Đề tài "Semistable sheaves in positive characteristic " doc

Semistable sheaves in positive characteristicBy Adrian Langer* Abstract We prove Maruyama’s conjecture on the boundedness of slope semistablesheaves on a projective variety defined over a

Annals of Mathematics

Semistable sheaves in positive characteristic

By Adrian Langer

Semistable sheaves in positive characteristic

By Adrian Langer*

Abstract

We prove Maruyama’s conjecture on the boundedness of slope semistablesheaves on a projective variety defined over a noetherian ring Our approachalso gives a new proof of the boundedness for varieties defined over a charac-teristic zero field This result implies that in mixed characteristic the modulispaces of Gieseker semistable sheaves are projective schemes of finite type Theproof uses a new inequality bounding slopes of the restriction of a sheaf to ahypersurface in terms of its slope and the discriminant This inequality alsoleads to effective restriction theorems in all characteristics, improving earlierresults in characteristic zero

F ⊂ E we have µ(F ) ≤ µ(E).

Semistability was introduced for bundles on curves by Mumford, and latergeneralized by Takemoto, Gieseker, Maruyama and Simpson This notion wasused to construct the moduli spaces parametrizing sheaves with fixed topo-logical data As for the construction of these moduli spaces the boundedness

of semistable sheaves is a fundamental problem equivalent for these modulispaces to be of finite type over the base field (see [Ma2, Th 7.5])

*The paper was partially supported by a Polish KBN grant (contract number 2P03A05022).

In the curve case the problem is easy In higher dimensions this problemwas successfully treated in characteristic zero using the Grauert-M¨ulich the-orem with important contributions by Barth, Spindler, Maruyama, Forster,Hirschowitz and Schneider In positive characteristic Maruyama proved theboundedness of semistable sheaves on surfaces and the boundedness of sheaves

of rank at most 3 in any dimension

In another direction Mehta and Ramanathan proved their restriction orem saying that the restriction of a semistable sheaf to a general hypersurface

the-of a sufficiently large degree is still semistable This theorem is valid in anycharacteristic but the result does not give any information on the degree of thishypersurface It was well known that an effective restriction theorem wouldprove the boundedness In the characteristic zero case such a theorem wasproved by Flenner Ein and Noma tried to use a similar approach in positivecharacteristic but they succeeded only for rank 2 bundles on surfaces

About the same time as people were studying the boundedness of semistablesheaves, Bogomolov proved his famous inequality saying that

∆(E) = 2 rk E c2 E − (rk E − 1)c2

1E

is nonnegative if E is a semistable bundle on a surface over a characteristic

zero base field This result can easily be generalized to higher dimensions bythe Mumford-Mehta-Ramanathan restriction theorem Bogomolov’s inequal-ity was generalized by Shepherd-Barron [SB1], Moriwaki [Mo] and Megyesi[Me] to positive characteristic but only in the surface case The higher dimen-sional version of this inequality follows only from the boundedness of semistablesheaves (see [Mo], the proof of Theorem 1), which is what we want to prove

In this paper we prove the boundedness of semistable sheaves andBogomolov’s inequality in positive characteristic Moreover, we prove effec-tive restriction theorems Our methods also give new proofs of these results incharacteristic zero

Our approach to these problems is through a theorem combining theGrauert-M¨ulich type theorem and Bogomolov’s inequality at the same time

To explain the basic idea let us state a special case of our Theorems 3.1 and

3.2 We say that E is strongly semistable if either char k = 0 or char k > 0 and all the Frobenius pull backs of E are semistable.

Theorem 0.1 Assume that n ≥ 2 Let E be a strongly semistable sion-free sheaf Let µ i (r i ) denote slopes (respectively: ranks) of the Harder -

tor-Narasimhan filtration of the restriction of E to a general divisor D ∈ |H|.

i<j

r i r j (µ i − µ j)2≤ H n · ∆(E)H n −2 .

In particular, ∆(E)H n −2 ≥ 0.

