Globally generated vector bundles on the Segre threefold with Picard number two

0.24 MB·English

by  Edoardo Ballico

GLOBALLY GENERATED VECTOR BUNDLES ON THE
SEGRE THREEFOLD WITH PICARD NUMBER TWO
5
1
E. BALLICO, S.HUH ANDF. MALASPINA
0
2
n Abstract. We classify globally generated vector bundles on P1 ×P2
a with small first Chern class, i.e. c1 = (a,b), a+b ≤ 3. Our main
J
method is to investigate the associated smooth curves to globally gen-

2 erated vector bundlesvia theHartshorne-Serreconstruction.
2

]
G
A 1. Introduction
.
h Globally generated vector bundles on projective varieties play an impor-
at tant role in classical algebraic geometry. If they are nontrivial they must
m have strictly positive first Chern class. The classification of globally gener-
[ ated vector bundles with low first Chern class has been done over several

rational varieties such as projective spaces [1, 13] and quadric hypersur-
1
v faces [3]. There is also a recent work over complete intersection Calabi-Yau
0 threefolds by the authors [4].
0 There are three types of Segre varieties of dimension 3: P3, P1 ×P2 and
6
P1×P1×P1. Inthispaperweexaminethesimilarproblemofclassification of
5
0 globally generated vector bundles for the Segre variety P1×P2, the product
1. of a projective line and a projective plane. Note that the classification is
0 already dealt in the case of P3 in [1, 13].
5
The Hartshorne-Serre correspondence states that the construction of vec-
1
tor bundles of rank r ≥ 2 on a smooth variety X with dimension 3 is closely
:
v
related with the structure of curves in X and it inspires the classification
i
X of vector bundles on smooth projective threefolds. There have been several
r works on the classification of arithmetically Cohen-Macaulay (ACM) bun-
a
dles on the Segre threefolds [8, 6, 7] and so it is sufficiently timely to classify
the globally generated vector bundles on the Segre threefolds.
Ourmainresultis toclassify globally generated vector bundleson P1×P2
with low first Chern class c , up to trivial factor. When c = (1,2) or
1 1
c = (2,1), we have the following result (see Theorem 5.12 and Theorem
1
2010 Mathematics Subject Classification. 14J60; 14J32; 14H25.
Key words and phrases. Segre variety, Vector bundles, Globally generated, Curves in
projective spaces.
ThefirstandthirdauthorsarepartiallysupportedbyMIURandGNSAGAofINDAM
(Italy). ThesecondauthorissupportedbyBasicScienceResearchProgram2010-0009195
throughNRFfundedbyMEST.ThethirdauthorissupportedbytheframeworkofPRIN
2010/11 ‘Geometria delle variet`a algebriche’, cofinanced byMIUR.
1
2 E.BALLICO,S.HUHANDF.MALASPINA
4.8 for a more general form in which for some of the invariants (s;f ,f ;r)
1 2
there bundles are described, up to isomorphisms):
Theorem 1.1. Let E be a globally generated vector bundle of rank r at least
2 on X = P1 ×P2 with the first Chern class c = (1,2) or c = (2,1) and
1 1
the second Chern class c (E) = (f ,f ). If E has no trivial factor, then the
2 1 2
quadruple (s;f ,f ;r) is one of the following:
1 2
(s is the number of connected components of associated curve.)
(1;1,1;2), (1;0,2;2),
(1;4,4;3 ≤ r ≤ 11), (1;3,3;3 ≤ r ≤ 7), (1;2,3;3 ≤ r ≤ 8),

(1;c,2;3 ≤ r ≤ c+2) with c∈ {1,2,3,4}

when c = (1,2); and
1
(s;0,s;r) with 1 ≤ s≤ 3 and 2 ≤ r ≤ s+1,
(1;1,b;r) with 1 ≤ b ≤ 4,r = 2 if b = 1, and 3 ≤ r ≤2b if b ≥ 2
(cid:26)
when c = (2,1).
