In this tutorial we describe one way to represent divisors on a smooth projective subvariety
X of
Pr, and show methods for computing the group operations, computing the vector space of sections, and determining whether two divisors are linearly equivalent. We also construct the canonical divisor on
X.
We consider smooth varieties only, although most of this can be extended to normal varieties. Cartier and Weil divisors on normal varieties might be the subject of a further tutorial.
Other possible future topics would be: intersection numbers, determining whether a divisor is very ample, and finding the base point locus of the divisor class.
The simplest case is when the homogeneous coordinate ring
SX of
X satisfies the
S2 condition of Serre: We say that a domain
R is
S2 if every proper nonzero principal ideal has pure codimension 1 (all associated primes of the ideal are of codimension 1).
In this tutorial, we consider the case when this holds (e.g., this holds for complete intersections). In a further tutorial, we will make the necessary extensions to handle the non
S2-case.
An example that we will use throughout is the plane cubic curve
E, whose homogenoeus coordinate ring is
SE:
i1 : KK = ZZ/31991
o1 = KK
o1 : QuotientRing
|
i2 : SE = KK[x,y,z]/(y^2*z - x*(x-z)*(x+3*z))
o2 = SE
o2 : QuotientRing
|
The sections in this tutorial are A. Representation of divisors
B. Group operations on divisors
C. Global Sections
D. Linear Equivalence
E. The canonical divisor
A. Representation of divisors
Let
X be a smooth irreducible variety. A (Weil) divisor on
X is an integral linear combination of irreducible subvarieties of
X of codimension
1. The divisor is called effective if all the coefficients are non-negative. To any ideal
I in the homogeneous coordinate ring
SX of
X we associate the effective divisor that is the sum of the pure codimension
1 components of
I, each taken with the multiplicity it has in the primary decomposition of
I.
Let
D = E - F be a divisor, where
E and
F are effective. Because
X is normal, there is a unique homogeneous ideal
I in
SX such that
V(I) = E, and
I is either
(1), or has pure codimension one. Similarly, there is a unique such ideal
J with
V(J) = F. Our plan is to represent the divisor
D by the pair of ideals
(I,J).
This representation is not unique. If
(I,J) and
(I’,J’) are two pairs of ideals (such that each ideal is either
(1) or has pure codimension one), then
(I,J) and
(I’,J’) represent the same divisor iff
sat(I J’) = sat(I’ J),
where
sat(K) is the saturation of
K (the largest ideal
L such that a power of the irrelevant ideal times
L is in
K) Write
(I,J) ≡(I’,J’) if
sat(I J’) = sat(I’ J).
This correspondence defines a bijection between
Div(X) and
(I,J) | I,J are homogeneous ideals in
SX either trivial, or pure codim one
/≡.
As we will often have to saturate ideals of codimension 1, we give here the most efficient method we know, which has the additional advantage of throwing away all components not of codimension 1. That is, we define
purify1S2(I), a function that takes an arbitary ideal
I in a ring satisfying
S2, and returns the ideal which is the intersection of the codimension 1 primary components of
I. In the next divisor tutorial (not yet written), we will write a routine
purify1(I) which does not require the ring to be
S2.
i3 : purify1S2 = I -> (
-- Assuming ring I is S2, and I is not 0, returns the
-- pure codimension 1 part of I.
-- Find a nonzero element of I:
M := compress gens I;
-- Explanation: gens I is
-- the matrix of generators of I; compress
-- removes the entries that are 0
-- and := makes M a local variable.
if numgens source M == 0
then error "purify1S2: expected nonzero ideal";
f := ideal M_(0,0);
-- f is the ideal generated by the first entry.
-- Since ring I is S2, the ideal f is
-- pure codimension 1. Thus
f:(f:I)
-- is the pure codimension 1 part. (The last
-- expression given in a function is the returned
-- value, provided the semicolon is left off.)
)
o3 = purify1S2
o3 : FunctionClosure
|
For example, in the ring
i4 : R = ZZ/5[a,b]
o4 = R
o4 : PolynomialRing
|
we have
i5 : purify1S2 ideal(a^2,a*b)
o5 = ideal(a)
o5 : Ideal of R
|
B Unfortunately at this moment the quotient code B makes this slow. Another possibility is B purify1S2 = saturation B which is (9/23/96) slightly faster (88 vs 106 sec).
