Lectures on Chevalley groups
Arun Ram
Department of Mathematics and Statistics
University of Melbourne
Parkville, VIC 3010 Australia
aram@unimelb.edu.au
Last update: 8 July 2013
§8. Variants of the Bruhat lemma
Let be a Chevalley group, as usual. We recall (Theorems 4 and 4'):
(a) |
a disjoint union.
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(b) |
For each
with, uniqueness of
expression on the right. Our purpose is to present some analogues of (b) with applications.
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For each simple root, we set
a group of
and assume that the
representative of in also denoted
is chosen in
Theorem 15: For each simple root let be a system of representatives
for
or more generally for For each
choose a minimal expression
as a product of reflections relative to simple roots
Then
with uniqueness of expression on the right.
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Proof. |
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Since
the second case above really
is more general than the first. We have
Now assume
with etc. Then
We have
or The second case can not occur since then the left side
would be in and the right side in
(by Lemma 25). From the definition of it follows that
and then by induction that
whence, the uniqueness in Theorem 15.
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Lemma 43: Let
be the canonical homomorphism (see Theorem 4', Cor. 6). Then satisfies the conditions of Theorem 15 in each of
the following cases.
(a) |
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(b) |
(resp. and
is the image under of the elements of
(resp.
(standard compact forms) of the form
with
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(c) |
If is a principal ideal domain (commutative with
is the group of units, is the quotient field, and
is the image under of the elements of
of the form
with running through a set of representatives for
and for each a running over a set of representatives for the residue classes of
mod
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Proof. |
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We have (a) by Theorem 4' applied to To verify (b) and (c) we may assume that
is and
the superdiagonal subgroup since
Any element of can be converted to
one of by adding a multiple of the second row to the first and normalizing the lengths of the rows.
Thus
Then
whence (b). Now assume
with as in (c). We choose in relatively prime and
such that (using unique factorization), and then
in so that
Multiplying the preceding
matrix on the right by
we get an element of Thus
and (c) follows.
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Remarks: (a) The case (a) above is essentially Theorem 4' since
in the notation of Theorem 15, by Appendix II 25, or else by induction on the length of the expression. (b) In (c) above the choice can be made precise in the
following cases:
(1) |
choose
so that
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(2) |
a field); choose so that is monic and
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(3) |
integers); choose a power of and an integer such that
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In what follows we will give separate but parallel developments of the consequences of (b) and (c) above. In (b) we will treat the case
for definiteness, the case being similar.
Lemma 44: Let and
be as in Theorem 1.
(a) |
There exists an involutory semiautomorphism of
(relative to complex conjugation of
such that
and
for every root
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(b) |
On the form defined by
in
terms of the Killing form is negative definite.
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Proof. |
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This basic result is proved, e.g., in Jacobson, Lie algebras, p. 147.
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Theorem 16: Let be a Chevalley group over viewed as a Lie group over
(a) |
There exists an analytic automorphism of such that
and
for all and
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(b) |
The group of fixed points of is a
maximal compact subgroup of
and the decomposition holds (Iwasawa decomposition).
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Proof. |
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Let be in Lemma 44 composed with complex conjugation, and
the representation of used to define Applying Theorem 4',
Cor. 5 to the Chevalley groups (both equal to constructed from the representations and
of we get an
automorphism of which aside from complex conjugation satisfies the equations of (a), hence composed with conjugation satisfies these equations.
From Theorem 7 adapted to the present situation (see the remark at the end of §5) it follows that is analytic, whence (a). We observe
that if is defined by the ajoint representation of then is
effected by conjugation by the semiautomorphism of Lemma 44.
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Lemma 45: Let
for each simple root
(a) |
(see Lemma 43(b)), hence
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(b) |
maximal torus in
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Proof. |
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The kernel of
is contained in and pulls back to
the inverse transpose conjugate, say on
Since the equation
has no solutions we get (a).
Since
(here and are corresponding weights on and
and the
generate we have
for all so that if and
only if
for all weights If
then
Since there are linearly independent weights we see that if
for all then
for some
whence
for all If is universal, then is the
product of the circles
hence is a torus; if not, we have to take the quotient by a finite group, thus still have a torus. Now if
is general enough, so that the numbers
are distinct and different from
then the centralizer of in
is by the uniqueness in Theorem 4', so that
is in fact a maximal Abelian subgroup of
which proves the lemma.
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Exercise: Check out the existence of and the property
above.