1
Note thatwe have c (E) = (f ,f )= (e ,e )where(e ,e )is thebidegree
2 1 2 2 1 1 2
of a curve associated to E via the Hartshorne-Serre correspondence (see
Definition 3.4). For cases c ∈ {(1,1),(a,0),(0,b) | a,b ∈ N} we also obtain
1
similar classification of possible quadruples (s;f ,f ;r) in Proposition 3.7,
1 2
Proposition 3.8 and Corollary 2.8. These results give us a complete answer
fortheclassificationofgloballygeneratedvectorbundleswiththefirstChern
class c with (0,0) ≤ c < (2,2). We also completely describe the globally
1 1
generated vector bundles with respect to quadruple (s;f ,f ;r) in most
1 2
cases. In the rank 2 case the list is so short that we may put it in the
introduction.
Theorem 1.2. Let E be a globally generated vector bundle of rank 2 on
X = P1 × P2 with no trivial factor and the first Chern class c such that
1
(0,0) ≤ c < (2,2). Then E is isomorphic to either
1
(1) a direct sum of two line bundles, or
(2) a Ulrich bundle arising from a non-trivial extension
0 −→ O (0,1) −→ E −→ O (2,0) −→ 0,
X X
(3) a twist of π∗(TP2(−1)).
2
So in the rank 2 case we found no unexpected bundle, finitely many iso-
morphism classes of bundles and these isomorphic classes are distinguished
by their c and c . The situation is quite different in rank higher than 2, al-
1 2
though sometimes an expected bundleof high rank gives a full classification
for lower ranks (see Proposition 5.11).
Let us here summarize the structure of this paper. In Section 2, we intro-
duce the definitions and main properties that will be used throughout the
paper, mainly the Hartshorne-Serre correspondence that relates the glob-
ally generated vector bundles with smooth curves contained in the Segre
threefolds. In Section 3 we collect several general results when one factor of
GLOBALLY GENERATED VECTOR BUNDLES 3
bidegree of associated curve is 1, and deal with the case of c = (1,1). In
1
Section 4 and 5 we keep classifying the globally generated vector bundles
with c = (2,1) and c = (1,2) respectively.
1 1
2. Preliminaries
In this section we introduce the definitions and main properties of Segre
varieties as well as the Hartshorne-Serre construction.
Definition 2.1. For n ,...,n ∈ N, let us define the Segre variety X =
1 s
Σ to be the product Pn1 ×···×Pns of s projective spaces.
n1,...,ns
Letting π : Pn1×···×Pns −→ Pni be the projection to the i-th factor, we
i
denote π1∗OPn1(a1)⊗···⊗πs∗OPns(as) by OX(a1,...,as). For our simplicity
letusdenoteO (a,...,a)byO (a), ifthereisnoconfusion. For acoherent
X X
sheaf E on X, we denote E ⊗O (a ,...,a ) by E(a ,...,a ) and the dual
X 1 s 1 s
sheaf of it by E∨.
The intersection ring A(X) is isomorphic to A(Pn1)⊗···⊗A(Pns) and so
we have
(1) A(X) ∼= Z[t ,...,t ]/(tn1+1,...,tns+1).
1 s 1 s
ThenX isembeddedintoPN withN = s (n +1)−1bythecompletelin-
i=1 i
earsystem|O (1)|. NotethatA1(X) ∼= Z⊕sbya t +···a t 7→ (a ,...,a ).
X 1 1 s s 1 s
Q
Let E be a globally generated vector bundle of rank r on X with the first
Chern class c (E) = (a ,...,a ). Then it fits into the exact sequence
1 1 s
⊕(r−1)
(2) 0−→ O −→ E −→ I (a ,...,a ) −→ 0,
X C 1 s
whereC isalocallycompleteintersection ofcodimension2onX by[11,Sec-
⊕(r−1)
tion 2. G]. If C is empty, then E is isomorphic to O ⊕O (a ,...,a ).
X X 1 s
In this article we will mainly work on X = P1 × P2 and so the locally
complete intersection C would be a curve in X.
Proposition 2.2. [12] If E is a globally generated vector bundle of rank r
on X with the first Chern class c such that H0(E(−c )) 6=0, then we have
1 1
⊕(r−1)
E ≃ O ⊕O (c ).
X X 1
In particular, in the classification of globally generated vector bundles on
X, we may assume that C is not empty and H0(E(−c )) =0.
