METODOLOGÍA

Fix

Q

a vector bundle on

X

of rank

n−r

and let

G ∈

Fl

S

Q. It will be useful to connect

Definition 2.2.1 to the partial flag notation we have been using up to this point.

Lemma 2.7.1.

With the notation as in Remark 2.4.1, ifφ∈Hom

OX

(V,Q), we haveφ(F

p

a

)⊂G

p_Ip a−a

for

a= 1, ..., r

if and only if

φ( ˜F

_ip

)⊆G

p_{(n−r)(1−W}p i+1)

Proof.

It is clear from Remark 2.4.1 that

W

_ip₊₁

=w

p 1+Pi j=1m p j

=

1

n−r(n−

i

X

j=1

(m

p_j

−I

p r−Pi j=1m p j

))

(2.17)

So, using the above and the fact that ˜F

_ip

=

F

p r−Pi

j=1m

p j

, the Lemma amounts to the claim that

φ(F

ap

)⊆G

p_Ip

a−a

for all

a= 0,1..., r

if and only if

φ(F

p (r−Pi j=1m p j)

)⊆G

p Ip (r−Pi j=1m p j) −(r−Pi j=1m p j)

for all

i= 0,1, ..., l

p

. One implication is immediate. For the other, suppose

a

is strictly between

r

−P

i+1 j=1

m

p j

and

r

−

P

i j=1

m

p

j

for some

i

= 0, ..., l

p

−1. Then, by definition of multiplicity,

w

p_r−a+1

=W

_ip₊₁

. Examining the formula forw

p

in Remark 2.4.1 and equation (2.17), we obtain the

equation

I

_ap

−a=I

p (r−Pi j=1m p j)

−(r−

i

X

j=1

m

p_j

).

It follows that

φ(F

_ap

)⊆φ(F

p (r−Pi j=1m p j)

)⊆G

p Ip (r−Pi j=1m p j) −(r−Pi j=1m p j)

=G

p_Ip a−a

,

as needed.

Remark

2.7.1.

If (Q,G) is given an MS parabolic structure with multiplicities ˜m

p_j

= (n−r)(W

_jp

−

W

_j−p ₁

) forj

= 1, ..., l

p

+ 1, then by Remark 2.4.1, ˜G

p_i

=G

p_{(n−r)(1−W}p i)

. If the MS parabolic weights of

(Q,G) are ˜W

_ip

=W

_i−p ₁

fori= 1, ..., l

p

+1, then Lemma 2.7.1 shows that Hom

I

(V,Q,F,G) is precisely

the set of MS parabolic morphisms (see [MS80, 1.5]) from (V,F˜,{W

_ip

, m

p_i

}) to (Q,G˜,{W˜

_ip

,m˜

p_i

}).

We now define a sheafHomI(Q,G) onR

SS

I,M S

×Xwith the following property. For eachq:= (q

:

O

N

_{V(k),F) inR}

SS

I

with image (also denotedq) (q,F˜) inR

I,M SSS

, the sheafHomI(Q,G)|

{q}×X

has global sections isomorphic to Hom

I

(V,Q,F,G). This is a “family” version of the construction

given in 2.2.

To this end, note that

V

is locally free onR

SS

I,M S

×X, as it has constant fiber dimension on a

smooth scheme. For each

p∈S, let

ip

:R

SS

a surjective map of sheaves

Hom(V|

_RSS I,M S×X

, π

∗ X

Q)⊕

p∈Sip∗Hom

(V|

RSS I,M S×{p}

, π

∗ X

(Q|

p

)).

For convenience, denoteV|

_RSS

I,M S×{p}

byV|

p

. Fori= 1, ..., l

p

_{, let ˜F}

p

i

denote the (locally free) kernel

of

V|

p

Q

p_i

|

RSS

I,M S

→0, and set ˜F

p

0

=V|

p

. Then we have a map

Hom(V|

_p

, π

∗_X

(Q|

_p

))→ ⊕

lp i=0

Hom( ˜F

p i

, π

∗ X

(Q|

p

/G

p_{(n−r)(1−W}p i+1)

)).

