### A Modular Compactification of the General Linear Group

Ivan Kausz

Received: June 2, 2000 Communicated by Peter Schneider

Abstract. We define a certain compactifiction of the general linear group and give a modular description for its points with values in arbi- trary schemes. This is a first step in the construction of a higher rank generalization of Gieseker’s degeneration of moduli spaces of vector bundles over a curve. We show that our compactification has simi- lar properties as the “wonderful compactification” of algebraic groups of adjoint type as studied by de Concini and Procesi. As a byprod- uct we obtain a modular description of the points of the wonderful compactification of PGln.

1991 Mathematics Subject Classification: 14H60 14M15 20G

Keywords and Phrases: moduli of vector bundles on curves, modular compactification, general linear group

1. Introduction

In this paper we give a modular description of a certain compactification KGln

of the general linear group Gln. The variety KGln is constructed as follows:

First one embeds Gln in the obvious way in the projective space which contains the affine space of n×n matrices as a standard open set. Then one succes- sively blows up the closed subschemes defined by the vanishing of the r×r subminors (1≤r≤n), along with the intersection of these subschemes with the hyperplane at infinity.

We were led to the problem of finding a modular description of KGln in the course of our research on the degeneration of moduli spaces of vector bundles.

Let me explain in some detail the relevance of compactifications of Gln in this context.

LetB be a regular integral one-dimensional base scheme andb0∈B a closed point. LetC →B be a proper flat familly of curves over B which is smooth outside b0 and whose fibre C0 at b0 is irreducible with one ordinary double point p0 ∈C0. Let ˜C0 →C0 be the normalization of C0 and let p1, p2 ∈C˜0

the two points lying above the singular point p0. Thus the situation may be depicted as follows:

( ˜C0, p1, p2)

uukkkkkkkkkkkkk

))

SS SS SS SS SS SS S

C˜0 C0

//C

b0 //B

where the left arrow means “forgetting the points p1, p2”. There is a corre- sponding diagram of moduli-functors of vector bundles (v.b.) of rankn:

v.b. Eon ˜C0

together with an
isomorphismE[p1]→^{∼} E[p2]

f1

vvlllllll

f2

∼=

((

RR RR RR R

v.b. on C˜0

v.b. on C0

//

v.b. on C/B

b0 //B

where E[pi] denotes the fibre ofE at the point pi (cf. section 3 below). The morphismf1is “forgetting the isomorphism between the fibres” andf2is “glue- ing together the fibres atp1 andp2along the given isomorphism”. The square on the right is the inclusion of the special fibre. It is clear that f is a lo- cally trivial fibration with fibre Gln. Consequently,f1 is not proper and thus {v.b. onC/B}is not proper overB. It is desirable to have a diagram (∗):

generalized v.b.-data on ( ˜C0, p1, p2)

f1

zzuuuuuuuu

f2KKKK%%

KK

v.b. on C˜0

generalized v.b. on

C0

//

generalized v.b. on

C/B

b0 //B

where the functors of “generalized” objects contain the original ones as open subfunctors and where {generalized v.b. onC/B}is proper overB or at least satisfies the existence part of the valuative criterion for properness. The mo- tivation is that such a diagram may help to calculate cohomological invariants of{v.b. onY}(Y a smooth projective curve) by induction on the genus ofY

(notice that the genus of ˜C0 is one less than the genus of the generic fibre of C/B).

In the current literature there exist two different approaches for the construc- tion of diagram (∗). In the first approach the “generalized v.b.” on C0 are torsion-free sheaves (cf. [S1], [F], [NR], [Sun]). The second approach is by Gieseker who considered only the rank-two case (cf. [G]). Here the “general- ized v.b.” onC0are certain vector bundles onC0,C1or C2, whereCi is built from C0 by inserting a chain ofi copies of the projective line atp0. (Cf. also [Tei] for a discussion of the two approaches). Of course, this is only a very rough picture of what is going on in these papers since I do not mention con- cepts of stability for the various objects nor the representability of the functors by varieties or by algebraic stacks.

In both approaches the morphismf2is the normalization morphism (at least on the complement of a set of small dimension) andf1is a locally trivial fibration with fibre a compactification of Gln. In the torsion-free sheaves approach this compactification is Gr(2n, n), the grassmanian ofn-dimensional subspaces of a 2n-dimensional vector space. In Gieseker’s construction the relevant compacti- fication of Gl2is KGl2. An advantage of Gieseker’s construction is that in con- trast to the torsion-free sheaves approach, the space{generalized v.b. onC/B}

is regular and its special fibre overb0 is a divisor with normal crossings.

Very recently, Nagaraj and Seshadri have generalized Gieseker’s construction of the right part of diagram (∗), i.e. the diagram

generalized v.b. on

C0

//

generalized v.b. on

C/B

b0 //B

to arbitrary rank n (cf. [NS], [S2]). Nagaraj’s and Seshadri’s “generalized vector bundles” onC0 are certain equivalence classes of vector bundles on one of the curves C0, . . . , Cn, whose push-forward to C0 are stable torsion free sheaves.

Without worrying about stability I have recently (and independently from Na- garaj and Seshadri) constructed the full diagram (∗) at least at the level of functors (details will appear in a forthcoming paper) and I have reasons to believe that the fibres of the corresponding morphismf1should be represented by KGln. The present paper is the first step in the proof of this fact.

The compactification KGlnof Gln has properties similar to those of the “won- derful compactification” of algebraic groups of adjoint type as studied by De Concini and Procesi (cf. [CP]). Namely:

1. The group Gln×Glnacts on KGln, extending the operation of Gln×Gln

on Gln induced by right and left multiplication (cf. 5.6).

2. The complement of Gln in KGln is a divisor with normal crossings with irreducible componentsYi,Zj (i, j∈ {0, . . . , n−1}) (cf. 4.2).