1
Let πˆ : X −→ Pn1 ×···Pni ×···×Pns denote the natural projection.
i
Proposition 2.3. Let c = (a ,...,a ) ∈ Z⊕s with a = 0. Then there is
1 c 1 s ≥0 s
a bijection given by E 7→ πˆ∗(E) between the set of spanned vector bundles E
s
of rank r on Pn1 ×···×Pns−1 with c (E) = (a ,...,,a ) and the spanned
1 1 s−1
vector bundles of rank r on X with the first Chern class c . Moreover we
1
have
(1) hi(Pn1 ×···×Pns−1,E) = hi(X,π∗(E)) for all i ≥ 0;
s
(2) for any spanned bundle G on X with c (G) = 0, we have G ∼=
1
πˆ∗(πˆ (G)) with πˆ (G) a spanned bundle on Pn1 ×···×Pns−1.
s s∗ s∗
4 E.BALLICO,S.HUHANDF.MALASPINA
Proof. Thelaststatement follows fromtheprojection formulaandtheLeray
spectralsequenceofπˆs,becauseπˆs∗(OX) ∼= OPn1×···×Pns−1 andRiπs∗(OX) =
0for all i ≥ 0. Fix a spannedvector bundleG of rankr on X with c (G) = 0
1
and let ϕ : X −→ G(r,m) be the associated morphism to a Grassmannian
with m = h0(G). To prove the first assertion it is sufficient to prove that ϕ
is constant on every fiber of πˆ . Let L ∼= Pns be any fiber of πˆ and then the
s s
vector bundleG is spanned with trivial determinant. Therefore G ∼= O⊕r
|L |L L
and so ϕ(L) is a point. Since G ∼= πˆ (E) for some E on Pn1 ×···×Pns−1,
s∗
the projection formula gives E ∼= πˆ (G). (cid:3)
s∗
Let us say that a line bundle R on X has property ⋄ if the following
conditions are satisfied:
(1) R is globally generated;
(2) R 6= O ;
X
(3) If A is a globally generated line bundle with H0(R⊗A∨)6= 0, then
we have either A∼= O or A ∼= R.
X
When we have dim(X) = 3, the Hartshorne-Serre correspondence can be
stated as follows:
Theorem 2.4. [2, Theorem 1] Let R be a line bundle on X with ⋄.
(i) There is a bijection between the set of pairs (E,j), where E is a
spanned vector bundle of rank 2 on X with det(E) ∼= R and j :
K −→ H0(E) is non-zero map, up to linear automorphisms of K,
and the smooth curves C ⊂ X with I ⊗R spanned and ω ∼= R ,
C C |C
except that C = ∅ corresponds to O ⊕R with a section s nowhere
X
vanishing.
(ii) Fix an integer r ≥ 3. There is a bijection between the set of triples
(E,V,j), where E is a spanned vector bundle of rank r on X with
det(E) ∼= R, V is an (r−1)-dimensional vector space and j : V −→
H0(E) is a linear map, up to a linear automorphism of V, with
dependency locus of codimension 2, and the smooth curves C ⊂ X
with I ⊗R spanned and ω ⊗O ⊗R∨ spanned, except that C = ∅
C C X
corresponds to O⊕(r−1)⊕R with V = H0(O⊕(r−1)). E has no trivial
X X
factors if and only if j is injective.
(iii) There is a bijection between the set of all pairs (F,s), where F is
a spanned reflexive sheaf of rank 2 on X with det(E) ∼= R and 0 6=
s ∈ H0(E), and reduced curves C ⊂ X with I ⊗ R spanned and
C
ω ⊗ R∨ spanned outside finitely many points, except that C = ∅
C
corresponds to O ⊕R with s nowhere vanishing.
X
Example 2.5. On a smooth and connected projective threefold X, let us
fixa globally generated line bundleLwith h0(L) ≥ 2and setr := h0(L)−1.
SinceLisglobally generated, sotheevaluation mapψ : H0(L)⊗O −→ Lis
X
surjective and ker(ψ) is a vector bundle of rank r on X. The vector bundle
GLOBALLY GENERATED VECTOR BUNDLES 5
F := ker(ψ)∨ fits in an exact sequence
(3) 0 −→ L∨ −→ O⊕(r+1) −→ F −→ 0
X
and it determines the Chern classes of F. If h1(L∨) = 0, e.g. L is ample,
then the sequence (3) gives h0(F) = r + 1. If Y ⊂ X is the complete
intersection of two elements of |L|, then we get Y as the dependency locus
of a certain (r−1)-dimensional linear subspace of H0(F).
