Table Of ContentCurves on Rational Surfaces with Hyperelliptic
1
1
Hyperplane Sections
0
2
n
a Ovidiu P˘as˘arescu
J ∗
3
]
G
Abstract
A
.
h In this article we study, given a pair of integers (d,g), the problem
at of existence of a smooth, irreducible, non-degenerate curve in Pn of
m degree d and genus g (the Halphen-Castelnuovo problem). We define
[ two domains from the (d,g)-plane, Dn and Dn, and we prove that
1 2
1 there is no gap in D1n. This follows constructing curves on some ra-
v tional surfaces with hyperelliptic hyperplane sections and from some
7
previous theorems of Ciliberto, Sernesi and of the author. Moreover,
7
5 in thelast section, based on some results of Horrowitz, Ciliberto, Har-
0 ris, Eisenbud, we conjecture that Dn is the right lacunary domain.
2
.
1
0
1 Contents
1
:
v
1. Introduction ...................................................... 2
i
X
2. The functions α (d,n); the domains Dn,Dn and An; the Main Theo-
p 1 2 p
r
a rem: absence of gaps in Dn ..............................................5
1
3. The proof of Main Theorem .......................................8
3.1 Methods ...................................................8
3.2 Two smoothing criteria; the surfaces Xn .................. 12
p
3.3 Numerical properties of the functions α (d,n) .............17
p
3.4 Invertible sheaves Pic(Xn) whose (deg( ),p ( )) cover
L ∈ p L a L L
the domain An (n 5,p n/3) ........................................18
p ≥ ≥
∗
Work partially supported by JSPS (Japan Soc. for Promotionof Science), Romanian
Ministry of Education and Research (CERES Program) and Romanian Academy (GAR
Project No. 5 / 5.02.2008)
1
3.5 Curves C Xn whose (deg(C),g(C)) cover the domain An
⊂ p C p
(n 8,n/3 p n 4) ...............................................28
≥ ≤ ≤ −
3.6 Conclusion ................................................36
4. Comments and further developments; expected gaps in Dn ....... 36
2
References ..........................................................37
1 Introduction
In this article we work over an algebraically closed field K of characteristic
zero (e.g. K = C the complex field); see the section 4 also for arbitrary
characteristic. We’ll use the standard notations, see [Ha1].
By a curve (resp. a surface) we mean a K-algebraic integral scheme of
dimension 1 (resp. 2). If not otherwise specified the curves and surfaces
appearing in this paper will be supposed non-singular.
A (possible singular) curve C Pn (= the projective n-space) is called
⊂
non-degenerate if it is not contained in any hyperplane.
Given a property we call -Halphen Castelnuovo Theory in Pn the
P P
study of existence - for arbitrary, but fixed triplets if integers (n,d,g), n 2,
≥
d n, g 0 - of non-degenerate (possible singular) curves C Pn of degree
≥ ≥ ⊂
d and geometric genus g, and having the property . Then it is natural to
n,P P
study the (non-empty) families of curves as before.
Fd,g
The property could be, for instance, smoothness, linear normality,
P
projective normality, maximal rank, expected number of moduli, etc. If
=smootheness andHn istheHilbertschemeofsmoothcurvesfromPn (i.e.
P d,g
the closure in the general Hilbert scheme of the locus of smooth, irreducible,
non-degenerate curves from Pn), then one could take nP = Hn .
Fd,g d,g
When studying the non-degenerate embedded curves C Pn for some
⊂
n 2 one usually study the previous schemes Hn , the first step being to
≥ d,g
find the triplets of integers (n,d,g) for which Hn = . Let’s now denote, as
d,g 6 ∅
usual, by the moduli space of curves of genus g. Then, there is a natural
g
M
rational map π = πn : Hn whose fibers over the [C] are
d,g d,g −− → Mg ∈ Mg
related to the determinantal varieties Wn = Wn(C) := gs gs a base-point
d d { d| d
free linear system of degree d and dimension s,s n appearing in the Brill-
≥ }
Noether Theory for polarized curves (C,gs). As explained by J. Harris in [H],
d
the rational map π is very useful when studying the connections between the
extrinsic geometry (represented by the properties of projective embeddings
and of the Hilbert scheme) and the intrinsic geometry (represented by the
properties of the abstract curves and of the moduli space) of the families of
2
curves. Certainly, this is possible when π = (so, again when Hn = ).