3. The orbit closures of the operation of Gln×Gln on KGln are precisely
the intersections YI∩ZJ, whereI, J are subsets of {0, . . . , n−1}with
min(I) + min(J)≥nand whereYI :=∩_{i∈I}Yi,ZJ:=∩_{j∈J}Zj (cf. 9.4).

4. For eachI, Jas above there exists a natural mapping fromYI∩ZJto the product of two flag varieties. This mapping is a locally trivial fibration with standard fibre a product of copies of PGlnk(the wonderful compact- ification of PGlnk) for some nk ≥1 and of one copy of KGlm for some m≥0 (cf. 9.3).

Our main theorem 5.5 says that KGln parametrizes what we call “general- ized isomorphisms” from the trivial bundle of rank n to itself. A generalized isomorphism between vector bundlesE andF is by definition a diagram

E = E0

⊗

E1

oo

⊗

E2

oo . . . En−1

⊗

En

oo ^{∼}//Fn //Fn−1

⊗

. . . F_{2} //F1 //

⊗

F0

⊗

= F

with certain properties, where the Ei and Fj are vector bundles of the same
rank as E and F and where the arrow ^{⊗} // indicates a morphims of the
source into the target tensored with a line bundle to be specified. Cf. 5.2 for a
precise definition.

The wonderful compactification PGln of PGln is contained as an orbit closure in KGln, in fact Y0 ∼= PGln. Therefore theorem 5.5 implies a modular de- scription of PGln. One of the reasons why I decided to publish the present paper separately from my investigations on the degeneration of moduli spaces of vector bundles on curves is the fact that PGln has been quite extensively studied in the past (cf. [Lak1] for a historical overview and also the recent pa- per [Tha2]). Although some efford has been made to find a modular description for it, up to now only partial results in this direction have been obtained (cf.

[V], [Lak2], [TK]). In section 8 we explain the connection of these results with ours. Recently Lafforgue has used PGln to compactify the stack of Drinfeld’s shtukas (cf. [Laf1], [Laf2]).

Sections 4 and 5 contain the main definitions: In section 4 we give the construc- tion of KGln and in section 5 we define the notion of generalized isomorphisms.

At the end of section 5 we state our main theorem 5.5. Its proof is given in sections 6 and 7. In section 8 we define complete collineations and compare our notion with the one given by previous authors, in section 9 we study the orbit closures of the operation of Gln×Gln on KGln and in section 10 we define an equivariant morphism of KGln onto the Grassmannian compactification of Gln

and compute its fibres.

My interest in degeneration of moduli spaces of bundles on curves has been greatly stimulated by a workshop on conformal blocks and the Verlinde for- mula, organized in March 1997 by the physicists J¨urgen Fuchs and Christoph Schweigert at the Mathematisches Forschungsinstitut in Oberwolfach. Part of this work has been prepared during a stay at the Mathematical Institute of the University of Oxford. Its hospitality is gratefully acknowledged. Thanks are due to Daniel Huybrechts for mentioning to me the work of Thaddeus, to M. Thaddeus himself for sending me a copy of part of his thesis and to M.

Rapoport for drawing my attention to the work of Laksov and Lafforgue. I would also like to thank Uwe Jannsen for his constant encouragement.

2. An elementary example

This section is not strictly necessary for the comprehension of what follows.

But since the rest of the paper is a bit technical, I felt that a simple example might facilitate its understanding.

LetAbe a discrete valuation ring,Kits field of fractions,mits maximal ideal, t∈ma local parameter and k:=A/mthe residue class field of A. LetE and F be two freeA-modules of ranknand letϕK :EK ∼

→FK be an isomorphism between the generic fibers EK := E⊗AK and FK :=F ⊗AK of E and F.

We can chooseA-bases ofE andF such thatϕK has the matrix presentation
diag(t^{m}^{1}, . . . , t^{m}^{n}) with respect to these bases, wheremi∈Zandm1 ≤ · · · ≤
mn. Now let a0:= 0 =:b0 and for 1≤i≤nset ai :=−min(0, m_{n+1−i}) and
bi:= max(0, mi). Note that we have

0 =a0=· · ·=an−l≤an−l+1≤ · · · ≤an

and 0 =b0=· · ·=bl≤bl+1≤ · · · ≤bn

for somel∈ {0, . . . , n}. Let

En⊆ · · · ⊆E1⊆E0:=E and Fn⊆ · · · ⊆F1⊆F0:=F be theA-submodules defined by

Ei+1:=

t^{a}^{i+1}^{−a}^{i}I_{n−i} 0

0 Ii

Ei , Fi+1:=

I_{i} 0

0 t^{b}^{i+1}^{−b}^{i}I_{n−i}

Fi ,

where I_{i} denotes the i×i unit matrix. Then ϕK induces an isomorphism
ϕ:En ∼

→Fn and we have the natural injections
Ei,→m^{a}^{i}^{−a}^{i+1}Ei+1 , Ei ←- Ei+1

Fi+1,→Fi , m^{b}^{i}^{−b}^{i+1}Fi+1←- Fi .

Observe that the compositions Ei+1 ,→ Ei ,→ m^{a}^{i}^{−a}^{i+1}Ei+1 and Ei ,→
m^{a}^{i}^{−a}^{i+1}Ei+1 ,→ m^{a}^{i}^{−a}^{i+1}Ei are both the injections induced by the inclu-
sion A ,→ m^{a}^{i}^{−a}^{i+1}. Furthermore, if ai −ai+1 < 0 then the morphism of
k-vectorspacesEi+1⊗k→Ei⊗kis of rankiand the sequence

Ei+1⊗k→Ei⊗k→(m^{a}^{i}^{−a}^{i+1}Ei+1)⊗k→(m^{a}^{i}^{−a}^{i+1}Ei)⊗k
is exact. This shows that the tupel