Remark 2.6. Assume that C is a curve with ω ∼= O (c −c (X)). If C
C C 1 1
has s connected components, then we have h0(ω (c (X)−c )) = s and so
C 1 1
the Hartshorne-Serre correspondence shows that C gives a vector bundle E
with c (E) = c of rank r with no trivial factor if and only if 2≤ r ≤ s+1.
1 1
Remark 2.7. On a smooth threefold X, let us fix a very ample line bundle
L and a smooth curve C ⊂ X. Assume that I ⊗L is globally generated
C
and take two general divisors M ,M ∈ |I ⊗L|. Set Y := M ∩M . By
1 2 C 1 2
the Bertini theorem we have Y = C ∪D with either D = ∅ or D a reduced
curve containing no component of C and smooth outside C ∩D.
FromnowonletusfixX tobetheSegrethreefoldP1×P2,theproductofa
projective line anda projective plane. LetV andV be2 and3-dimensional
1 2
vector spaces with the coordinates [x ,x ] and [y ,y ,y ], respectively. Let
0 1 0 1 2
X ∼= P(V )×P(V ) and then it is embedded into P5 ∼= P(V) by the Segre
1 2
map where V = V ⊗V .
1 2
Theintersection ringA(X)isisomorphictoA(P1)⊗A(P2)andsowehave
A(X) ∼= Z[t ,t ]/(t2,t3).
1 2 1 2
We may identify A1(X) ∼= Z⊕2 by a t +a t 7→ (a ,a ). Similarly we have
1 1 2 2 1 2
A2(X) ∼= Z⊕2 by b t t +b t2 7→ (b ,b ) and A3(X) ∼=Z by ct t2 7→ c. Then
1 1 2 2 2 1 2 1 2
X is embedded into P5 as a subvariety of degree 3 by the complete linear
system |O (1,1)|, because (t +t )3 = 3t t2.
X 1 2 1 2
IfE isacoherentsheafofrankronX withtheChernclassesc = (a ,a ),
1 1 2
c = (b ,b ) and c we have:
2 1 2 3
c (E(s ,s )) = (a +rs ,a +rs )
1 1 2 1 1 2 2
c (E(s ,s )) = c +c (s ,s )+(s ,s )2
2 1 2 2 1 1 2 1 2
1
χ(E) = r+ 2a +(a +1)a (a +3)
1 1 2 2
2
h
−(a +2,a +3)(b ,b )+c
1 2 1 2 3
for (b ,b ) ∈ Z⊕2. i
1 2
Now Proposition 2.3 implies the following when X = P1×P2.
Corollary 2.8. We have the following one-to-one correspondences:
(1) There is a bijection F 7→ π∗(F) between the spanned vector bundles
1
of rank r on P1 with degree a and the spanned vector bundles of rank
r on X with c = (a,0).
1
6 E.BALLICO,S.HUHANDF.MALASPINA
(2) There is a bijection F 7→ π∗(F) between the spanned vector bundles
2
of rank r on P2 with c = b and the spanned vector bundles of rank
1
r on X with c = (0,b).
1
In particular we have a full classification of globally generated vector
bundles on X with c = (1,0) or (0,b) with b ≤ 2, since we have a full list of
1
globally generated vector bundles on P2 with c ≤ 2 in [13]. Thus we may
1
assume c = (a,b) with a,b > 0. We will mainly work on all the cases with
1
a+b ≤ 3.
3. Warm-up and case of c = (1,1)
1
Let E be a globally generated vector bundle of rank r ≥ 2 on X with
the first Chern class c = (a ,a ), a ≥ a > 0. Then it fits into the exact
1 1 2 1 2
sequence
⊕(r−1)
(4) 0 −→ O −→ E −→ I (c ) −→ 0
X C 1
for a general r −1 sections of E, where C is a smooth curve. Write C =
C ⊔···⊔C with each C connected.
1 s i
If the rank is r = 2, we have E∨ ∼= E(−c ) and ω ∼= O (c −(2,3)). In
1 C C 1
this case we have h1(E∨) = h1(I ) since h1(O (−c )) = 0 by the Ku¨nneth
C X 1
formula. Since s is the number of the connected components of C, we have
h1(E∨) = s−1. If r ≥ 3, then we have c (E) = deg(ω ((2,3) −c )) and
3 C 1
ω ((2,3)−c ) is spanned.