Throughout this tutorial, we will treat divisors as equivalence classes of pairs, and our operations will operate on pairs. So let’s define a divisor type in
Macaulay2. The following declaration provides a new data type, the
Divisor.
i6 : Divisor = new Type of BasicList
o6 = Divisor
o6 : Type
|
Let’s write a routine to create a divisor, from either a single ideal, or a pair of ideals. (This routine should check that its arguments are pure codimension one, or trivial, and in the same ring, but we will ignore that).
Defining
divisor to be a method allows us to define different versions of this routine which take different arguments.
i7 : divisor = method()
o7 = divisor
o7 : MethodFunction
|
The following allows us to define an object of class
Divisor from a pair of ideals.
i8 : divisor(Ideal,Ideal) := (I,J) -> new Divisor from {purify1S2 I,purify1S2 J};
|
The following routine defines an (effective) divisor from a single ideal.
i9 : divisor Ideal := I -> divisor(I, ideal 1_(ring I));
|
The divisors of some rational points on the elliptic curve
E include
i10 : P = divisor ideal(x,z)
o10 = Divisor{ideal (z, x), ideal 1}
o10 : Divisor
|
i11 : R = divisor ideal(x,y)
o11 = Divisor{ideal (y, x), ideal 1}
o11 : Divisor
|
i12 : R1 = divisor ideal(x-z,y)
o12 = Divisor{ideal (y, x - z), ideal 1}
o12 : Divisor
|
i13 : R2 = divisor ideal(x+3*z,y)
o13 = Divisor{ideal (y, x + 3z), ideal 1}
o13 : Divisor
|
i14 : Q1 = divisor ideal(y-6*z, x-3*z)
o14 = Divisor{ideal (y - 6z, x - 3z), ideal 1}
o14 : Divisor
|
Testing equality of divisors is often made simpler by having a “normal form” for divisors. The normal form of a divisor
D is
E - F where
E and
F are both effective and have disjoint support. It is easy to see that the normal form of
(I,J) is
(I:J, J:I).
In the following code, the expressions
D#0 and
D#1 refer to the first and second ideals in the list representing
D. (
D#0 is the first because
Macaulay2 counts everything starting from 0.)
i15 : normalForm = method()
o15 = normalForm
o15 : MethodFunction
|
i16 : normalForm Divisor := D -> new Divisor from {D#0 : D#1, D#1 : D#0};
|
Two pairs
(I,J), (I’,J’) define the same divisor exactly when their normal forms are equal. The following code establishes a method for testing the equality of divisors. The function "toList" converts a divisor to a list of ideals, and then we let the built in method for comparing lists take over: it compares corresponding elements.
i17 : Divisor == Divisor := (D,E) -> toList normalForm D == toList normalForm E;
|
An important point here is that the built in method for comparing two ideals used by the operator "==" succeeds even if the given generators differ. We shall later show that with
R1 and
R2 as above, the divisor
(R1 + R2) - R1 is represented by
i18 : D = divisor(ideal(y, x^2+2*x*z-3*z^2), ideal(x-z, y))
2 2
o18 = Divisor{ideal (y, x + 2x*z - 3z ), ideal (y, x - z)}
o18 : Divisor
|
so that the normal form of
D is
R2:
i19 : normalForm D
o19 = Divisor{ideal (y, x + 3z), ideal 1}
o19 : Divisor
|
and we can directly test equality by
B. Group operations on divisors
To add divisors we multiply the corresponding ideals and then saturate. This may be coded as follows (the products are saturated in the
divisor routine):
i21 : Divisor + Divisor := (D,E) -> divisor(D#0 * E#0, D#1 * E#1);
|
Negation is even simpler, since all we need do is exchange the two ideals. We don’t use the
divisor routine, since our ideals are already saturated.
i22 : - Divisor := (D) -> new Divisor from {D#1, D#0};
|
Let’s also include functions to compute differences and to multiply by integers.
i23 : Divisor - Divisor := (D,E) -> D + (-E);
|
i24 : ZZ Divisor := ZZ * Divisor := (n,D) -> divisor((D#0)^n, (D#1)^n);
|
Some arithmetic of divisors on our elliptic curve
i25 : 2P
2
o25 = Divisor{ideal (z, x ), ideal 1}
o25 : Divisor
|
i26 : 3P
o26 = Divisor{ideal(z), ideal 1}
o26 : Divisor
|
Notice that
3P is the hyperplane section
z=0, which is the equation of the flex line to the cubic at the point
P.
i27 : D = P-R1
o27 = Divisor{ideal (z, x), ideal (y, x - z)}
o27 : Divisor
|
i28 : D2 = 2P - 2R1
2 2
o28 = Divisor{ideal (z, x ), ideal (x - z, y )}
o28 : Divisor
|
C. Global Sections
Since we have assumed
X smooth, Weil divisors can all be represented by Cartier divisors, that is, by sections of an invertible sheaf. If
D = (I,J) is a divisor, and
sheaf(I) denotes the sheaf of
OX-modules corresponding to
I, then we put
OX(D) = sheaf(I)-1 ⊗sheaf(J).