LetK

p

denote the kernel. We then have a composite surjection:

Hom(V|

_RSS I,M S×X

, π

∗ X

Q)⊕

p∈S

i

p∗

Hom(V|

_p

, π

∗_X

(Q|

_p

))

K

p

(2.18)

Denote the kernel of (2.18) byHomI(Q,G). Then

HomI(Q,G) is locally free (Proposition 2.7.2)

and is easily seen to have the desired property (Lemma 2.7.1).

We now record the important properties of

HomI(Q,G). Similar results appear in [Pau98]

and [Gav04].

Proposition 2.7.2.

HomI(Q,G)

is locally free on

R

SS

I,M S

×X

of rank equal to the rank of

Hom(V|

_RSS

I,M S×X

, π

∗ X

Q).

Proof.

Denote

R

:=

R

SS

I,M S

. We shorthand

HomI(Q,G) and

Hom(V|

R×X

, π

_X∗

Q) by

H

and

H, respectively. Let (q, x) be a point of

R ×X. Clearly we have an isomorphism of stalks

H

(q,x)

→H

(q,x)

whenx

is not among the points of

S. It suffices then to show that for each marked

point

p∈S

that the stalk

H

_(q,p)

is free.

By the local criterion for flatness, we will be done if we can show that

Tor

O₁R×X,(q,p)

(H

(q,p)

, k(q, p)) = 0.

Passing to (q, p) stalks in (2.18) and taking the Tor sequence, we see that the condition above is

equivalent to

0 = Tor

O₂R×X,(q,p)

((ip∗H|

p/Kp

)

(q,p)

, k(q, p)) = Tor

OR,q

This holds, since by Lemma 2.7.4, the sheaf

H|

_p

/K

p

is locally free on

R.

Proposition 2.7.3.

Assume

g= 0 (X

=P

1

),

I

satisfies the codimension condition (2.22), and

degQ=d−D. Then,

detRπ

_RSS

I,M S∗H

omI(Q,G)

and

˜

Θ|

_RSS

I,M S

are canonically isomorphic.

Proof.

We will omit reference to the semistable locus for convenience. Also we will use the shorthand

H

andH

for

HomI(Q,G) and

Hom(V, π

∗_X

Q), respectively. By the defining expression (2.18), we

have

detRπ∗H

= detRπ∗H⊗ ⊗

p∈S

(detRπ∗(i

p∗H

|

p

/K

p

))

−1

(2.19)

Since

ip∗H|

p/Kp

_{is only supported onR}

SS

I,M S

× {p}, we have:

detRπ∗(i

p∗H

|

_p

/K

p

) = (det(H|

_p

/K

p

))

−1

.

Equation (2.19) can now be rewritten:

detRπ∗H

= detRπ∗H⊗ ⊗

p∈S

(det(H|

p

/K

p

)).

(2.20)

Use Lemma 2.7.4 to write:

det(H|

_p

/K

p

) = (detV|

_p

)

−(n−r)(W1p)−( Plp i=1d p i)

⊗ {⊗

lp i=1

(detQ

p i

)

dp_i

_}.

It is easy to see from the definitions that the first tensor factor above can be rewritten as

(detQ

_lpp₊₁

)

d

p

lp+1

⊗_(detV|

p

)

−(n−r)

. Substituting into (2.20), we obtain:

detRπ∗H

= detRπ∗H⊗δ⊗ {⊗

p∈S

(detV|

p

)

−(n−r)

}.

(2.21)

By Mumford’s Seesaw Theorem, we have detV|

_p

canonically isomorphic to detV|

_y

for any

p∈X.

By Serre duality, detRπ∗H

= detRπ∗(V

⊗π

_X∗

(Q

∨

_⊗K

X

)). From [Pau98, Lemme 3.5] it follows that

the right hand side of this equation is canonically isomorphic to (detRπ∗V)

n−r

⊗(detV|

_y

)

d−D+2(n−r)

.

Combine this with (2.21) and the codimension condition (2.22), and compare with definitions (2.15),

(2.16), and (2.11) to complete the proof.

the Seesaw Theorem. In higher genus, since we consider bundles

V

of fixed degree but not fixed

determinant, the line bundles detV|

x

will depend a priori on

x.