C 1
Remark 3.1. The intersection theory on X is given by two variables t ,t
1 2
with the relations t2 = 0, t3 = 0 and t t2 = 1. Thus we have
1 2 1
(at +bt )(at +bt )(t +t ) = (2abt t +b2t2)(t +t )= 2ab+b2.
1 2 1 2 1 2 1 2 2 1 2
Remark 3.2. Let Y ⊂ X be the complete intersection of two divisors of
type |O (a,b)| containing C. By Remark 3.1 we have deg(Y) = 2ab+b2,
X
where deg(Y) is the degree of Y as a curve in P5. By the Bertini theorem
Y is a curve containing C and smooth outside C. Note that C occurs with
multiplicity one in Y, because I (a,b) is spanned and so, affixing P ∈ C ,
C i i
1 ≤ i ≤ s, we may find a divisor T ∈ |I (a,b)| not containing the tangent
C
lineofC atP . AKoszulcomplexstandardexactsequencegivesh0(O ) = 1
i i Y
andsoY isconnected. Theadjunctionformulagivesω ∼= O (2a−2,2b−3)
Y Y
and so we have
2p (Y)−2= (at +bt )(at +bt )((2a−2)t +(2b−3)t )
a 1 2 1 2 1 2
= (−3+2b)2ab+b2(−2+2a)
= 2b(3ab−3a−b).
Example 3.3. Let Y ⊂ X be any complete intersection of two elements
of |O (a,b)| with (a,b) ∈ N2 \{(0,0)}. We have ω ∼= O (2a−2,2b−3)
X Y Y
and hence ω (2−a,3−b) ∼= O (a,b). Therefore Y is not the zero-locus
Y Y
of a section of a bundle of rank 2. Note that h0(ω (2 − a,3 − 2b)) =
Y
GLOBALLY GENERATED VECTOR BUNDLES 7
(a+1) b+2 −2. As in Example 2.5 we see that Y gives a globally generated
2
bundle with c = (a,b) and Y is a dependency locus for a bundle of rank r
1
(cid:0) (cid:1)
if and only if 3 ≤ r ≤ (a+1) b+2 −1. If r = (a+1) b+2 −1, there is a
2 2
unique bundle E associated to some complete intersection curve. Since E is
unique, we have g∗(E) ∼= E for(cid:0)eac(cid:1)h g ∈ Aut(X) = Aut(cid:0)(P1)(cid:1)×Aut(P2), i.e.
E is homogeneous.
Let T be an integral projective curve. By the universal property of the
projective spaces, there is abijection between theset of morphismsu: T −→
X and the set of pairs (u ,u ) of morphisms u : T −→ P1 and u :T −→ P2.
1 2 1 2
If u is an embedding, then we have
deg(O (a,b)) = adeg(u )+bdeg(u )deg(u (T))
u(T) 1 2 2
with the convention that deg(u ) = deg(u (T)) = 0 if u (T) is a point.
2 2 2
Now assume that u (T) is a curve and T is smooth. Let u′′ :J −→ u (T)
2 2
denote the normalization map and then there is a unique morphism u′ :
T −→ J such that u = u′◦u′′. We have deg(u ) = deg(u′).
2 2
Now take as T a connected component C of C. Write
i
e[i] := deg(u ) and e[i] = deg(u )deg(u (C )).
1 1 2 2 2 i
If e[i] 6= 0, set e′ := deg(u′) and e′′ := deg(u (T)). If e[i] = 0, set
2 2 2 2 2
e′ = e′′ = 0.
2 2
Definition 3.4. For a curve C ⊂ X, the bidegree of C is defined to be the
pair of integers (e ,e ) with
1 2
s s
(e ,e ) := e[i] , e[i] .
1 2 1 2
!
i=1 i=1
X X
Indeed, we have e = deg(O (1,0)) and e = deg(O (0,1)).
1 C 2 C
Lemma 3.5. Assume c = (a,1) with a ≥ 0.
1
(i) We have e[i] ∈ {0,1} for each i∈ {1,...,s}.
