a
K3-surface X is a proper flat and surjective morphism
π : X(C) → ∆
over the open disc, whose general fibre Xt = π−1
(t), for t = 0 is a
smooth K3-surface and X0 is reduced with normal crossings.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
51. Modification of a degeneration
Definition
A modification of π : X(C) → ∆ is a degeneration of surfaces
π : X(C) → ∆ such that there exists a birational map
φ : X(C) → X (C) given an isomorphism form
(X(C) − X0) −→ (X (C) − X0) and such that the diagram:
X(C)
φ
//
π
X (C)
π
||
∆
commutes.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
52. Kulikov degenerations
We have the following theorem:
Theorem (Kulikov, Persson, Pinkham)
Let π : X(C) → ∆ be a semistable degeneration of a K3-surface, then
there exists a modification π : X (C) → ∆ such that the canonical
divisor of the total space X (C) is trivial.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
53. Kulikov degenerations
We have the following theorem:
Theorem (Kulikov, Persson, Pinkham)
Let π : X(C) → ∆ be a semistable degeneration of a K3-surface, then
there exists a modification π : X (C) → ∆ such that the canonical
divisor of the total space X (C) is trivial.
A degeneration with trivial canonical divisor is called a good
degeneration or a Kulikov degeneration.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
54. Kulikov criterion
Theorem
Let X(C) → ∆ be a good degeneration of a K3-surface. The degenerate
fibre X0 is one of the following three types:
I. X0 is a nonsingular K3 surface.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
55. Kulikov criterion
Theorem
Let X(C) → ∆ be a good degeneration of a K3-surface. The degenerate
fibre X0 is one of the following three types:
I. X0 is a nonsingular K3 surface.
II. X0 = ∪n
i=1Vi where the Vi are rational surfaces and V2, . . . , Vn−1 are
elliptic ruled surfaces.
III. X0 = ∪n
i=1Vi where all the Vi are rational surfaces.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
56. In therms of monodromy
Moreover, the three cases can be distinguished from each other by means
of the monodromy T acting on H2
(Xt, Z):
For Type I we have N := ln T = 0 that is T = id.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
57. In therms of monodromy
Moreover, the three cases can be distinguished from each other by means
of the monodromy T acting on H2
(Xt, Z):
For Type I we have N := ln T = 0 that is T = id.
For Type II, N = 0 but N2
= 0.
For Type III, N2
= 0 but N3
= 0
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
58. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
59. The plan:
We need:
A p-adic semistable degeneration of our K3-surface.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
60. The plan:
We need:
A p-adic semistable degeneration of our K3-surface.
A monodromy operator N on the log-crystalline cohomology.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
61. The plan:
We need:
A p-adic semistable degeneration of our K3-surface.
A monodromy operator N on the log-crystalline cohomology.
We relate it with the monodromy operator on
Dst(H2
et(X¯K , Qp)) H2
log−cris(XK /W ) appearing on Fontain’s
theory.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
62. The plan:
We need:
A p-adic semistable degeneration of our K3-surface.
A monodromy operator N on the log-crystalline cohomology.
We relate it with the monodromy operator on
Dst(H2
et(X¯K , Qp)) H2
log−cris(XK /W ) appearing on Fontain’s
theory.
By p-adic Hodge theory if H2
´et(X¯K , Qp) is crystalline then N = 0.
Indeed BN=0
st = Bcris and Dst(V )N=0
= Dcris(V ) , so if a p-adic
representation is crystalline we most have N = 0.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
63. Finally we base change to the complex numbers and use Kulikov’s
classification theorem to deduce that our crystalline K3-surface has good
reduction.
Here we use the Deligne’s work on the Monodromy expressed as the
residue at zero of the GM-conexion.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
64. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
65. Logarithmic geometry is concerned with a method of finding and
using“hide smoothness”in singular varieties.
Let X be a nonsingular irreducible complex variety, S a smooth
curve with a point s and f : X → S a dominant morphism smooth
away from s, the fiber Xs := f −1
(s) = Y1 ∪ · · · ∪ Yn reduced simple
normal crossing divisor.
ΩX/S = ΩX /f ∗
ΩS fails to be locally free at the singular points of f .
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
66. Consider ΩX/S (log(Xs)) the sheaf of differentials with at most
logarithmic poles along the Yi , and similarly ΩS (log(s)), there is an
injective sheaf homomorphism
f ∗
ΩX (log(Xs)) −→ ΩS (log(s))
and the quotient sheaf ΩX (log(XS ))/f ∗
ΩX (log(Xs)) is locally free.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
67. pre-log.st
Definition
1 Let X be a scheme. A pre-log structure on X, is a sheaf of
monoids MX together with a morphism of sheaves of monoids:
α : MX −→ OX , called the structure morphism.
2 A pre-log structure is called a log structure (log.st for short) if
α−1
(O∗
X ) O∗
X via α.
3 The pair (X, MX ) is called a log scheme and it will be denoted by
X×
.
4 Morphisms are morphisms of sheaves which are compatible with the
structure morphism.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
68. Induced log.st
We have the forgetful functor i from the category of log.st of X to the
category of pre-log.st of X by sending a log.st M in X to itself considered
as a pre-log.st i(M).