Let us note that theorems of this type do not immediately give eventhe usual Mumford-Mehta-Ramanathan theorem However, together withKleiman’s criterion, this theorem gives the boundedness of semistable sheaves

on surfaces Later we will prove a much stronger theorem (see Section 3) plying the boundedness of all semistable pure sheaves with bounded slopesand fixed Hilbert polynomial in all dimensions and in any characteristic (seeTheorem 4.1) In fact, we prove a stronger statement of boundedness in mixedcharacteristic, which was conjectured by Maruyama (see [Ma1, Question 7.18],[Ma2, Conj 2.11]) Then a standard technique (see [HL, Ch 4]; see also [Ma3])implies the following corollary

im-Theorem 0.2 Let R be a universally Japanese ring Let f : X → S be a projective morphism of R-schemes of finite type with geometrically connected fibers and let O X (1) be an f -ample line bundle Then for a fixed polynomial

P there exists a projective S-scheme M X/S (P ) of finite type over S, which

uniformly corepresents the functor

M X/S (P ) : {schemes over S} o → {sets}





.

Moreover, there is an open scheme M X/S s (P ) ⊂ M X/S (P ) that universally

corepresents the subfunctor of families of geometrically stable sheaves.

Universally Japanese rings are also called Nagata rings In the abovetheorem semistability is defined by means of the Hilbert polynomial Apartfrom that exception semistability in this paper is always defined using theslope

Let us also remark that quotients of semistable points in mixed teristic are uniform categorical and universally closed but not necessarily uni-

charac-versal Therefore the moduli space M X/S (P ) does not in general universally

corepresent M X/S (P ) (but it does in characteristic 0) However, M s

X/S (P )

universally corepresents the corresponding subfunctor, because in this case the

corresponding quotient is in fact a PGL(m)-principal bundle in fppf topology

(but not in ´etale topology; see [Ma1, Cor 6.4.1])

As a final application of our theorems we give a new effective restrictiontheorem, which works in all characteristics (see Section 5) In characteristiczero our result is a stronger version of Bogomolov’s restriction theorem (see[HL, Th 7.3.5]) It has immediate applications to the study of moduli spaces

of Gieseker semistable sheaves

The paper is organized as follows In Section 1 we recall some basic factsand prove some useful inequalities In Section 2 we explain that Frobenius pullbacks of semistable sheaves are semistable (although the notion of semistabilityhas to be altered) and we use it to explain some basic properties of the Harder-Narasimhan filtrations in positive characteristic Section 3 is the heart of thepaper and it contains formulations and proofs of our restriction theorem and afew versions of Bogomolov’s inequality We prove our theorems by induction onthe rank of a sheaf In Section 4 we use these results to prove the boundedness

of semistable sheaves In Section 5 we prove effective restriction theorems inall characteristics In Section 6 we further study semistable sheaves in positivecharacteristic

Notation used in this paper is consistent with that in the literature Forbasic notions, facts and history of the problems we refer the reader to theexcellent book [HL] by Huybrechts and Lehn

1 Preliminaries

Let X be a normal projective variety of dimension n and let O X(1) be a

very ample line bundle Let [x]+ = max(0, x) for any real number x If E is

a torsion-free sheaf then µmax(E) denotes the maximal slope in the Narasimhan filtration of E (counted with respect to the natural polarization).

Harder-Theorem 1.1 (Kleiman’s criterion; see [HL, Th 1.7.8]) Let {E t } be a family of coherent sheaves on X with the same Hilbert polynomial P Then the family is bounded if and only if there are constants C i , i = 0, , deg P , such that for every E t there exists an E t -regular sequence of hyperplane sections

H1, , H deg P , such that h0(E t |

j≤i H j)≤ C i

Lemma 1.2 (see [HL, Lemma 3.3.2]) Let E be a torsion-free sheaf of

rank r Then for any E-regular sequence of hyperplane sections H1, , H n

the following inequality holds for i = 1, , n:

Proof For m = 1, 2 the inequality is easy to check For m = 3 the required

This proves the lemma for m = 3.