6 ∅ d,g 6 ∅
We recall now the Castelnuovo bound (found in 1893, for a modern proof,
see [H], ch.3):
Theorem 1.1 (Castelnuovo [C]). If n Z, n 3 and C Pn is a
∈ ≥ ⊂
non-degenerate (possibly singular) curve of degree d and geometric genus g,
then d n and 0 g π (d,n).
0
≥ ≤ ≤
This result has been a generalization of aprevious similar result, obtained
by Halphen and Noether in 1882 for n = 3 ([Hl], [N]).
Here π (d,n) is the first one of the Harris-Eisenbud numbers π (d,n),
0 p
introduced in [H], ch.3 (for p 1), and given (for 0 p n 2) by:
≥ ≤ ≤ −
m (m 1)
p p
(1.1) π = π (d,n) = − (n+p 1)+m (ε +p)+µ , where
p p p p p
2 −
(1.2) m = m (d,n) = [(d 1)/(n+p 1)]
p p ∗
− −
(we denote, during this article, by [x] the integer part of x R = the field
∗
∈
of the real numbers)
(1.3) ε = ε (d,n) = d 1 m (n+p 1)
p p p
− − −
(1.4) µ = µ (d,n) = max(0,[(p n+2+ε )/2] ).
p p p ∗
−
We remark that µ = 0 and π = d2/(2(n+p 1))+O(d).
0 p
−
We recall, as an example, that if =nodality, the complete answer in
P
-Halphen-Castelnuovo Theory is given by the following:
P
Theorem 1.2 (Tannenbaum [T ],[T ]). For n 2, n Z and any
1 2
≥ ∈
d,g Z, d n and 0 g π (d,n) there is a non-degenerate curve C Pn
0
∈ ≥ ≤ ≤ ⊂
of degree d and geometric genus g having only nodes as singularities.
This Theorem generalizes a similar result of Severi from 1915 for n = 2
([Sv]).
If =smoothnesswearrivetotheHalphen-Castelnuovo Problem(related
P
to the classification of the abstract curves and to the comparison between
the intrinsic and extrinsic geometry of curves), denoted by HC(n), for 2,
≥
n Z:
∈
HC(n): For which pairs of integers (d,g), d n, 0 g π (d,n) do
0
≥ ≤ ≤
we have Hn = (equivalently, for which pairs (d,g) as before there is a
d,g 6 ∅
(smooth, irreducible) non-degenerate curve C Pn of degree d and genus
⊂
g?).
3
This is the Problem which we’ll consider in this article.
Definition 1.3. A pair of integers (d,g), d n, 0 g π (d,n) is
0
≥ ≤ ≤
called a gap for HC(n) if Hn = (i.e. there is no non-degenerate (smooth,
d,g ∅
irreducible) curve C Pn of degree d and genus g).
⊂
FromtheTheorem1.2followsthatthereisnogapin -Halphen-Castelnuovo
P
Theory when = nodality. However, the situation is different for HC(n),
P
so the Problem is more complicated.
Indeed, HC(2) is simple: H2 = d 2 and g = (d 1)(d 2)/2.
d,g 6 ∅ ⇔ ≥ − −
But, for n 3, HC(n) becomes highly non-trivial. A (correct) answer to
≥
HC(3) has been stated by Halphen in 1882 ([Hl]), but his proof has been,
partially, incorrect. A complete proof has been given, 100 years later, by
Gruson and Peskine ([GP1], [GP2]). The solution of HC(3) is the following:
Theorem 1.4 (Halphen-Gruson-Peskine). For HC(3) there are two
domains in the (d,g)-plane: the non-lacunary domain D3, where there is
1
no gap for HC(3) and the lacunary domain D3, where there are gaps. Here
2
D3 : 0 g π (d,3), d 3
1 ≤ ≤ 1 ≥ .