1
(ii) If e[i] = 0, then e[i] = 1 and there are P ∈ P1 and a line L ⊂ P2
1 2 i i
such that C = {P }×L .
i i i
(iii) If e[i] = 1 for some i, then s = 1 and C is rational.
1
Proof. If e[i]1 = 0, then there are Pi ∈ P1 and Li ∈ |OP2(e[i]2)| such that
C = {P }×L . Since I (a,1) is globally generated, we get e[i] = 1.
i i i C 2
Assume e[i] 6= 0 and then π (C ) = P1. For a fixed point P ∈ P1, the
1 1 i
degreeofO (a,1) is1. SinceI (a,1)isglobally generated, soweget
X |{P}×P2 C
deg(C ∩π−1(P)) = 1, i.e. e[i] = 1. We also see that π : C −→ P1 has
1 1 1|Ci i
degree 1 and so C is rational. If e 6= 0, we get deg(π ) = 1 and so there
i 1 1|C
is a unique index i with e[i] = 1. Assume s≥ 2 and fix j ∈{1,...,s}\{i}.
1
Since e[j] = 0, there is P ∈ P1 and a line L ⊂ P2 with C = {P }×L .
1 j j j j j
Set {P ,O} := C ∩{P }×P2. Since C ∩C = ∅, we have O ∈/ L and so
j i j i j j
{P }×P2 is in the base locus of I (1,1). (cid:3)
j C
8 E.BALLICO,S.HUHANDF.MALASPINA
Lemma 3.6. Assume c = (1,b) with b ≥ 0.
1
(i) π :C −→ P2 is an embedding.
2|C
(ii) e > 0 and C is isomorphic to a smooth plane curve of degree e .
1 2
Proof. Let us fix a point P ∈ P2 and set T := P1 ×{P}. Since O (1,3−
T
b) ∼= O (1,0) has degree 1, so ω (1,3 − b) has negative degree and in
T T
particular it is not spanned. Thus T is not a connected component of C.
Since I (1,b) is globally generated, we get deg(T ∩C) ≤ 1 for all P ∈ P2
C
andsoπ : C −→ P2 is an embedding. Sinceeach planecurveis connected,
2|C
we get s = 1. Since π is an embedding, we also get that C is isomorphic
2|C
to a smooth plane curve of degree e . (cid:3)
2
Proposition 3.7. Let E be a globally generated vector bundle of rank 2 with
c1(E) = (1,1) and no trivial factor. Then we have E ∼= OX(1,0)⊕OX(0,1).
Proof. Thezero-locusofageneralsectionofO (1,0)⊕O (0,1)hasbidegree
X X
(0,1), because c (O (1,0) ⊕ O (0,1)) = t t . Since I (1,1) is spanned,
2 X X 1 2 C
so C is contained in the complete intersection Y of two hypersurfaces in
|I (1,1)|. Note that we have deg(Y) = 3 and ω ∼= O (0,−1); we have
C Y Y
2p (Y)−2 = (t +t )(t +t )(−t ) = −2 and so p (Y) = 0. Thus we have
a 1 2 1 2 2 a
deg(C) ≤ 3 with the equality if and only if C = Y is a linear section of
codimension 2.
The case of deg(C) = 3 is not possible since ω ∼= O (−1,−2). Now
C C
assume deg(C) ≤ 2. Since c (O (1,0)⊕O (0,1)) = t t , the curves asso-
2 X X 1 2
ciated to O (1,0)⊕O (0,1) have bidegree (0,1). Since ω ∼= O (−1,−2)