(m^{a}^{i}^{−a}^{i+1}, 1∈m^{a}^{i}^{−a}^{i+1} , Ei+1,→Ei , m^{a}^{i}^{−a}^{i+1}Ei+1 ←- Ei , i)

is what we call a “bf-morphism” of rank i (cf. definition 5.1). Observe now that ifai−ai+1<0 and (f, g) is one of the following two pairs of morphisms:

E⊗k−→^{f} (m^{−a}^{i}Ei)⊗k−→^{g} (m^{−a}^{i+1}Ei+1)⊗k ,
Ei⊗k←−^{g} Ei+1⊗k←−^{f} En⊗k ,

then im(g◦f) = im(g). The above statements hold true also if we replace the Ei-s by theFi-s and the ai-s by thebi-s. Observe finally that in the diagram

0

ker(En⊗k→E0⊗k)

++WWWWWWWWW

0 //ker(Fn⊗k→F0⊗WWk)WWWWWW//EW++n⊗k∼=Fn⊗k

//im(Fn⊗k→F0⊗k) //0

im(En⊗k→E0⊗k)

0

the oblique arrows are injections.

All these properties are summed up in the statement that the tupel
Φ := ((m^{b}^{i}^{−b}^{i+1}, 1), (m^{a}^{i}^{−a}^{i+1}, 1), Ei,→m^{a}^{i}^{−a}^{i+1}Ei+1, Ei←- Ei+1,

Fi+1,→Fi, m^{b}^{i}^{−b}^{i+1}Fi+1←- Fi (0≤i≤n−1), ϕ:En ∼

→Fn)
is a generalized isomorphism fromE toF in the sense of definition 5.2, where
fora≤0 we considerm^{a}as an invertibleA-module with global section 1∈m^{a}.
Observe that Φ does not depend on our choice of the bases forEandF. Indeed,
it is well-known that the sequence (m1, . . . , mn) is independent of such a choice
and it is easy to see that En=ϕ^{−1}_{K} (F)∩E, Fn =ϕK(En) and

Ei =En+m^{a}^{i}E , Fi=Fn+m^{b}^{i}F

for 1≤i≤n−1, where the +-sign means generation inEKandFKrespectively.

Observe furthermore that by pull-back the generalized isomorphism Φ induces
a generalized isomorphism f^{∗}Φ on a scheme S for every morphism f : S →
Spec (A). Of course the morphismsf^{∗}Ei+1 →f^{∗}Ei etc. will be in general no
longer injective, but this is not required in the definition.

3. Notations

We collect some less common notations, which we will use freely in this paper:

• For two integersa≤bwe sometimes denote by [a, b] the set{c∈Z|a≤ c≤b}.

• For an×n-matrix with entries aij in some ring, and for two subsetsA and B of cardinality r of {1, . . . , n}, we will denote by detAB(aij) the determinant of ther×r-matrix (aij)i∈A,j∈B.

• For a schemeX we will denote byKX the sheaf of total quotient rings of OX.

• For a schemeX, a coherent sheafE onX and a pointx∈X, we denote
byE[x] the fibreE ⊗_{O}_{X}κ(x) ofE atx.

• Forn∈N, the symbolSn denotes the symmetric group of permutations of the set{1, . . . , n}.

4. Construction of the compactification

LetX^{(0)}:= ProjZ[x00,xij (1≤i, j≤n)]. We define closed subschemes
Y_{0}^{(0)} //Y_{1}^{(0)} // . . . //Y_{n−1}^{(0)}

Z_{n−1}^{(0)} //

?OO

. . . //Z_{1}^{(0)} //

?OO

Z_{0}^{(0)}

of X^{(0)}, by setting Yr^{(0)} := V^{+}(Ir^{(0)}), Zr^{(0)} := V^{+}(Jr^{(0)}), where Ir^{(0)} is the
homogenous ideal inZ[x00, xij (1≤i, j≤n)], generated by all (r+1)×(r+1)-
subdeterminants of the matrix (xij)1≤i,j≤n, and whereJr^{(0)}= (x00) +I_{n−r}^{(0)} for
0 ≤ r ≤ n−1. For 1 ≤ k ≤ n let the scheme X^{(k)} together with closed
subschemes Yr^{(k)}, Zr^{(k)} ⊂ X^{(k)} (0 ≤ r ≤ n−1) be inductively defined as
follows:

X^{(k)} → X^{(k−1)} is the blowing up of X^{(k−1)} along the closed subscheme
Y_{k−1}^{(k−1)}∪Z_{n−k}^{(k−1)}. The subscheme Y_{k−1}^{(k)} ⊂ X^{(k)} (respectively Z_{n−k}^{(k)} ⊂ X^{(k)})
is the inverse image of Y_{k−1}^{(k−1)} (respectively of Z_{n−k}^{(k−1)}) under the morphism
X^{(k)} → X^{(k−1)}, and for r 6= k−1 (respectively r 6= n−k) the subscheme
Yr^{(k)} ⊂ X^{(k)} (respectively of Zr^{(k)} ⊂ X^{(k)}) is the complete transform of
Yr^{(k−1)}⊂X^{(k−1)} (respectivelyZr^{(k−1)}⊂X^{(k−1)}). We set

KGln :=X^{(n)} and Yr:=Y_{r}^{(n)}, Zr:=Z_{r}^{(n)} (0≤r≤n−1).
We are interested in finding a modular description for the compactification
KGln of Gln = SpecZ[xij/x00 (1≤i, j≤n), det(xij/x00)^{−1}].