D3 : π (d,3) < g π (d,3), d 3.
2 1 ≤ 0 ≥
Moreover, a pair (d,g) D3 is not a gap for HC(3) iff (d 2)2 4g is the
∈ 2 − −
square of some integer (see the Theorem 1.1 also).
HC(n) has been also solved for n = 4,5 by Rathmann ([Ra]), n = 6
by Ciliberto ([Ci]) and ”almost” solved by Ciliberto if n = 7 ([Ci]). The
situation is similar to the case n = 3 in the sense that there are two domains:
Dn, where there is no gap, and Dn, where there are gaps. The domains Dn
1 2 1
and Dn (n 4,5,6,7 ) are explicitely defined and the gaps from Dn are
2 ∈ { } 2
also determined (with the exception of 24 pairs for n = 7 for which is not
yet known if they are gaps or not). Results for the general HC(n) belongs
to Ciliberto, Sernesi ([Ci], [CS]), Harris, Eisenbud ([H]), Rathmann ([R]),
Pa˘sa˘rescu ([P1], [P2], [P3]) and others. We’ll use some of them in the next
section (see Section 4 also).
We end this section remarking that the case n = 3 of HC(n) has some
intrinsic importance, because any (smooth, irreducible) curve from some Pn,
n 4, can be projected (so, preserving the degree) isomorphically (so, pre-
≥
serving the genus) on P3. But, for more sophisticated reasons, consisting in
the comparison between the extrinsic and intrinsic properties, the natural
problem is the general HC(n), n 3.
≥
Aknowledgement: Duringthe preparation ofthis article, Ihave worked
to the Institute of Mathematics of the Romanian Academy and I have visited
4
the University of Ferrara, the Mittag-Lehfler Institute from Stockholm, the
University of Nice, the University of Angers, TIFR Mumbai, MSRI Berkeley,
the National Australian University Canberra, the University of Iowa and the
University of Tokyo (in this chronological order). I have had interesting and
stimulating discussions on the subject with J. Alexander, C. Ciliberto, J.
Coanda˘, D. Eisenbud, Ph. Ellia, R. Hartshorne, A. Hirschowitz, T. Katsura,
J. Kleppe, D. Laksov, A. Lascu, Y. Miyaoka, C. Peskine, M. Reid, R. Strano.
I want to thank to S¸. Basarab, A. Buium, A. Dimca, D. Popescu also for
general stimulating discussions.
2 The functions α (d,n); the domains Dn,Dn
p 1 2
and An; the Main Theorem: absence of
p
gaps in Dn
1
Before stating the Main Theorem which will be proved in this article, we
need a number of definitions.
For p,n Z, p 0, n 3 we define the following numerical functions
∈ ≥ ≥
α (d,n) = d2/(2(n+p 1))+O(d) given by:
p
−
x (x 1)
p p
(2.1) α = α (d,n) = − (n+p 1)+x (t +p)+u , where
p p p p p
2 −
(2.2) x = x (d,n) := [(d an)/(n+p 1)]
p p − p − ∗
(2.3) a = an := [(n p)/2] +1
p p − ∗
(2.4) t = t (d,n) := d 1 x (n+p 1)
p p p
− − −
(2.5) u = u (d,n) := [(p n+1+t )/2] .
p p p ∗
−
We also consider the functions
(2.6)
1
max(2n+1, (3+(4k 1)√24k 33)), n 0 (mod 3)
6 − − ≡
d1(n) := max(2n+1, 4k4+k 1(−4k +√32k3 +16k2 −2k −1)), n ≡ 1 (mod 3)
max(2n+1,5k+3+(2k +1)√48k +6), n ≡ 2 (mod 3)
5
where k = [n/3] , n 4.
∗
≥
We’ll need also (with k from before):
(2.7)
π (d,n), if n 0,1 (mod 3)
k
A(d,n) := ≡
π (d,n) µ +max(0,ε 3k 1), if n 2 (mod 3)
k k k
− − − ≡
(see (1.1)-(1.4)).