X X C C
and O (1,2) is ample, each C is rational and e[i] +2e[i] = −deg(ω ) =
X i 1 2 Ci
2. Thus each connected component C of C is a curve of bidegree (0,1).
i
Lemma 3.6 gives s = 1. Since h0(O (0,1)) = 2 < h0(O (0,1)), we
C X
have h0(I (0,1)) > 0 and so any associated bundle E has a non-zero map
C
f : O (1,0) −→ E because of h1(O (−1,0)) = 0. Since h0(I (−1,1)) =
X X C
h0(I (0,0)) = 0, so coker(f) is torsion-free, i.e. coker(f) ∼= I (0,1) with
C T
either T = ∅ or T a locally complete intersection curve. Since c (O (1,0)⊕
2 X
O (0,1)) = t t and C has bidegree (0,1), so we have T = ∅. It implies
X 1 2
E ∼= O (1,0)⊕O (0,1), since h1(O (1,−1)) = 0. (cid:3)
X X X
Proposition 3.8. Let E be a globally generated vector bundle of rank r ≥ 3
on X with c = (1,1) and no trivial factor. Then we have r ∈ {3,4,5} and
1
E is one of the following:
⊕(r+1)
(i) 0 −→ O (−1,−1) −→ O −→ E −→ 0 with r ∈ {3,4,5},
X X
the associated curve is the complete intersection Y of two hypersur-
faces in |O (1,1)|;
X
(ii) E ∼= O (1,0)⊕π∗(TP2(−1)).
X 2
Proof. If the associated curve is the complete intersection Y of two hyper-
surfaces in |O (1,1)| we are in the case (i) by Example 3.3 and so we may
X
assume that the associated curve C is not a complete intersection.
GLOBALLY GENERATED VECTOR BUNDLES 9
Since in the case r = 2 no bundle has a section with s > 1 by Proposition
3.7, we only need to check the smooth curves C with I (1,1) globally gen-
C
erated and with ω (1,1) globally generated, butnot trivial. It only remains
C
to check the cases with s = 1 and deg(C) = 2. C cannot have bidegree
(2,0), because it is irreducible.
Nowtakeaconicwithbidegree(0,2). ThereareP ∈P1 andC′ ∈ |OP2(2)|
with C = {P}×C′. Then {P}×P2 is in the base locus of I (1,1).
C
Hence the only bundles not associated to the complete intersection curve
may come from a connected C with bidegree (1,1). For any such a curve C
we have deg(ω (1,2)) = 1 and h0(ω (1,2)) = 2. Thus it gives a bundle if
C C
and only if r = 3. Any connected curve C with bidegree (1,1) is associated
to a pair (u ,u ) with u : P1 −→ P1 an isomorphism and u : P1 −→ P2
1 2 1 2
an embedding as a line. Hence for any two such curves C and C′, there is
f ∈ Aut(X) with f∗(C) = C′. The bundle E := O (1,0)⊕π∗(TP2(−1))
0 X 2
has the Chern polynomial (1+t )(1+t +t2) = 1+t +t +t t +t2+t t2
1 2 2 1 2 1 2 2 1 2
and so it is associated to a connected curve with bidegree (1,1). For any
f ∈ Aut(X) we have f∗E ∼= E and so E is the only bundle associated to a
0 0 0
curve of bidegree (1,1). (cid:3)
Remark 3.9. If r = 5 and C = Y, then we have E ∼= TP5(−1) , be-
|X
cause Remark 2.5 gives the uniqueness of such a bundle. We denote by
S the family of bundles obtained from (i). We may compute dim(S ) =
r r
h0(O⊕(r+1)(1,1))−(r+1)2 and so we get
X
dim(S ) = 0 , dim(S ) = 5 , dim(S ) = 8.
5 4 3
InfactS is given bythepull-backs of TP4(−1) byaprojection fromapoint
4
of P5\X and S is given by the pull-backs of TP3(−1) by a projection from
3
a line of P5 not meeting X.
4. Case of c = (2,1)
1
Let Y be any complete intersection of two hypersurfaces in |O (2,1)|.
X
Since (2t + t )(2t + t )t = 1 and (2t + t )(2t + t )t = 4, we have
1 2 1 2 1 1 2 1 2 2
deg(O (1,0)) = 1 and deg(O (0,1)) = 4. In particular the connected curve
Y Y
Y ⊂ P5 has degree 5. We have ω ∼= O (2,−1). Since (2t + t )(2t +
Y Y 1 2 1
t )(2t −t ) = −2, we have p (Y) = 0. It implies that the sectional genus
2 1 2 a
of O (2,1) is zero. Notice ω 6∼= O (0,−2), because deg(O (0,−2)) = −4.
X Y Y Y
Example 3.3 shows that Y gives a globally generated vector bundle of rank
r if and only if 3 ≤ r ≤ 8.
Lemma 4.1. Assume that I (2,1) is globally generated. Then one of the
C
following cases occurs:
(i) (s;e ,e )= (1;1,b) with 1≤ b ≤ 4.