Let (α, β)∈Sn×Sn and set

x^{(0)}_{ij} (α, β) := xα(i),β(j)

x00 (1≤i, j≤n) . For 1≤k≤nwe define elements

yji(α, β), zij(α, β) (1≤i≤k, i < j≤n)
x^{(k)}_{ij} (α, β) (k+ 1≤i, j≤n)

of the function field Q(X^{(0)}) =Q(xij/x00 (1≤i, j ≤n)) of X^{(0)} inductively
as follows:

yik(α, β) := x^{(k−1)}_{ik} (α, β)

x^{(k−1)}_{kk} (α, β) (k+ 1≤i≤n) ,

zkj(α, β) := x^{(k−1)}_{kj} (α, β)

x^{(k−1)}_{kk} (α, β) (k+ 1≤j≤n) ,

x^{(k)}_{ij} (α, β) := x^{(k−1)}_{ij} (α, β)

x^{(k−1)}_{kk} (α, β)−yik(α, β)zkj(α, β) (k+ 1≤i, j≤n) .
Finally, we sett0(α, β) :=t0:=x00 and

ti(α, β) :=t0· Yi j=1

x^{(j−1)}_{jj} (α, β) (1≤i≤n) .

Observe, that for each k∈ {0, . . . , n}, we have the following decomposition of the matrix [xij/x00]:

xij

x00

=nα

1_ _ _ _ 9 9 9

_ _ _

0 1 yij(α, β)

I_{n−k}

9 9 9

_ _

t1 (α,β)

t0 0

0
0 ^{tk}^{(α,β)}_{t}

0

0 ^{tk}^{(α,β)}_{t}_{0} [x^{(k)}_{ij} (α, β)]

1_ _ _ _ _

=

=

=

_ _ _

z_{ij}(α, β)
1

0 I_{n−k}

=

=

=

_ _

n^{−1}_{β}
Here, nα is the permutation matrix associated to α, i.e. the matrix, whose
entry in thei-th row and j-th column isδi,α(j). For convenience, we define for
eachl∈ {0, . . . , n}a bijectionιl:{1, . . . , n+ 1}→ {0, . . . , n}, by setting^{∼}

ιl(i) =

i if 1≤i≤l 0 if i=l+ 1

i−1 if l+ 2≤i≤n+ 1

for 1≤i≤n+ 1. With this notaton, we define for each triple (α, β, l)∈Sn×
Sn×[0, n] polynomial subalgebrasR(α, β, l) of Q(KGln) =Q(X^{(0)}) together
with idealsIr(α, β, l) andJr(α, β, l) (0≤r≤n−1) ofR(α, β, l) as follows:

R(α, β, l) := Z

t_{ι}_{l}_{(i+1)}(α, β)

t_{ι}_{l}_{(i)}(α, β) (1≤i≤n), yji(α, β), zij(α, β) (1≤i < j≤n)

, Ir(α, β, l) :=

t_{ι}_{l}_{(r+2)}(α, β)
tι_{l}(r+1)(α, β)

if l≤r≤n−1 and Ir(α, β, l) := (1) else, Jr(α, β, l) :=

tι_{l}(n−r+1)(α, β)
tι_{l}(n−r)(α, β)

ifn−l≤r≤n−1 andJr(α, β, l) := (1) else.

Proposition 4.1. There is a covering of KGln by open affine piecesX(α, β, l) ((α, β, l)∈ Sn×Sn×[0, n]), such that Γ(X(α, β, l),O) = R(α, β, l) (equality as subrings of the function field Q(KGln)). Furthermore, for 0 ≤ r≤ n−1 the ideals Ir(α, β, l)andJr(α, β, l)of R(α, β, l)are the defining ideals for the

closed subschemesYr(α, β, l) :=Yr∩X(α, β, l)andZr(α, β, l) :=Zr∩X(α, β, l) respectively.

Proof. We make the blowing-up procedure explicit, in terms of open affine coverings. For eachk∈ {0, . . . , n}we define a finite index setPk, consisting of all pairs

(p, q) =

p0

: pk

,

q0

: qk

∈ {0, . . . , n}^{k+1}× {0, . . . , n}^{k+1}

with the property thatpi6=pj andqi6=qj fori6=j and thatpi = 0 for somei, if and only ifqi= 0. Observe that for eachk∈ {0, . . . , n}there is a surjection Sn×Sn× {0, . . . , n} → Pk, which maps the triple (α, β, l) to the element

(p, q) =

α(ιl(1)) : α(ιl(k+ 1))

,

β(ιl(1)) : β(ιl(k+ 1))

ofPk. (Here we have used the convention thatα(0) := 0 for any permutation
α ∈ Sn). Furthermore, this surjection is in fact a bijection in the case of
k =n. Let (p, q) ∈ Pk and chose an element (α, β, l) in its preimage under
the surjection Sn×Sn × {0, . . . , n} → Pk. We define subrings R^{(k)}(p, q) of
Q(xij/x00(1≤i, j≤n)) together with idealsIr^{(k)}(p, q),Jr^{(k)}(p, q) (0≤r≤n),
distinguishing three cases.

First case: 0≤l≤k−1

R^{(k)}(p, q) := Z

"

t_{ιl}(i+1)(α, β)

t_{ιl}(i)(α, β) (1≤i≤k), yji(α, β), zij(α, β)

1≤i≤k, i < j≤n

,
x^{(k)}_{ij} (α, β) (k+ 1≤i, j≤n)i

I_{r}^{(k)}(p, q) :=

(1) ifr∈[0, l−1]

_{tι}

l(r+2) (α,β) tιl(r+1) (α,β)

ifr∈[l, k−1]

detAB(x^{(k)}_{ij} (α, β))

A, B⊆ {k+ 1, . . . , n}

]A=]B=r+ 1−k ifr∈[k, n−1]

J_{r}^{(k)}(p, q) :=

(1) ifr∈[0, n−l−1]

tιl(n−r+1) (α,β) tιl(n−r) (α,β)

ifr∈[n−l, n−1]

Second case: l=k

R^{(k)}(p, q) := Z

"

t_{ιl}(i+1)(α, β)

t_{ιl}(i)(α, β) (1≤i≤k), yji(α, β), zij(α, β)

1≤i≤k, i < j≤n

, tk(α, β)

t0

x^{(k)}_{ij} (α, β) (k+ 1≤i, j≤n)