We define
L(D) to be the space of global sections of the sheaf
OX(D). Note that a global section is the same as a sheaf homomorphism
OX → OX(D). If we write
D = E-F, where
E and
F are effective, then global sections of
OX(E-F) can be identified with homomorphisms
OX(-E) → OX(-F).
If we write
D = (I,J), then
L(D) and
Hom(I,J) can be identified with subsets of the field of fractions of
SX. Since
SX satisfies
S2, these sets are equal. The following proposition allows us to compute
Hom(I,J):
Proposition. Suppose
X is a smooth projective variety whose homogeneous coordinate ring
SX is
S2. If
D is the divisor
(I,J) and
f is any non-zero element of
I, then
L(D) is the degree zero part of
sat((f*J) : I) /f.
Proposition. If
s = g/f is section of the divisor
D = (I,J) as above, then the zero scheme of
s is defined by the ideal
sat(f I : g) : J.
Consider the divisor
2P on our curve
E:
i29 : D = 2P
2
o29 = Divisor{ideal (z, x ), ideal 1}
o29 : Divisor
|
In this case,
I = (x2, z), and
J = (1). Compute the vector space of sections
L(2P):
i30 : I = D#0
2
o30 = ideal (z, x )
o30 : Ideal of SE
|
i31 : J = D#1
o31 = ideal 1
o31 : Ideal of SE
|
i32 : f = z
o32 = z
o32 : SE
|
The degree 0 part in the proposition is the degree
d part of
sat((fJ) : I), divided by
f, where
d = deg f.
We can use the command
basis to obtain a vector space basis of a module or ideal in a given degree and thus compute the global sections (For an explanation of this use of the
basis routine, see the tutorial on canonical embeddings of plane curves and gonality)
i33 : LD = basis(degree f, purify1S2((f*J) : I))
o33 = {1} | 1 0 |
{1} | 0 1 |
o33 : Matrix
|
i34 : LD = super (LD ** (ring target LD))
o34 = | z x |
1 2
o34 : Matrix SE <--- SE
|
so the vector space
L(2P) is generated by
1=z/z, and
x/z. Since
J = (1), the zero locus of the section
(z+x)/z is defined by the ideal
i35 : imI = purify1S2(((z+x)*I) : z)
2 2
o35 = ideal (x + z, y - 4z )
o35 : Ideal of SE
|
and its degree is:
Let’s now package this into a routine
globalSections which takes an argument
D of class
Divisor, and computes a basis of
L(D), represented as fractions with a common denominator. The output is a row vector of the numerators, followed by the denominator.
i37 : globalSections = method()
o37 = globalSections
o37 : MethodFunction
|
i38 : globalSections Divisor := (D) -> (
-- First let's grab the parts (I,J) of D.
I := D#0;
J := D#1;
-- Let 'f' be the first element of the
-- matrix of generators of the ideal I.
f := (gens I)_(0,0);
-- Now compute the basis of global sections
-- just as above
LD := basis(degree f, purify1S2((f*J) : I));
LD = super (LD ** (ring target LD));
-- Return both this vector space and the denominator
{LD, f});
|
Another important task is to find the ideal of zeros of a section
s = f/g of a divisor
D.
i39 : sectionIdeal = (f,g,D) -> (
I := D#0;
J := D#1;
purify1S2((f*I):g) : J
);
|
Let’s find the image of the elliptic curve
E under the linear system
4P. To do this we define a ring homomorphism from the global sections with the command map. Its kernel defines the image of
E.
i40 : D = 4P
2
o40 = Divisor{ideal (z , x*z), ideal 1}
o40 : Divisor
|
i41 : L = globalSections D
2
o41 = {| xz yz z2 x2 |, z }
o41 : List
|
i42 : phi = map(SE, ZZ/31991[a..d], L#0)
2 2
o42 = map(SE,KK [a, b, c, d],{x*z, y*z, z , x })
o42 : RingMap SE <--- KK [a, b, c, d]
|
i43 : ker phi
2 2
o43 = ideal (b + 3a*c - a*d - 2c*d, a - c*d)
o43 : Ideal of KK [a, b, c, d]
|
The image in
P3 is a complete intersection of two quadrics: the elliptic normal curve in
P3.