Now assume that m ≥ 3 Set r

assume that µ1 ≥ µ2 ≥ · · · ≥ µ l Let P be the polygon obtained by joining p i

to p i+1 for i = 0, , l − 1 and p l to p0 By assumption, P is the convex hull conv(p0 , , p l ) of points p0 , , p l

Lemma 1.5 Let P and P
be two such convex polygons (possibly erated ) and assume that they have the same beginning and end points (i.e.,

degen-p0 = p
0 and p l = p
l  ) If P
is contained in P then

r i µ2i ≥
r
i (µ
i)2 Proof We prove the lemma by induction on l

If l
= 1 then the inequality follows from

r i µ2i ≥ (

r i µ i)2

r i

.

Assume that l
= k ≥ 2 and the lemma holds for all pairs of polygons

with l
< k In this case for each nonnegative number α let us set p
0(α) = p
0,

p
i (α) = (x
i , y
i + α) for i = 1, , l
−1 and p

l (α) = p
l Then the corresponding

sequence µ
i (α) is still decreasing Consider the largest nonnegative α such that the polygon P

= conv(p
0(α), , p
l  (α)) is still contained in P Then there exists a vertex p
j (α), j = 0, l
, which lies on the (upper) boundary of P

Now let us note that

l  Therefore the inequality for the pair P and P

is stronger

than the one for P and P
But the inequality for P and P

follows (bysumming) from the inequalities for two pairs of smaller polygons, which hold

by the induction assumption

2 Semistability of Frobenius pull backs

In this section we assume that X is a smooth n-dimensional projective variety defined over an algebraically closed field k of characteristic p > 0 Let X (i) = X × Spec k Spec k, where the product is taken over the ith power

of an absolute Frobenius map on Spec k Then the factorization of the absolute Frobenius morphism F : X → X gives the geometric Frobenius morphism

F g : X → X(1)

If E is a coherent sheaf on X and ∇ : E → E ⊗ Ω X is an integrable

k-connection then one can define its p-curvature ψ : Der k (X) → End X (E) by

ψ(D) = (∇(D)) p − ∇(D p ) (note that ψ is not an O X-homomorphism, but it

is p-linear).

If E is a coherent sheaf on X(1) then one can construct a canonical nection∇can on F g ∗ E (by using the usual differentiation in tangent directions).

con-Now let us recall Cartier’s theorem (see, e.g., [Ka, Th 5.1])

Theorem 2.1 (Cartier) There is an equivalence of categories between

the category of quasi -coherent sheaves on X(1) and the category of quasi coherent O X -modules with integrable k-connections, whose p-curvature is zero.

-This equivalence is given by E → (F ∗

g E, ∇can) and (G,∇) → ker ∇.

Gieseker [Gi] gave examples of semistable bundles whose Frobenius pullbacks are no longer semistable However, Theorem 2.1 allows for inseparabledescent and it allows us to explain the behaviour of semistable sheaves underFrobenius pull-backs

Let us recall that a coherent O X -sheaf E with a W -valued operator η :

E → E ⊗ W is called η-semistable if the inequality on slopes is satisfied for all

nonzero subsheaves of E preserved by η.

Proposition 2.2 A coherent sheaf E on X(1) is semistable with respect

to H if and only if the sheaf F g ∗ E is ∇can-semistable with respect to Fg ∗ H.

Lemma 2.3 Let E be a torsion-free sheaf with a k-connection ∇ Assume that E is ∇-semistable and let 0 = E0 ⊂ E1 ⊂ · · · ⊂ E m = E be the usual

Harder -Narasimhan filtration Then the induced maps E i → (E/E i)⊗ Ω X are

O X -homomorphisms and they are nonzero for i = 1, , m − 1.