I^{(k)}r (p, q) :=

(1) ifr∈[0, l−1]

detAB

_{tk}_{(α,β)}

t0 x^{(k)}_{ij} (α, β) A, B⊆ {k+ 1, . . . , n}

]A=]B=r+ 1−k ifr∈[l, n−1]

J_{r}^{(k)}(p, q) :=

(1) ifr∈[0, n−l−1]

tιl(n−r+1) (α,β) tιl(n−r) (α,β)

ifr∈[n−l, n−1]

Third case: k+ 1≤l≤n

R^{(k)}(p, q) := Z

"

t_{ιl}(i+1)(α, β)

t_{ιl}(i)(α, β) (1≤i≤k), t0

tk+1(α, β), yji(α, β), zij(α, β)

1≤i≤k+ 1, i < j≤n

, x^{(k+1)}_{ij} (α, β) (k+ 2≤i, j≤n)

I^{(k)}r (p, q) :=

(1) ifr∈[0, k]

detAB

x^{(k+1)}_{ij} (α, β) A, B⊆ {k+ 2, . . . , n}

]A=]B=r−k ifr∈[k+ 1, n−1]

Jr^{(k)}(p, q) :=

_{t}

0

tk+1 (α,β), detAB(x^{(k+1)}_{ij} (α, β))

A, B⊆ {k+ 2, . . . , n}

]A=]B=n−r−k ifr∈[0, n−k−1]

tιl(n−r+1) (α,β) tιl(n−r) (α,β)

ifr∈[n−k, n−1]

Observe that the objects R^{(k)}(p, q),Ir^{(k)}(p, q), Jr^{(k)}(p, q) thus defined, depend
indeed only on (p, q) and not on the chosen element (α, β, l). By induction on
kone shows thatX^{(k)}is covered by open affine piecesX^{(k)}(p, q) ((p, q)∈ Pk),
such that Γ(X^{(k)}(p, q),O) =R^{(k)}(p, q) (equality as subrings of the function field
Q(X^{(k)})), and such that the ideals Ir^{(k)}(α, β) andJr^{(k)}(α, β) are the defining
ideals of the closed subschemesYr^{(k)}∩X^{(k)}(p, q) andZr^{(k)}∩X^{(k)}(p, q) respec-
tively.

Corollary 4.2. The scheme KGln is smooth and projective over SpecZand contains Gln as a dense open subset. The complement of Gln in KGln is the union of the closed subschemes Yi, Zi (0≤i≤n−1), which is a divisor with normal crossings. Furthermore, we haveYi∩Zj =∅for i+j < n.

Proof. This is immediate from the local description given in 4.1.

We will now define a certain toric scheme, which will play an important role
in the sequel. Let M :=Z^{n}, with canonical basis e1, . . . , en. For m∈ M we
denote byt^{m}the corresponding monomial in the ring Z[M]. Furthermore, we
write ti/t0 for the canonical generatort^{e}^{i} ofZ[M]. LetN :=M^{∨} be the dual
of M with the dual basise^{∨}_{1}, . . . , e^{∨}_{n}. For 0≤l≤nletσl ⊂NQ :=N⊗Qbe

the cone generated by the elements −Pi

j=1e^{∨}_{j} (1≤i≤l) and the elements
Pn

j=ie^{∨}_{j} (l+ 1≤i≤n). In other words:

σl= Xl i=1

Q_{+}·

−

Xi j=1

e^{∨}_{j}

+ Xn i=l+1

Q_{+}·

Xn

j=i

e^{∨}_{j}

.

Let Σ be the fan generated by allσl(0≤l≤n) and letTe:=XΣthe associated
toric scheme (overZ). See e.g. [Da] for definitions. Teis covered by the open
setsTel:=Xσ^{∨}_{l} = SpecZ[t^{m}(m∈σ_{l}^{∨}∩M)] = SpecZ[tιl(i+1)/tιl(i)(1≤i≤n)].

Observe that there are Cartier divisorsY_{r,}_{T}_{e}, Z_{r,}_{T}_{e}(0≤r≤n−1) onTe, such
that for eachl∈ {0, . . . , n}over the open part Tel⊂Te,

Y_{r,}_{T}_{e} is given by the equation

1 if 0≤r≤l−1
tιl(r+2)/tιl(r+1) ifl≤r≤n−1
Z_{r,}_{T}_{e} is given by the equation

1 if 0≤r≤n−l−1
tιl(n−r+1)/tιl(n−r) ifn−l≤r≤n−1
Observe furthermore that Y_{i,}_{T}_{e}∩Z_{j,}_{T}_{e} = ∅ for i+j < n and that for each
r∈ {1, . . . , n}, multiplication bytr/t0 establishes an isomorphism

O_{T}_{e} −

n−rX

i=0

Z_{i,e}_{T}

!

−→ O∼ _{T}_{e} −

r−1X

i=0

Y_{i,e}_{T}

! .

Lemma 4.3. The toric schemeTe together with the “universal” tupel
(O_{T}_{e}(Y_{i,e}_{T}), 1_{O}

e

T(Y_{i,}_{T}_{e}), O_{T}_{e}(Z_{i,e}_{T}), 1_{O}

e

T(Z_{i,}_{T}_{e})(0≤i≤n−1), tr/t0(1≤r≤n))
represents the functor, which to each schemeSassociates the set of equivalence
classes of tupels

(Li, λi, Mi, µi (0≤i≤n−1), ϕr (1≤r≤n)) ,

where the Li andMi are invertibleOS-modules with global sectonsλi andµi

respectively, such that fori+j < nthe zero-sets of λi andµj do not intersect, and where theϕr are isomorphisms

n−rO

i=0

M^{∨}_{i} −→^{∼}

r−1O

i=0

L^{∨}_{i} .