For a less obvious example, consider the divisor
4P - R, which is not effective. Since it has degree 3 as a divisor on an elliptic curve, the Riemann Roch theorem tells us that it is equivalent to an effective divisor; in fact that it has three sections. We can check this as follows:
i44 : D = 4P - R
2
o44 = Divisor{ideal (z , x*z), ideal (y, x)}
o44 : Divisor
|
i45 : L = globalSections D
2
o45 = {| yz xz x2 |, z }
o45 : List
|
i46 : II = sectionIdeal(y*z+x*z+x^2, z^2, D)
2 2 2 2
o46 = ideal (y + 3x*z + y*z + 3z , x*y + x*z - 3z , x + x*z + y*z)
o46 : Ideal of SE
|
i47 : degree II
o47 = 3
|
D. Linear Equivalence
Testing whether two divisors
E and
F are linearly equivalent boils down to testing whether
D = E-F is principal and thus linearly equivalent to 0.
One method to determine whether
D is principal is to compute the global sections of
D. A divisor
D is principal iff
L(D) has dimension one, and the zero locus of its generator is the empty set.
For example, on the elliptic curve
E, consider
P - R:
i48 : globalSections (P-R)
o48 = {0, z}
o48 : List
|
P-R has no global sections, so it is not equivalent to 0. Now consider
2 P - 2 Ri49 : D = 2 P - 2 R
2 2
o49 = Divisor{ideal (z, x ), ideal (x, y )}
o49 : Divisor
|
i50 : LB = globalSections D
o50 = {| x |, z}
o50 : List
|
Since the divisor
D = 2P-2R has degree 0 and has a section,
D is linearly equivalent to 0. The result shows that the rational function
x/z has divisor
2P-2R.
To check that a divisor of unknown degree is equivalent to 0, we attempt to find a section and show it does not vanish anywhere. We include this in the routine below.
Remember that in this tutorial we are assuming that
SX is
S2 and that
X is smooth. These computations are easily modified in the non-
S2 case. See the corresponding tutorial, once it is written!
i51 : linearlyEquivalent = (D,E) -> (
F := normalForm(D-E);
LB := globalSections F;
L := LB#0;
-- L is the matrix of numerators. Thus numgens source L
-- is the dimension of the space of global sections.
if numgens source L != 1
then false
else (
R := ring L;
V := sectionIdeal(L_(0,0), LB#1, F);
if V == ideal(1_R)
then L_(0,0)/LB#1
else false)
);
|
We get the same answers as before:
i52 : linearlyEquivalent(P,R)
o52 = false
|
i53 : linearlyEquivalent(2P,2R)
x
o53 = -
z
o53 : frac(SE)
|
We now look at the group law on the cubic: We take the point
P to be 0; we can then identify the natural group of divisor classes of degree 0 with the set of points on the curve. With this identification, the group law
++ on points of the curve is defined by:
R ++ S = the unique point
T for which the divisor
(R-P)+(S-P) is linearly equivalent to
(T-P). i.e.
R ++ S := unique effective divisor in
R+S-P.
What we need to do is: given a divisor
R+S-P, find an effective divisor equivalent to it.
i54 : effective = (D) -> (
LB := globalSections D;
L := LB#0; -- the matrix of numerators
if numgens source L == 0
then error(toString D + " is not effective")
else divisor sectionIdeal(L_(0,0), LB#1, D));
|
i55 : effective(2R-P)
o55 = Divisor{ideal (z, x), ideal 1}
o55 : Divisor
|
i56 : addition = (R,S) -> effective(R + S - P);
|
i57 : addition(R1,R2)
o57 = Divisor{ideal (y, x), ideal 1}
o57 : Divisor
|
Some points are in the torsion subgroup:
i58 : Q2 = addition(Q1, Q1)
o58 = Divisor{ideal (y, x - z), ideal 1}
o58 : Divisor
|
i59 : Q3 = addition(Q2, Q1)
o59 = Divisor{ideal (y + 6z, x - 3z), ideal 1}
o59 : Divisor
|
i60 : Q4 = addition(Q3, Q1)
o60 = Divisor{ideal (z, x), ideal 1}
o60 : Divisor
|
i61 : Q4a = addition(Q2,Q2)
o61 = Divisor{ideal (z, x), ideal 1}
o61 : Divisor
|
So the point
Q1 = (3,6,1) is a point of order 4 in the group.