Lemma 2.3 together with Proposition 2.2 lead to the following lemmaproved by N Shepherd-Barron (and many others)

Corollary 2.4 (see [SB2, Prop 1]) Let E be a semistable torsion-free

sheaf such that F ∗ E is unstable Let 0 = E0 ⊂ E1 ⊂ · · · ⊂ E m = F ∗ E

be the Harder -Narasimhan filtration Then the O X -homomorphisms E i →

(E/E i)⊗ Ω X induced by ∇can are nontrivial.

Note that Shepherd-Barron’s proof is much less elementary and it usesEkedahl’s results on quotients by foliations in positive characteristic

Let us fix a collection of nef divisors D1 , , D n −1 The maximal

(minimal) slope in the Harder-Narasimhan filtration of E (with respect to (D1 , , D n −1 )) is denoted by µmax(E) (µmin(E), respectively) Since it will

usually be clear which polarizations are used, we suppress D1 , , D n −1 in the

(respec-Moreover, Lmax(E) ≥ µmax(E) and Lmin(E) ≤ µmin(E) Let us also remark

that if E is semistable then Lmax(E) = µ(E) (or Lmin(E) = µ(E)) if and only

if E is strongly semistable.

Let us also set

α(E) = max(Lmax(E)− µmax(E), µmin(E)− Lmin(E)).

Corollary 2.5 Let A be a nef divisor such that T X (A) is globally

gen-erated Then for any torsion-free sheaf E of rank r

α(E) ≤ r − 1

p − 1 AD1 D n −1 .

Proof First we prove that if E is semistable then µmax(F∗ E) −µmin(F∗ E)

≤ (r − 1)AD1 D n −1 (cf [SB2, Cor 2]) To prove this take the

Harder-Narasimhan filtration 0 = E0 ⊂ E1 ⊂ · · · ⊂ E m = F ∗ E By Corollary 2.4

µmin(Ei)≤ µmax((F∗ E/E i)⊗Ω X ) By assumption Ω X embeds into a direct sum

of copies ofO X (A), so that µmax((F ∗ E/E i)⊗Ω X)≤ µmax((F∗ E/E i)⊗O X (A)) Summing inequalities µ(E i /E i −1) ≤ µ(E i+1 /E i ) + AD1 D n −1 yields therequired inequality

Now we get µmax((F k)∗ E)

p k ≤ µmax+r p −1 −1 AD1 D n −1 by simple induction.

Passing to the limit yields the required inequality for Lmax(E) − µmax(E).

Similarly one can show that the corresponding inequality holds for µmin(E) −

strongly semistable

If E has an fdHN property (we say that “E is fdHN” for short) and E • is

the Harder-Narasimhan filtration of (F k) E for some k ≥ k0, then F∗ (E •) is

the Harder-Narasimhan filtration of (F k+1) E.

Let E be a torsion-free sheaf and let 0 = E0 ⊂ E1 ⊂ · · · ⊂ E m = E be the Harder-Narasimhan filtration of E To any sheaf G we may associate the point

p(G) = (rk G, deg G) in the plane Now consider the points p(E0), , p(Em)

and connect them successively by line segments connecting the last point with

the first one The resulting polygon HNP(E) is called the Harder-Narasimhan

polygon of E (see [Sh]).

Let us recall that HNP(E) lies above the corresponding polygon obtained from any other filtration of E with torsion free quotients (see, e.g., [Sh, Th 2]).

If char k = p then we may associate to E a sequence of polygons HNP k (E),

where HNPk (E) is defined by contracting HNP((F k) E) along the degree axis

by the factor 1/p k By the above remark the polygon HNPk (E) is contained

in HNPk+1 (E) Moreover, all these polygons are bounded, by Corollary 2.5.

Therefore there exists the limit polygon HNP∞ (E) Using it one can define

µ i ∞ (E) and r i ∞ (E) in the obvious way.

Note that E is fdHN if and only if there exists k0 such that HNPk0(E) =

HNP∞ (E).

Theorem 2.7 Every torsion-free sheaf is fdHN.