Here two tupels (Li, λi, Mi, µi (0 ≤ i ≤ n−1), ϕr (1 ≤ r ≤ n)) and
(L^{0}_{i}, λ^{0}_{i}, M^{0}_{i}, µ^{0}_{i} (0≤i≤n−1), ϕ^{0}_{r} (1≤r≤n))are called equivalent, if there
exist isomorphisms Li ∼

→ L^{0}_{i} andMi ∼

→ M^{0}_{i} for 0≤i ≤n−1, such that all
the obvious diagrams commute.

Proof. LetSbe a scheme and (Li, λi, Mi, µi(0≤i≤n−1), ϕr(1≤r≤n)) a tupel defined over S, which has the properties stated in the lemma. Let us first consider the case, where all the sheavesLi,Mi are trivial and where there exists anl∈ {0, . . . , n}, such thatλi andµj is nowhere vanishing for 0≤i < l and 0 ≤ j < n−l respectively. Observe that under theses conditions there

exists a unique set of trivializations Li ∼

→ OS, Mi ∼

→ OS, (0 ≤ i≤ n) such that λi 7→ 1 for 0 ≤ i < l, µj 7→ 1 for 0 ≤ j < n−l, and such that the diagrams

Nn−r
i=0 M^{∨}_{i}

∼

$$I

II II II II

ϕr

∼ //Nr−1
i=0L^{∨}_{i}

∼

zzvvvvvvvvv

OS

commute for 1 ≤ r ≤ n. Let aν ∈ Γ(S,OS) (1 ≤ ν ≤ n) be defined by
λi 7→ai+1 forl ≤ i ≤n−1 andµj 7→an−j for n−l ≤ j ≤n−1, and let
fl:S →Tel be the morphism defined by f_{l}^{∗}(tιl(ν+1)/tιl(ν)) =aν (1 ≤ν ≤n).

It is straightforward to check that the induced morphismf :S →Te does not depend on the chosen number l and that it is unique with the property that the pull-back underf of the universal tupel is equivalent to the given one on S.

Returning to the general case, observe that there is an open coveringS=∪kUk,
such that for eachkthere exists anlwith the property that overUk all theLi,
Mi are trivial and thatλi and µj is nowhere vanishing overUk for 0≤i < l
and 0 ≤j < n−l. The above construction shows that there exists a unique
morphism f : S → Te such that for each k the restriction to Uk of the pull-
back under f of the universal tupel is equivalent to the restriction to Uk of
the given one. Thus it remains only to show that the isomorphisms defining
the equivalences over theUk glue together to give a global equivalence of the
pull-back of the universal tupel with the given one. However, this is clear, since
it is easy to see that there exists at most one set of isomorphisms Li → L∼ ^{0}_{i},
Mi ∼

→ M^{0}_{i}establishing an equvalence between two tuples (Li, λi, Mi, µi, ϕr)
and (L^{0}_{i}, λ^{0}_{i}, M^{0}_{i}, µ^{0}_{i}, ϕ^{0}_{r}).

For each pair (α, β)∈Sn×Sn we define the open subsetX(α, β)⊆KGln as the union of the open affinesX(α, β, l) (0≤l≤n). Let

U^{−} := SpecZ[yji(1≤i < j≤n)] ,
U^{+} := SpecZ[zij (1≤i < j≤n)] .

Let y :X(α, β)→U^{−} (respectivelyz :X(α, β)→U^{+}) be the morphism de-
fined by the property thaty^{∗}(yji) =yji(α, β) (respectivelyz^{∗}(zij) =zij(α, β))
for 1 ≤i, j ≤n. Observe that just as in the case of Te, multiplication by the
rational functiontr(α, β)/t0 provides an isomorphism

O_{X(α,β)} −

n−rX

i=0

Zi(α, β)

!

−→ O∼ _{X(α,β)} −
Xr−1

i=0

Yi(α, β)

!

for 1≤r≤n, whereYi(α, β) (respectivelyZi(α, β)) denotes the restriction of Yi (respectivelyZi) to the open set X(α, β). Thus, by lemma 4.3, the tupel

(O(Yi(α, β)), 1, O(Zi(α, β)), 1(i∈[0, n−1]), tr(α, β)/t0 (r∈[1, n])) defines a morphism t:X(α, β)→Te.

Lemma 4.4. The morphism (y, t, z) :X(α, β)→U^{−}×Te×U^{+} is an isomor-
phism.

Proof. Let Ω(α, β) ⊂ X(α, β) be the preimage of Gln under the morphism
X(α, β),→KGln→X^{(0)}. By definition of KGln, we have for alll∈ {0, . . . , n}:

Ω(α, β) = X(α, β, l)\

n−1[

i=0

(Yi(α, β, l)∪Zi(α, β, l))

= SpecZ[yji(α, β), zij(α, β) (1≤i < j≤n),

(ti(α, β)/t0)^{±1}(1≤i≤n)] .
Let T := SpecZ[(ti/t0)^{±1}]⊂Te be the Torus inTe. We have an isomorphism
Ω(α, β)→^{∼} U^{−}×T ×U^{+} defined byyji7→ yji(α, β), zij 7→zij(α, β),ti/t0 7→

ti(α, β)/t0, and a commutative quadrangle

X(α, β) −−−−→^{(y,t,z)} U^{−}×Te×U^{+}
x

x

Ω(α, β) −−−−→^{∼} U^{−}×T×U^{+} ,

where the vertical arrows are the natural inclusions. Furthermore, the map
(y, t, z) induces an isomorphism X(α, β, l) →^{∼} U^{−}×Tel×U^{+} for 0 ≤ l ≤ n.

Using the fact thatX(α, β) is separated and that Ω(α, β) dense inX(α, β), the lemma now follows easily.