(2.8)
α (d,n), if n 1,2 (mod 3)
k+1
≡ , if 2n+1 d < d1(n)
B(d,n) := αk(d,n), if n ≡ 0 (mod 3) ≤
A(d,n), if d d (n)
≥ 1
We consider now the following graphs (contained in the (d,g)-plane), of
equations:
Bn : g = π (d,n)
0 0
π (d,n), n d 2n
0
Bkn : g = ≤ ≤ , k = [n/3]∗
B(d,n), d 2n+1
≥
Cn : g = α (d,n), p n/3, p,n Z, n 3
p p ≥ ∈ ≥
Now we are ready to define the domains Dn (bounded by g = 0 and Bn)
1 k
and Dn (bounded by Bn and Bn), contained in the (d,g)-plane:
2 k 0
π (d,n), n d 2n
0
(2.9) Dn : 0 g ≤ ≤
1 ≤ ≤ B(d,n), d 2n+1
≥
(2.10) Dn : B(d,n) < g π (d,n), d 2n+1
2 ≤ 0 ≥
TheMainTheorembelongingtoHalphen-Castelnuovo Theorywhichwe’ll
prove in this article is:
MAIN THEOREM. In the domain Dn there is no gap for HC(n),
1
n 4, n Z.
≥ ∈
Remark 2.1. The Main Theorem is known to be true for d > D(n) =
a quadratic function in n (see [Ci], [CS], [P1]). So, our main contribution
is for ”small” degrees, 2n + 1 d < D(n). Moreover, as we recalled in
≤
section 1 the Main Theorem is known to be true for n 4,5,6,7 , due to
∈ { }
6
the contributions of Rathmann ([Ra]), Ciliberto, Sernesi ([Ci], [CS]), and of
the author ([P1]). So, we’ll suppose that n 8 (but our arguments work, in
≥
principle, for 5 n 7 also).
≤ ≤
During the proof of the Main Theorem, the domain Dn will be divided
1
in subdomains An, defined by:
p
(2.11) A′n : 0 g α (d,n), d 2n+1 (between g = 0 and Cn )
n−3 ≤ ≤ n−3 ≥ n−3
(2.12) An : α (d 1,n) g α (d,n), d 2n+1,p n/3
p p+1 − ≤ ≤ p ≥ ≥
(containing the domain between Cn and Cn for p n 1: see subsection
p+1 p ≤ −
3.3 from section 3).
(2.13) A˜n : α (d 1,n) < g B(d,n), d 2n+1,k = [n/3]
k k+1 − ≤ ≥ ∗
(containing the domain between Cn and Bn)
k+1 k
(2.14) An : 0 g π (d,n), n d 2n.
0 ≤ ≤ 0 ≤ ≤
Lemma 2.2. In the domain An there is no gap for HC(n), n 8.
0 ≥
Proof. We use projections of non-special curves. For d = 2n and g =
π (d,n) = π (2n,n), we consider the extremal curve.
0 0
q.e.d.
The following equality holds (see Lemma 3.3.1b)):
(2.15) Dn = An A′n An A˜n
1 0 ∪ n−3 ∪ p ∪ k
n≤p[≤n−4
3
We just solved the elementary step (curves in An) of HC(n), n 8 in
0 ≥
the previous lemma. By (2.15), the Main Theorem is now the consequence
of the following three Theorems:
Theorem A. If n Z, n 8, then there is no gap for HC(n) in the
domain A˜n. ∈ ≥
k
The proof of this Theorem essentially belongs to Ciliberto ([Ci], Theorem
1.1). However the statement of the Theorem A differs slightly from the
statement of the Theorem of Ciliberto. The proof of the Theorem A can be
found in [P2].