Proof The proof is by induction on rank For rank 1 the assertion is

obvious Assume that the theorem holds for every sheaf of rank less than r and let E be a rank r sheaf Let 0 = p0 ∞ , p1 ∞ , , p (l −1)∞ , p l ∞ = (r, deg E) be

the vertices of HNP∞ (E) Let 0 = E0k ⊂ E 1k ⊂ · · · ⊂ E l k k = (F k) E be the

Harder-Narasimhan filtration of (F k) E and let p ik denote the correspondingvertices of HNPk (E) For every j = 0, , l there exists a sequence {p i j k }

which tends to p j ∞.

Claim 2.7.1 There exists k0 such that E i1k = (F k −k0) E i1k0 for all

k ≥ k0.

Proof First let us note that for every ε > 0 there exists k(ε) such that

||p i j k −p j ∞ || < ε for k ≥ k(ε) If we take ε < 1 then rk E i1k = r1 ∞ for k ≥ k(ε).

Let us take k ≥ k(ε) and consider HNP ∞ (E i1k) If the first line segment

s of HNP ∞ (E i1k ) lies on the line segment p0 ∞ p1∞ then by the induction

as-sumption there exists l and a subsheaf G of (F l) E i1k ⊂ (F k+l) E such that

the point p(G) lies on p0 ∞ p1∞ Then E is fdHN since (F k+l) E/G is fdHN by

the induction assumption

Therefore we can assume that the segment s lies below p0 ∞ p1∞ In

par-ticular there exists l such that the line segment p0 ∞ p i1(k+l) lies above s Then there exists j > i1 such that

µmax((Fl) (E jk /E (j −1)k )) > µmax((F l) E i1k ).

Otherwise, µmax((F k+l) E) ≤ µmax((Fl) E i1k), a contradiction

There also exists a saturated subsheaf G ⊂ (F l) E jk containing

(F l) E (j −1)k such that

µ(G/(F l) E (j −1)k ) = µmax((F l) (E jk /E (j −1)k )).

Consider the point p(G) = (rk G, deg G p k+1 ) Then HNP k+l (E) contains the est convex polygon W containing HNP k (E) and p(G) Here we again use the

small-fact that any polygon whose vertices are saturated subsheaves of a fixed sheaflies below the Harder-Narasimhan polygon

But the difference of areas of W and HNP k (E) is at least

It is easy to make the above procedure more efficient

By the claim, p i1k = p1 ∞ for k ≥ k0and hence E i1k0 is strongly semistable

(since p1 ∞ is the first nonzero vertex of HNP∞ (E) and HNP k0(E) is convex) Since (F k0) E/E i1k0 is fdHN by the induction assumption, the sheaf E is also

fdHN

3 Restriction to hypersurfaces and Bogomolov’s inequality

Notation Let k be an algebraically closed field of any characteristic.

Let X be a smooth projective variety of dimension n ≥ 2 over k and let

D1, , D n −1 be nef divisors on X such that the 1-cycle D1 D n −1 is

numer-ically nontrivial Set d = D12D2 D n −1 ≥ 0.

Let E be a rank r torsion-free sheaf on X Set ∆(E) = 2rc2(E) −

(r − 1)c1(E)2, µ = µ(E), µmin = µmin(E) and µmax = µmax(E) For simplicity

we usually ignore the dependence of slopes on the collection (D1 , , D n −1).

In the following, F always denotes the absolute Frobenius morphism or identity if the characteristic is zero If char k = p > 0 then we already defined

Lmax(E) and Lmin(E) in Section 2 If char k = 0 then we set Lmax(E) =

µmax(E) and Lmin(E) = µmin(E).

Corollary 2.5 and Theorem 2.7 imply that Lmax(E) and Lmin(E) are well defined rational numbers For simplicity, we set Lmax = Lmax(E) and Lmin =

Now choose a nef divisor A on X such that T X (A) is globally generated.

To simplify notation, we usually write β r = β r (A; D1 , , D n −1).