5. bf-morphisms and generalized isomorpisms

Definition 5.1. LetS be a scheme,E andFtwo localy freeOS-modules and ra nonnegative integer. Abf-morphism of rank rfrom E toF is a tupel

g= (M,µ, E → F, M ⊗ E ← F, r),

where M is an invertible OS-module andµ a global section of M such that the following holds:

1. The composed morphismsE → F → M ⊗ E andF → M ⊗ E → M ⊗ F are both induced by the morphismµ:OS → M.

2. For every pointx∈S withµ(x) = 0, the complex E[x]→ F[x]→(M ⊗ E)[x]→(M ⊗ F)[x]

is exact and the rank of the morphismE[x]→ F[x] equals r.

The letters “bf” stand for “back and forth”. As a matter of notation, we
will sometimes write g^{]} for the morphism E → F and g^{[} for the morphism
F → M ⊗ E occuring in the bf-morphism g. Note that in case µ is nowhere
vanishing, the number rkg:=rcannot be deduced from the other ingredients

of g. Sometimes we will use the following more suggestive notation for the bf-morphismg:

g=

E ^{r} //

(M,µ)F

⊗

.

In situations where it is clear, what (M, µ) andrare, we will sometimes omit these data from our notation:

g=

E //F

⊗

.

Definition 5.2. LetS be a scheme,E andF two locally freeOS-modules of rankn. Ageneralized isomorphism from E toF is a tupel

Φ = (Li,λi,Mi,µi, Ei→ Mi⊗ Ei+1, Ei← Ei+1,

Fi+1→ Fi, Li⊗ Fi+1← Fi (0≤i≤n−1), En ∼

→ Fn), where E =E0, E1, . . . ,En, Fn, . . . ,F1, F0 =F, are localy free OS-modules of ranknand the tupels

(Mi,µi, Ei+1→ Ei, Mi⊗ Ei+1← Ei, i) and (Li,λi, Fi+1→ Fi, Li⊗ Fi+1← Fi, i)

are bf-morphisms of ranki for 0 ≤ i ≤n−1, such that for each x ∈ S the following holds:

1. Ifµi(x) = 0 and (f, g) is one of the following two pairs of morphisms:

E[x]−→^{f} ((⊗^{i−1}_{j=0}Mj)⊗ Ei)[x]−→^{g} ((⊗^{i}_{j=0}Mj)⊗ Ei+1)[x],
Ei[x]←− E^{g} i+1[x]←− E^{f} n[x],

then im(g◦f) = im(g). Likewise, if λi(x) = 0 and (f, g) is one of the following two pairs of morphisms:

Fn[x]−→ F^{f} i+1[x]−→ F^{g} i[x],

((⊗^{i}_{j=0}Lj)⊗ Fi+1)[x]←−^{g} ((⊗^{i−1}_{j=0}Lj)⊗ Fi)[x]←− F[x]^{f} ,
then im(g◦f) = im(g).

2. In the diagram:

0

ker(En[x]→ E0[x])

**UUUUUUUU

0 //ker(Fn[x]→ F0[x])UUUUUUU//UE**n[x]∼=Fn[x]

//im(Fn[x]→ F0[x]) //0

im(En[x]→ E0[x])

0

the oblique arrows are injections.

Definition 5.3. Aquasi-equivalence between two generalized isomorphisms Φ = (Li,λi,Mi,µi, Ei→ Mi⊗ Ei+1, Ei← Ei+1,

Fi+1→ Fi, Li⊗ Fi+1← Fi (0≤i≤n−1), En ∼

→ Fn),
Φ^{0} = (L^{0}_{i},λ^{0}_{i},M^{0}_{i},µ^{0}_{i}, E_{i}^{0} → M^{0}_{i}⊗ E_{i+1}^{0} , E_{i}^{0}← E_{i+1}^{0} ,

F_{i+1}^{0} → F_{i}^{0}, L^{0}_{i}⊗ F_{i+1}^{0} ← F_{i}^{0} (0≤i≤n−1), E_{n}^{0} → F^{∼} _{n}^{0})
fromEtoFconsists in isomorphismsLi ∼

→ L^{0}_{i}andMi ∼

→ M^{0}_{i}for 0≤i≤n−1,
and isomorphismsEi ∼

→ E_{i}^{0}andFi ∼

→ F_{i}^{0}for 0≤i≤n, such that all the obvious
diagrams are commutative. A quasi-equivalence between Φ and Φ^{0} is called an
equivalence, if the isomorphismsE0 ∼

→ E_{0}^{0} andF0 ∼

→ F_{0}^{0} are in fact the identity
onE andF respectively.

After these general definitions, we now return to our scheme KGln. The nota- tions are as in the previous section.

From the matrix-decomposition on page 560 (for k=n) we see that the ma- trix [xij/x00]1≤i,j≤n has entries in the subspace Γ(KGln,O(Pn−1

i=0 Zi)) of the function fieldQ(KGln) of KGln. Therefore it defines a morphism

x:E0−→ O

n−1X

i=0

Zi

!

·F0 ,

whereE0=F0=⊕^{n}OKGln.

LetEn⊂E0be the preimage underxofF0⊂ O(Pn−1

i=0 Zi)·F0and letFn⊂F0

be the image underxofEn. Thusxinduces a morphism En −→Fn ,

which we again denote by x. For 1≤i≤n−1 we defineOKGln-submodules
Ei andFi of⊕^{n}KKGlnas follows:

Ei := En+O

−

Xi−1 j=0

Zj

·E0

Fi := Fn+O

−

Xi−1 j=0

Yj

·F0

(the plus-sign means generation in⊕^{n}KKGln). Observe that for 0≤i≤n−1
we have the following natural injections:

Ei,→ O(Zi)·Ei+1 , Ei←- Ei+1

Fi+1,→Fi , O(Yi)·Fi+1 ←- Fi

Proposition 5.4. The tupel
Φuniv := (O(Yi), 1_{O(Y}

i), O(Zi), 1_{O(Z}

i), Ei ,→ O(Zi)·Ei+1, Ei←- Ei+1, Fi+1,→Fi, O(Yi)·Fi+1←- Fi (0≤i≤n−1), x:En →Fn)

is a generalized isomorphism from⊕^{n}OKGl_{n} to itself.