Theorem B. If n Z, n 8, then there is no gap for HC(n) in the
domain A′n . ∈ ≥
n−3
7
The proof of this Theorem essentially belongs to Ciliberto and Sernesi
([CS], Main Theorem), see also [P1]. However the statement of the Theorem
of Ciliberto and Sernesi differs slightly from the statement of the Theorem
B. The proof of the Theorem B can be found in [P3].
Theorem C. If n Z, n 8, then there is no gap for HC(n) in the
∈ ≥
domain An.
p
n≤p[≤n−4
3
This Theorem C represents our main contribution. It has been already
used during the proof of the Theorem A in [P2]. The complete proof of the
Theorem C is given in the next section 3. It is done constructing curves on
some rational surfaces Xn Pn with hyperelliptic hyperplane sections.
p ⊂
3 The proof of Main Theorem
3.1 Methods
Let’s fix some notations. Let there be Σ P2 a finite set of (distinct) points
⊂
Σ = P ,P ,...,P . We denote by S = BL (P2) P2 the blow up of P2
0 1 s Σ
{ } →
in Σ. Then Pic(S) = Z Zs+1 with (l; e , e ,..., e ) a Z-basis (here l is
∼ 0 1 s
⊕ − − −
the class of the inverse image in S of a line L P2 and e are the classes of
i
⊂
the exceptional divisors E S corresponding to the points P , 0 i s).
i i
⊂ ≤ ≤
We recall that the intersection form on Pic(S) is given by:
(l e ) = 0, (e2) = 1, i = 0,s, (e e ) = 0, ( )i = j, (l2) = 1.
· i i − i · j ∀ 6
s
If = al b e Pic(S), we write = (a;b ,b ,...,b ).
i i 0 1 s
L − ∈ L
Xi=0
If = (D), D Div (S), we denote by [ ] = D the complete
S
L O ∈ L | |
linear system associated to (resp. D). We write [(a;b ,b ,...,b )] :=
0 1 s
L
[a;b ,b ,...,b ]. We say that a,b ,b ,...,b are the coefficients of (or [ ]).
0 1 s 0 1 s
L L
Let’s now analyse the Gruson-Peskine type construction given by Cilib-
erto ([Ci]). The surfaces Xn used by Ciliberto, containing the necessary
k
curves (k = [n/3] ) are obtained blowing up 3k n + 6 points from P2 in
∗
−
general position (i.e. any 3 non-collinear and not all of them on a smooth
conic) and embedding the abstract surface obtained in such a way using
the very ample invertible sheaf (k+2;k,13k−n+5) (we denoted (a;b,c,...,c)
stimes
by (a;b,cs)). Because 3k n + 6 4,5,6 there is an well-kno|wn{zcri}-
− ∈ { }
terion giving sufficient conditions for a linear system in order to contain
8
(smooth, irreducible) curves. Precisely, if = (a;b ,b ,...,b ) Pic(Xn),
L 0 1 snk ∈ k
sn = 3k n+5 and
k −
(3.1.1) a b +b +b , b b ... b 0, a > b
0 1 2 0 1 sn 0
≥ ≥ ≥ ≥ k ≥
then [ ] = [a;b ,b ,...,b ] contains a (smooth, irreducible) curve.
0 1 sn
L k
Due to this criterion (depending only on coefficients), the construction
of curves on the surfaces Xn has two parts: an arithmetical part, where the
k
necessary degrees and arithmetical genera are obtained using the well-known
formulae of genus and degree for various Pic(Xn), and a geometrical
L ∈ k
part, where the necessary (smooth, irreducible) curves are obtained applying
(3.1.1) to the sheaves used in order to solve the arithmetical part.
L
The main contribution of this paper to HC(n) is the construction of
invertible sheaves (and then curves) having the degree and genus in the
domains An (see (2.12)), on some surfaces Xn. Precisely, let’s denote by
p p
S = Sn := Bl (P2), Σ = Σn = P ,P ,...,P P2, sn = 3p n+5,
p p Σp p p { 0 1 snp} ⊂ p −
Σn containing general points. Then n := (p + 2;p,1snp) Pic(Sn) is very
p Hp ∈ p
ample (see subsection 3.2, Prop. 3.2.2) and then Xn := Imϕ Pn,
p [Hnp] ⊂
degXn = n+p 1. If p = k one obtains the surfaces Xn used in [Ci].
p − k
Ifwetrytouseanargumentsimilartotheargumentfrom[GP2]or[Ci]for
Xn, so todivide theproofintwo parts, anarithmetical andageometrical one
p
(the smoothing) we need a criterion similar to (3.1.1) for an arbitrary number
of points from Σ . But this is, anyway, very complicated by itself (see, for
p
instance [Hi]). On the other hand, such a general criterion must include the
conditions (3.1.1) (normalization, using quadratic transformations, see [Hi]);
but these conditionsallow usto construct onlycurves whose degreed isd > a
function of degree 3/2 in n, as it can be seen if we try to apply an argument
as in [Ci]. So, in order to produce the necessary curves for small degrees, we
need a new technique, which we shall explain here.