Let Num(X) = Pic(X) ⊗R/ ∼, where ∼ is an equivalence relation defined

by L1 ∼ L2 if and only if L1 AD2 D n −1 = L2 AD2 D n −1 for all divisors

A Then we define an open cone

K+={D ∈ Num(X) : D2D2 D n −1 > 0

and DD1 D n −1 ≥ 0 for all nef D1}.

As in the surface case, by the Hodge index theorem, this cone is “self-dual” inthe following sense:

if and only if DLD2 D n −1 > 0 for all L ∈ K+− {0}.

Theorem 3.1 Assume that D1 is very ample and the restriction of E

to a general divisor D ∈ |D1| is not µ-semistable (with respect to (D2| D , ,

D n −1 | D)) Let µ i (r i ) denote slopes (respectively: ranks) of the Harder Narasimhan filtration of E| D Then

-(3.1.1)

i<j

r i r j (µ i − µ j)2 ≤ d∆(E)D2 D n −1 + 2r2(Lmax − µ)(µ − Lmin).

The inequality in Theorem 3.1 is sharp for unstable sheaves Equalityholds, e.g., for OPn (k) ⊕ OPn(−k) For semistable sheaves of rank 2 it can be

slightly improved (see (3.10.1))

The following theorems generalize Bogomolov’s instability theorem

Theorem 3.2 Let E be a strongly (D1, , D n −1 )-semistable torsion-free

Strategy of the proof Let T i (r), i = 1, , 4 denote the statement:

Theo-rem 3.i holds for all sheaves of rank ≤ r on any smooth variety Let T5(r) note the statement: Theorem 3.2 holds if D1 , , D n −1 are ample and rk E ≤ r.

de-We will prove that T1(r) implies T5(r), T5(r) implies T3(r), T3(r) implies

T4(r), T4(r) implies T2(r) and finally T2(r) implies T1(r + 1) Since T1(1) istrivial this will prove all the theorems at the same time by simple induction

Proofs.

3.5 T1(r) implies T5(r).

Let us assume that D1 , , D n −1 are very ample, E is strongly semistable

and ∆(E)D2 D n −1 < 0 Then Lmax= Lmin = µ and T1(r) implies that the restriction of E to D1 is semistable Since (F k) E is also strongly semistable

the restriction of (F k) E to a general element of |D1| is also semistable

There-fore the restriction of E to a very general element of |D1| is strongly semistable.

By induction, the restriction of (F l) E to a very general complete intersection

X i=|D1| ∩ · · · ∩ |D i | is strongly semistable for i = 1, , n − 1.

Now without loss of generality we can assume that X is a surface, E

is locally free (because ∆(E ∗∗) ≤ ∆(E) and E ∗∗ is locally free on a smooth

surface) and the restriction of E to a very general curve C ∈ |D1| is strongly

semistable Then T5(r) follows from Bogomolov’s inequality if char k = 0 and from [Mo, Th 1] if char k = p However, we prefer to give a different proof,

which does not depend on the characteristic of the base field We use themethod of Y Miyaoka in [Mi]

On a curve, bundles associated to representations of a strongly semistablebundle are strongly semistable (see [Mi, §§5 and 6]) Therefore S kr E | C isstrongly semistable The standard short exact sequence

respect to D1 Recall that h0(G) ≤ [deg G + rk G]+ for any semistable vector

bundle G over a curve Hence h0(S kr E(−kc1E)) = O(k r) Similarly, by Serre

duality h2(S kr E(−kc1E)) = h0((S kr E(−kc1E)) ∗ ⊗ ω X ) = O(k r)

On the other hand, the Riemann-Roch theorem gives

χ(X, S kr E(−kc1E)) = − r r ∆(E)

and connect them successively by line segments connecting the last point with

the first one The resulting polygon HNP(E) is called... class="text_page_counter">Trang 9

(respec-Moreover, Lmax(E) ≥ µmax(E) and Lmin(E) ≤ µmin(E) Let us also remark

that... X-homomorphism, but it

is p-linear).