Proposition 2.7.3 gives a way of producing a SL

N

invariant sections of ˜Θ|

RSS

I,M S

and hence

of Θ. With assumptions as in 2.7.3, let

Q

be a vector bundle of rank

n−r, degree

d−D,

with flags

G ∈

Fl

S

Q

such that there exists a semistable parabolic vector bundle (V

0

,F

0

) with

Hom

I

(V

0

,Q,F

0

,G) = 0 (we will produce such (Q,G) in Section 2.8). Using the assumptions, one

computes

χ(HomI(Q,G)|

_q

) = 0 for any

q∈ R

SS

I,M S

.

We proceed as in the det/div theory of [KM76], see [Pau98, Section 3] for the relevant aspects.

Supposef

:L

0

→L

1

is a map of locally frees onR

SSI,M S

such that

kerf

=π∗HomI(Q,G),

cokerf

=R

1

π∗HomI(Q,G).

By cohomology and base change, at a point

q

₀

such that

H

i

(HomI(Q,G)|

q0

) = 0 for

i= 0,1, the

mapf

is an isomorphism. Any preimage of (V

0

,F

0)

∈ M(I) will do for such aq

0

. In particular,

L

0

andL

1

have the same rank, so we obtain a map det(f) of their determinant bundles. Tensoring

both sides of det(f) by detL

−₀1

, we obtain a section of

Rπ∗HomI(Q,G). Up to a nonzero complex

scalar, it does not depend on the choice ofL

0

,

L

1

, orf, so we denote the (scalar class of the) section

θ(Q,G). The vanishing locus ofθ(Q,G) is the set of

qsuch that

f

fails to be surjective on the

q

fiber, equivalently

R

1

π∗HomI(Q,G)|

_q

6= 0. Using the trivial Euler characteristic of

HomI(Q,G)

and the theory of cohomology and base change, one sees that this is precisely the set ofq

such that

if the image ofqin

M(I) is (V,F) then Hom

I

(V,Q,F,G)6= 0.

All of the above works equally well in families (see e.g. [Pau98, Section 5]) so

θ(Q,G) descends

to a section of Θ. It is clear in any case from our construction that the divisor ofθ(Q,G) is SL

N

invariant.

We now prove a technical lemma that was needed above.

Lemma 2.7.4.

Let

V

be a vector bundle on a schemeR

with a filtration by subbundles

Let

Q

be a vector space with a filtration

Q⊇Gα

0

⊇Gα

1

⊇...⊇Gα

l

.

Consider the map:

Hom(V, Q⊗ O)→ ⊕

l_i=0

Hom( ˜F

i, Q/Gαi

⊗ O).

Denote its image by

H/K

. Then, after choosing a splitting of the vector space

Q=Q/G

α0

⊕G

α0

/G

α1

⊕...⊕G

αl

,

one has a canonical isomorphism

H/K

=⊕

l_i=0

( ˜F

_i∨

⊗Gα

i−1

/Gα

i

),

where we take

G

α−1

=Q.

Proof.

We induct on

l. The casel= 0 is clear. Assume

l≥1 and assume that the image of

Hom( ˜F

₁

,G

_α0

⊗ O)−→ ⊕

φ l

i=1

Hom( ˜F

i

, G

α0

/G

αi

⊗ O)

is isomorphic to⊕

l

i=1

( ˜F

i∨

⊗Gα

i−1

/Gα

i

). Now consider the commutative diagram

Hom(V, G

α0

⊗ O)

a

−−−−→ ⊕

l i=1

Hom( ˜F

i

, G

α0

/G

αi

⊗ O)

b





y





y

c

Hom(V, Q⊗ O)

−−−−→ ⊕

d l i=0

Hom( ˜F

i, Q/Gαi

⊗ O).

We would like to compute Im(d). An easy diagram chase shows that ker(d) =b◦ker(a). Thus, we

get an inclusion Im(a)→Im(d) whose cokernel, by the snake lemma, is isomorphic to the cokernel

of

b. We then have an exact sequence:

0→Im(a)→Im(d)→coker(b)→0.

It is easy to see that Im(a) = Im(φ) and that coker(b) =V

∨

_⊗Q/G

˜

F

∨

0

⊗G

α−1

/G

α0

). So by the inductive hypothesis, we have written Im(d) as an extension of the

desired form. Using the splitting ofQ, we have a section coker(b)→Hom(V, Q⊗ O), which then

splits the extension Im(d).

In document "Propuesta de sistema de control de calidad para producto terminado en maíz dulce y minivegetales ( chilote, escalopin, zuchini y zanahoria) " (página 52-63)