Now we consider part (b) of Theorem 16. By Theorem 15 and Lemmas 43(b) and 45(a) we have
By the same results
a compact set since each factor is (the compactness of tori and is being used). Thus
is compact. (This also follows easily from Lemma 44(b)). Let
be a compact subgroup of
Assume
Write with
and then with
Since
is compact, all eigenvalues
are bounded, whence by Lemma 45(b). Then all coefficients of all are bounded so that
Thus so that
is maximal compact.
Remark: It can be shown also that is semisimple and that a complete set of semisimple compact Lie groups is got from the above construction.
Corollary 1: Let be of the same type as with a weight lattice
containing that of
and the natural projection. Then
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Proof. |
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This follows from the fact proved in Lemma 45 that is generated by the groups
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Examples:
(a) |
If
then
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(b) |
If
then fixes simultaneously the forms
and
hence equals (compact form) after a
change of coordinates. Prove this.
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(c) |
If
then fixes the forms
and
and is isomorphic to
(compact form, quaternions). For this see Chevalley, Lie groups, p. 22.
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(d) |
We have isomorphisms and central extensions,
(compact forms).
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This follows from (a), (b), (c), Corollary 1 and the equivalences
Corollary 2: The group is connected.
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Proof. |
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As already remarked, is generated by the groups
Since is connected, so is
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Corollary 3: If denotes the maximal torus then
is homeomorphic to under the natural map.
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Proof. |
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The map
is continuous and constant on the fibres of
hence leads to a continuous map of into
which is and onto since
and Since
is compact, the map is a homeomorphism.
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Corollary 4:
(a) |
is contractible to
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(b) |
If is universal, then is simply connected.
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Proof. |
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Let
Then we have so that
On the right there is
uniqueness of expression. Since is compact it easily follows that the natural map
is a homeomorphism. Since
is contractible to a point, is contractible to
If also is universal, then is simply connected by Theorem 13; hence so is
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Corollary 5: For set
and let be as in
Theorem 15. Then and
with uniqueness of expression on the right.
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Proof. |
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This follows from Theorem 15 and Lemma 43(b).
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Remark:
Observe that is essentially a cell since each
is homeomorphic to (consider the values of in Lemma 43(b)). A true cellular decomposition is obtained by writing
as a union of cells. Perhaps this decomposition can be used to give an elementary treatment of the cohomology of
Corollary 6: and have as their Poincaré
polynomials
They have no torsion.
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Proof. |
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We have homeomorphic to
a cell of real
dimension
Since each dimension is even, it follows that the cells represent independent elements of the homology group and that there is no torsion (essentially because the
boundary operator lowers dimensions by exactly 1), whence Cor. 6. Alternately one may use the fact that each
is homeomorphic to
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Remark: The above series will be summed in the next section, where it arises in connection with the orders of the finite Chevalley groups.
Corollary 7: For let
be a minimal expression
as before and let denote the set of elements of each of which is a product of some subsequence of the expression for
Then (topological closure)
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Proof. |
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If we have
by Lemma 45(a) and
by the corresponding result in Now
by Lemma 43(b). Hence
so that
and we have equality since each factor on the right is compact, so that the right side is compact, hence closed. Since
if and
is simple, by Lemma 25, Cor. 7 follows.
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Corollary 8:
(a) |
is the closure of every
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(b) |
is closed if and only if
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Corollary 9: The set of Cor. 7 depends only on not on the minimal expression
chosen, hence may be written
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Proof. |
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Because doesn't depend on the expression.
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Lemma 46: Let be the element of which makes all positive roots negative. Then
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Proof. |
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Assume and let
be a minimal expression as a product of simple reflections and similarly for
Then
is one for since if is the number of positive roots then m
and
Looking at the initial segment of we see that
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Corollary 10: If is as above and
is a minimal expression, then
(a) |
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(b) |
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Proof. |
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(a) By Cor. 7 and Lemma 46.
(b) By (a)
We may write
simple), then absorb the
in appropriate to get (b).
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Exercise: If is any Chevalley group and
are as
above, show that
Remarks: (a) If and are replaced by
and in accordance with Lemma 43(b), then everything above goes
through except for Cor. 4, Cor. 6 and the fact that is no longer a torus. In this case each
is a circle since is. The corresponding angles in Cor. 10(b), which we have to restrict
suitably to get uniqueness, may be called the Euler angles in analogy with the classical ease:
(b) If is replaced by
in Cor. 7, the formula for is obtained. (Prove this.) If
(or is replaced by any algebraically closed field and the Zariski topology
is used, the same formula holds. So as not to interrupt the present development, we give the proof later, at the end of this section.