The main idea is to consider the arithmetical and the geometrical parts
of the construction of curves on Xn entirely linked, i.e. to construct linear
p
systems containing the necessary curves directly and explicitly enough using
combinations between some simple sheaves (so, somehow easy to understand
them).
Explicitly, we’ll proceed as follows (in order to construct the necessary
curves on Xn whose (degree, genus) belongs to the domain An): we start
p p
with a simple initial family of sheaves
0
D
(3.1.2) = (a+2;a,1t,0snp−t) Pic(Xn), 0 t sn, a Z
D0 ∈ p ≤ ≤ p ∈
9
of arithmetical genus a and some degrees, realizing the necessary genera in
some initial intervals in d (like x (d,n) = 0, for instance - see (2.2) - but not
p
only). The families (3.1.2) are the only one which are good for the initial in-
tervals in our method (we’ll explain this later). After the initial construction
we continue using a number of inductive arguments (after x (d,n), for in-
p
stance, butnotonly), adding repeatedlyto (bytensorization) somesimple
0
D
(and well understood) invertible sheaves, similar to the class of hyperplane
section(seeLemma3.2.4). Let’sexplain, shortly, whytheinductive processes
works. Let’s suppose that we need to construct curves on the surfaces Xn of
p
degree d and genus g = π (d,n) (see (1.1)). Let’s suppose that we already
p
constructed the necessary curves for m (d,n) = 0 (see (1.2)). Because
p
π (d+(n 1)) = π (d)+(d+p 1);
p p p
(3.1.3) − −
m (d+(n+p 1)) = m (d)+1
p p
−
in order to construct curves C with (deg(C),g(C)) = (d,g), we proceed
as follows: if the curves C have degrees d and genera g = π (d,n) in the
p
domain m (d,n) = m, then the (smooth, irreducible!) curves C′ C+Hn ,
Hpn ∈ [p+2p;p,1snp]havedegreesd′ = d+(n+p−1) andgenera g′ =∈π|p(d′,n)pin|
the next domain m (d,n) = m+1 (use (3.1.3)). Hence, we obtain invertible
p
sheaves forall pairs(d,g), g = π (d,n)asfaras wesucceed toconstruct these
p
invertible sheaves in the domain m (d,n) = 0. Moreover, we can test when
p
the associated linear system contain (smooth, irreducible) curves, because
the appearing linear systems D +tHn , t 0 are simple enough (specialize
| 0 p| ≥
the points from Σn on a rational plane curve of degree p + 1, having one
p
ordinary singular point P with multiplicity p, see Prop. 3.2.2 and Lemma
0
3.2.4).
Inourcase, inordertoconstruct thenecessary curves covering by(degree,
genus) the domains An it is necessary to change in some way the functions π
p p
so that the initial verifications (corresponding to m (d,n) = 0) to be made
p
using sheaves as in (3.1.2) and a property as in (3.1.3) to still hold (and some
others appearing, see subsection 3.3, Lemma 3.3.1). We get in such a way
the (unique) functions α (d,n) from section 2 (see (2.1)).
p
Moreover, the method shortly explained here works only if #Σn = 3p
p −
n + 6 12 (#A means the number of elements of the finite set A) so,
≥
if p n/3 + 2. So, for #Σn 11 we need another method in order to
≥ p ≤
construct curves on Xn. Because we need such curves for small degrees also,
p
the inductive argument is used again, as possible. But in this second method
of construction we’ll be, partially inspired from [GP2], because in this case
10