Exercise: Write a routine that computes
n times a point in this group law.
E. The canonical divisor
The most important divisor class on a variety is the canonical class. For example, consider the twisted cubic curve whose ideal is the ideal of
2×2 minors of the “catalecticant” matrix
i62 : S = ZZ/31991[a,b,c,d];
|
i63 : catalect = map(S^2, 3, (i,j)->S_(i+j))
o63 = | a b c |
| b c d |
2 3
o63 : Matrix S <--- S
|
i64 : IC = minors(2, catalect)
2 2
o64 = ideal (- b + a*c, - b*c + a*d, - c + b*d)
o64 : Ideal of S
|
i65 : SX = S/IC
o65 = SX
o65 : QuotientRing
|
As a graded module, the canonical class is given as
KX = Extc(SX, S(-r-1)), where
c = codim X,
X ⊂Pr, and
S = k[x0,...,xr] is the polynomial ring.
i66 : KX = Ext^2(coker gens IC,S^{-4})
o66 = cokernel {1} | c b a |
{1} | -d -c -b |
2
o66 : S-module, quotient of S
|
i67 : canpres = substitute(presentation(KX), SX)
o67 = {1} | c b a |
{1} | -d -c -b |
2 3
o67 : Matrix SX <--- SX
|
i68 : betti canpres
0 1
o68 = total: 2 3
1: 2 3
o68 : BettiTally
|
Thus we need a routine that takes a rank 1 torsion free module over a domain and finds an ideal isomorphic to it. We wish to compute homomorphisms from the canonical module into
SX, and take the divisor whose first ideal is the image of a homomorphism of non-negative degree, and whose second ideal is an arbitrary nonzero element of
SX whose degree is equal to the degree of the homomorphism. First we find a homomorphism of lowest degree:
i69 : I1 = transpose (syz transpose canpres)_{0}
o69 = | d c |
1 2
o69 : Matrix SX <--- SX
|
The degree is
i70 : dg = (degrees (target I1))_0_0
o70 = 0
|
We need to balance the degree
dg with a power of the first nonzero generator of the ring. This is done in the following packaged version.
i71 : divisorFromModule = M -> (
-- given a module M, returns the divisor of the image
-- of a nonzero homomorphism to R, suitably twisted.
-- first get the presentation of M
I1 := transpose (syz transpose presentation M)_{0};
-- The degree is
d := (degrees target I1)_0_0;
-- We need to balance the degree d with a power
-- of the first nonzero generator of the ring.
var1 := (compress vars ring M)_{0};
-- Now fix up the degrees.
if d==0 then divisor ideal I1
else if d>0 then divisor(
ideal (I1**dual(target I1)),
ideal var1^d
)
else divisor ideal(
var1^(-d)**I1**dual target I1
)
);
|
We start from a module over the ring
SX:
i72 : M = coker canpres
o72 = cokernel {1} | c b a |
{1} | -d -c -b |
2
o72 : SX-module, quotient of SX
|
i73 : divisorFromModule M
o73 = Divisor{ideal (d, c), ideal 1}
o73 : Divisor
|
Some tests:
i74 : use SX
o74 = SX
o74 : QuotientRing
|
i75 : divisorFromModule image matrix{{d^2}}
2
o75 = Divisor{ideal 1, ideal(a )}
o75 : Divisor
|
i76 : divisorFromModule SX^{1}
o76 = Divisor{ideal(a), ideal 1}
o76 : Divisor
|
Here is the canonical divisor routine in packaged form:
i77 : canonicalDivisor= SX ->(
-- Given a ring SX, computes a canonical divisor for SX
I := ideal presentation SX;
S := ring I;
embcodim := codim I;
M := Ext^embcodim(coker gens I,S^{-numgens S});
M = coker substitute(presentation M, SX);
divisorFromModule M
);
|
i78 : canonicalDivisor SX
o78 = Divisor{ideal (d, c), ideal 1}
o78 : Divisor
|
There are other ways of computing the canonical class. Perhaps we have already written a tutorial on this subject.