Vice-versa given a pre-log.st we can construct a log.st Mls
out of it in
such a way that ( )ls
is left adjoint of i, hence Mls
is universal.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
69. Inverse image log.st
Definition
Let f : X → Y be a morphism of schemes. Given a log.st MY on Y we
can define a log.st on X, called the inverse image of MY , to be the log
structure associated to the pre-log.st
f −1
(MY ) → f −1
(OY ) → OX .
This is denoted by f ∗
(MY ).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
70. Morphisms of log-schemes
Definition
By a morphism of log-schemes X∗
−→ Y ∗
we understand a morphism of
the underlying schemes f : X → Y and a morphism f #
: f ∗
MY → MX of
log.st on X.
We denote by LSch the category of log.schemes.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
71. One of the main examples of interest for us is the following:
Example
Let X be a regular scheme (we can take for example a K3-surface over K
or a proper model of it). Let D be a divisor of X. We can define a log.st
M on X associated to the divisor D as
M(U) := g ∈ OX (U) : g|UD ∈ O∗
X (U D)
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
72. log.st to algebraizable formal schemes
Let X be a scheme and ˆX is a formal completion of X along a closed
subscheme Y , then we have a morphism of ringed spaces:
ˆX
φ
−→ X
for which φ is the inclusion Y → X on topological spaces, and on
sheaves, it is the natural projection
OX −→ lim
←−
OX /In
= OˆX
where I is the sheaf of ideals defined by the closed immersion Y → X.
If we have a log.st on X, say M, we can give a log structure on ˆX by
taking the inverse image of the log structure M so that ˆX becomes a
log-formal scheme:
(ˆX, φ∗
M).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
73. For K3-surfaces
1 When X is a proper model of a K3-surface XK , we have that the
special fibre ¯X of X is a closed divisor with normal crossings.
This divisor induces a log.st on X. We denote by X×
the log-formal
scheme obtained as in the previews paragraph; that is, by completing
X along ¯X and giving to it the inverse image log.st of X induced by
¯X.
2 Notice that we have an inclusion of ringed spaces ¯X → X×
. We
denote by ¯X×
the log-scheme obtained by giving to ¯X the inverse
image log.st of X×
.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
74. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
75. p-adic degeneration
Definition
A p-adic degeneration of a K3-surface with semistalbe reduction is a proper, flat morphism of
schemes X −→ Spec(W[[t]]) with geometrically connected fibres, such that:
1 We have an isomorphism of the semistable model X of XK with the fibre Xπ of
X → Spec(W[[t]]) induced by the ring homomorphism: W [[t]] → OK ; t −→ π.
2 We have an isomorphism (compatible with the previous one) of the special fibre ¯X of the
semistable model of XK with the fibre X0 induced by the projection
W [[t]] → W [[t]]/(p, t) k = OK /πOk .
3 X → Spec(W[[t]]) is smooth in the complement of ¯X that is
(X − ¯X) −→ (Spec(W[[t]]) − Spec(k))
is smooth.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
76. In a diagram
Then we have commutative Cartesian diagrams:
¯X //
X //
X
Spec(k) // Spec(OK) // Spec(W[[t]])
.
Note that Spec(W[[t]]) is the analogous, in p-adic settings, of the open
unit disc ∆ in the complex numbers, and so we call Spec(W[[t]]) the
p-adic unit disc denoted by D. Then X is a family of surfaces
parametrized by the p-adic unit disc D and removing the special fibre ¯X
smooth over D∗
= D − Spec(k).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
77. rig functor
1 Let Y be the fibre of f at t = 0, that is the fibre induced by the
morphism
W [[t]] → W ; t → 0.
This is a scheme over Spec(W) whose special fibre is again X.
Moreover Y is a normal crossing divisor (but now in characteristic
zero).
2 Call X = (X×
)rig, D = (D×
)rig, and f = (f ×
)rig the rigid analytic
spaces over K0.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
78. rigid version of the degeneration
Lemma
Under the previous settings we have:
1 X −→ Spec(K0) is smooth
2 Y := f −1
(0) = (Y×
)rig is a semistable surface over K0.
3 f |X∗ : X∗
:= (X − Y) −→ D∗
:= (D − {0}) is smooth.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
79. Layout
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
80. Complex of relative logarithmic differentials
Consider the complex of sheaves K·
X/D induced by the relative
logarithmic differential:
OX
d1
X/D
−−−→ OX ⊗X/K0
Ω1
X/D(log(Y))
d2
X/D
−−−→ OX ⊗X/K0
Ω2
X/D(log(Y))
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
81. The connection
Denote by Hi
the i-th logarithmic relative de Rham cohomology group of
X/D with coefficients in OX , i.e, it is the sheaf Rf∗(K·
X/D). For every i,
Hi
is a free OD-module with an integrable, regular-singular connection
i : Hi
−→ Hi
⊗OD
Ω1
D/K0
(log(0)).
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
82. Monodromy
If s is a point in D, let Hi
s be the fibre of H1
at s. We now define the
monodromy Ni as the residue at 0 of this connection. That is
Ni = res0( i ).
In our case the only important value is for i = 2, so we define the
monodromy as N := N2.
J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a
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J. Rogelio P´erez Buend´ıa A Crystalline Criterion for Good Reduction on Semi-stable K3-Surfaces over a