Proof. It suffices to show that for each (α, β) ∈ Sn×Sn the restriction of
Φunivto the open setX(α, β) is a generalized isomorphism from⊕^{n}O_{X(α,β)}to
itself. Letz(α, β) (y(α, β)) be the upper (lower) triangularn×nmatrix with
1 on the diagonal and entries zij(α, β) (yji(α, β)) over (under) the diagonal
(1≤i < j ≤n). For 0≤i≤nwe define

Ei(α, β) := z(α, β)·n^{−1}_{β} ·Ei|_{X(α,β)} ,
Fi(α, β) := y(α, β)^{−1}·n^{−1}_{α} ·Fi|X(α,β) .

Here we interprete the matricesz(α, β)·n^{−1}_{β} andy(α, β)^{−1}·n^{−1}_{α} as automor-
phisms of ⊕^{n}KX(α,β). Accordingly we view the sheavesEi(α, β) and Fi(α, β)
as subsheaves of⊕^{n}KX(α,β). We have to show that the tupel

Φ(α, β) := (O(Yi(α, β)), 1_{O(Y}

i(α,β)) , O(Zi(α, β)), 1_{O(Z}

i(α,β)) ,

Ei(α, β),→ O(Zi(α, β))·Ei+1(α, β), Ei(α, β)←- Ei+1(α, β), Fi+1(α, β),→Fi(α, β), O(Yi(α, β))·Fi+1(α, β)←- Fi(α, β) (0≤i≤n−1),

y(α, β)^{−1}n^{−1}_{α} xnβz(α, β)^{−1}:En(α, β)→^{∼} Fn(α, β))
is a generalized isomorphism from ⊕^{n}O_{X(α,β)} to itself.

We have for 0≤i≤nthe following equality of subsheaves of⊕^{n}K_{X(α,β)}:
Ei(α, β) =

Mn−i j=1

O − Xi−1 ν=0

Zν(α, β)

!

⊕ Mn j=n−i+1

O −

n−jX

ν=0

Zν(α, β)

! ,

Fi(α, β) = Mi j=1

O −

j−1X

ν=0

Yν(α, β)

!

⊕ Mn j=i+1

O − Xi−1 ν=0

Yν(α, β)

! .

This is easily checked by restricting both sides of the equations to the open subsets X(α, β, l), (0 ≤ l ≤ n) of X(α, β) and using 4.1. Observe that the morphisms

Ei(α, β),→ O(Zi(α, β))·Ei+1(α, β), Ei(α, β)←- Ei+1(α, β) are described by the matrices

I_{n−i} 0
0 µiI_{i}

and

µiI_{n−i} 0
0 I_{i}

, and the morphisms

Fi+1(α, β),→Fi(α, β), O(Yi(α, β))·Fi+1(α, β)←- Fi(α, β) by the matrices

I_{i} 0
0 λiI_{n−i}

and

λiI_{i} 0
0 I_{n−i}

respectively, where we have abbreviated 1_{O(Y}

i(α,β)) by λi, and 1_{O(Z}

i(α,β))

by µi. Furthermore the matrix-decomposition on page 560 (for k = n)

shows that y(α, β)^{−1}n^{−1}_{α} xnβz(α, β)^{−1} is the diagonal matrix with entries
(t1(α, β)/t0, . . . , tn(α, β)/t0). With this information at hand, it is easy to see
that Φ(α, β) is indeed a generalized isomorphism from⊕^{n}OX(α,β)to itself.

Theorem 5.5. LetSbe a scheme andΦa generalized isomorphism from⊕^{n}OS

to itself. Then there is a unique morphism f :S→KGln such that f^{∗}Φuniv is
equivalent toΦ. In other words, the scheme KGln together withΦunivrepresents
the functor, which to each schemeS associates the set of equivalence classes of
generalized isomorphisms from⊕^{n}OS to itself.

The proof of the theorem will be given in section 7.

Corollary 5.6. There is a (left) action of Gln×Gln on KGln, which extends
the action ((ϕ, ψ),Φ)7→ψΦϕ^{−1} of Gln×Gln on Gln. The divisors Zi andYi

are invariant under this action.

Proof. The the morphism (Gln×Gln)×KGln →KGln defining the action is given onS-valued points by

((ϕ, ψ),Φ)7→Φ^{0} ,

where Φ is a generalized isomorphism as in definition 5.2 from E0=⊕^{n}OS to
F0 = ⊕^{n}OS and Φ^{0} is the generalized isomorphism where for 2 ≤i ≤n the
bf-morphisms fromEi to Ei−1, the ones fromFi toFi−1 and the isomorphism
En ∼

→ Fn are the same as in the tupel Φ, and where the bf-morphisms (M0, µ0, E1→ E0, M0⊗ E0← E1, 0)

and (L0, λ0, F1→ F0, L0⊗ F0← F1, 0) in the tupel Φ are replaced by the bf-morphisms

(M0, µ0, E1→ E0

→ Eϕ 0, M0⊗ E1← E0
ϕ^{−1}

← E0, 0) and (L0, λ0, F1→ F0

→ Fψ 0, L0⊗ F1← F0
ψ^{−1}

← F0, 0)

respectively. The invariance of the divisorsZi and Yi is clear, since they are defined by the vanishing ofµi andλi respectively.

6. Exterior powers

Lemma 6.1. Let S be a scheme and E, F two locally free OS-modules of rank n. Let

g= (M, µ, E → F, M ⊗ E ← F, r)

be a bf-morphism of rank r from E to F. Then each point x ∈ S has an open neighbourhood U such that over U there exist local frames (e1, . . . , en) and (f1, . . . , fn) for E and F respectively with the property that the matrices for the morphisms

E −→ F and M ⊗ E ←− F with respect to these frames are

I_{r} 0
0 µ/mIn−r

and

µIr 0 0 mIn−r