1 2
(ii) (s;e ,e )= (s;0,s) with 1 ≤ s ≤ 3.
1 2
In particular there are s distinct points P ∈ P1 and lines L ⊂ P2
i i
such that C = {P }×L for each i; in this case ω ∼= O (0,−2).
i i i C C
10 E.BALLICO,S.HUHANDF.MALASPINA
Proof. Since Y has bidegree (1,4), we have e ≤ 1 and e ≤ 4. If e > 0, we
1 2 1
have s = 1 and 1≤ e ≤ 4 by Lemma 3.5.
2
Now assume e = 0. By Lemma 3.5 we have s = e and there are P ∈P1
1 2 i
and a line L ⊂ P2 such that C = {P } × L . If (e ,e ) = (0,4), then
i i i i 1 2
Y = C ∪D with D a line of type (1,0) since Y has bidegree (1,4). Since
h0(O )= 1 (see Remark 3.2), we get deg(D∩C)≥ 4. Therefore I (2,1) is
Y C
not globally generated. (cid:3)
Remark 4.2. Wedonotclaimthateach C listedinLemma4.1hasI (2,1)
C
globally generated. The case (e ,e ) = (1,4) corresponds to the complete
1 2
intersection curve Y. We check in Example 4.7 that the case e ∈ {2,3} is
2
realized. Since in both cases C is connected with arithmetic genus zero, we
have h0(ω (0,2)) = 2e −1 and ω (0,2) is spanned (and trivial if e = 1).
C 2 C 2
Hence the existence part for the case (s;e ,e ) = (1;1,1) gives globally
1 2
generated bundles with s = 1 and bidegree (1,1) if and only if r = 2, while
the existence part for (s;e ,e ) = (1;1,b) with 2 ≤ b ≤ 4, gives bundles if
1 2
and only if 3 ≤ r ≤ 2b.
Lemma 4.3. Consider a curve C with (s;e ,e ) = (2;0,2). Then the asso-
1 2
ciated bundle of rank r with no trivial factor is one of the following:
(i) an extension of O (2,0) by O (0,1) if r = 2; either O (2,0) ⊕
X X X
O (0,1) or one of the bundles in the 2-dimensional family of Ulrich
X
bundles in [8, Theorem 4.1];
(ii) O (0,1)⊕O (1,0)⊕2 if r = 3.
X X
In particular such a curve C does not give bundles of rank higher than 3
without trivial factors.
Proof. Each connected component of C is a line of bidegree (0,1). Since
ω ∼= O (0,−2) and s = 2, the associated bundles have rank r ∈ {2,3} by
C C
Remark 2.6. In all cases we have c (E) = 0.
3
(a) First assume r = 2. Since (s;e ,e ) = (2;0,2), so there are P ,P ∈
1 2 1 2
P1 andlines L ,L ⊂ P2 suchthat C = {P }×L ∪{P }×L . In particular,
1 2 1 1 2 2
we have h0(I (2,0)) > 0 and so there is a non-zero map f : O (0,1) −→
C X
E. Since L ∩ L 6= ∅, we have P 6= P and so h0(I (1,0)) = 0. Since
1 2 1 2 C
h0(I (2,−1)) = 0, so coker(f) is torsion-free, i.e. coker(f) = I (2,0) with
C T
either T = ∅ or T a locally complete intersection curve. Since c (O (2,0))·
1 X
c (O (0,1)) = 2t t and C has bidegree (0,2), we have T = ∅. Since
1 X 1 2
h1(O (−2,1)) = 3, we get non-trivial extensions of O (2,0) by O (0,1).
X X X
Since the two line bundles are both Ulrich, any extension of them is again
Ulrich (see [8, Theorem 4.1]).
(b) Assumer = 3. Let F bethe cokernel of a general map u: O −→ E
X
and v : E −→ F denote the quotient map. Since c (E) = 0, so F is locally
3
free and it is as described in (i). In particular O (0,1) is a subbundle of
X
F with O (2,0) as its cokernel. Set G := v−1(O (0,1)). Since G is an
X X
extension of O (0,1) by O , we have G ∼= O ⊕ O (0,1). Therefore E
X X X X
is an extension of O (2,0) by O ⊕O (0,1). In particular O (0,1) is a
X X X X