Theorem 17: (Cartan). Again let be a Chevalley group over or
as above, and
(a) |
(Cartan decomposition).
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(b) |
In (a) the is determined uniquely up to conjugacy under the Weyl group.
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Proof. |
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(a) Assume By the decompositions
and (Theorem 16), t
here exist elements in Given such an
element we write
uniquely determined by
then set
the Killing norm in This norm is invariant under
We now choose to maximize (recall that is compact). We must show that
This follows from: if
then can be increased.
We will reduce to the rank 1 case. Write
We may assume for some simple choose
of minimum height, say such that
then if
choose simple so that
and
then replace by
and proceed by induction on We write
with
(here is the set of positive roots). Then we write
choose so that
is orthogonal to set
Then
commutes with elementwise and is orthogonal
to relative to the bilinear form corresponding to the norm introduced above. By
for groups of rank 1, there exist
such that
and
Then
Since normalizes
(since and do),
Since
we have modulo the rank 1 case. This case, essentially
will be left as an exercise.
(b) Assume
as in (a). Then
so that
Here since
for all
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Lemma 47: If elements of are conjugate in (any Chevalley group), they are conjugate under the Weyl group.
This easily follows from the uniqueness in Theorem 4'.
By the lemma above uniquely determines up to conjugacy under the Weyl group, hence also
since square-roots in are unique.
Remark: We can get uniqueness in (b) by replacing by
This follows from Appendix III 33.
Corollary: Let consist of the elements of which satisfy
and have all eigenvalues positive.
(a) |
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(b) |
Every is conjugate under to some
uniquely determined up to conjugacy under (spectral theorem).
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(c) |
with uniqueness on the right (polar decomposition).
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Proof. |
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(a) This has been noted in (b) above.
(b) We can assume by the theorem. Apply
Thus commutes with hence also with
(Since is diagonal (relative to a basis of weight vectors) and positive, the matrices
commuting with have a certain block structure which does not change when it is replaced by
Then and
so that is unipotent by the definition of Since is compact,
Thus
The uniqueness in (b) follows as before.
(c) If then
as in the theorem, so that
Thus Assume
with and
By (b) we can assume that Then
As in (b) we conclude that
whence the uniqueness in (c).
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Example: If
so that
positive diagonal matrices
positive-definite Hermitean matrices
then (b) and (c) reduce
to classical results.
We now consider the case (c) of Lemma 43. The development is strikingly parallel to that for case (b) just completed although the results are basically arithmetic
in one case, geometric in the other. Throughout we assume that
are as in Lemma 43(c) and that the Chevalley group under discussion is based on We write
for the subgroup of elements of whose coordinates, relative to the original lattice
all lie in
Lemma 48: If is as in Theorem 4', Cor. 6, then
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Proof. |
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If is a Euclidean domain then
is generated by its unipotent superdiagonal and subdiagonal elements, so that the lemma follows from the fact that
acts on as an integral polynomial in In the general case it follows that if
is a prime in and is the localization of at
(all such that
with prime to
then
Since
e.g. by unique factorization, we have our result.
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Remark: A version of Lemma 48 is true if is any commutative ring since
is generically expressible as a polynomial in
with integral coefficients (proof omitted). The proof just given works if is any integral domain for which
maximal ideal), which includes most of the interesting cases.
Lemma 49: Write
(a) |
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(b) |
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(c) |
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(d) |
Hence
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Proof. |
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(a) If then its diagonal
relative to a basis of made up of weight vectors (see Lemma 18, Cor. 3), must be in
hence must also.
(b) If
then by induction on heights, the equation
and the primitivity of in
(Theorem 2, Cor. 2) we get all
(c) If in diagonal form as above, then
must be in for each
weight of the representation defining in fact in
since the sum of these weights is (the sum is invariant under
If we write
and use what has just been proved, we get for some
whence
by unique factorization.
(d) Set
By Lemma 48, Since
by Lemma 43(c) and the reverse inclusion
follows from:
Now if
then and by
(a), (b), (c) applied to so that
whence (d).
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Theorem 18: Let and
be as above. Then
(Iwasawa decomposition).
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Proof. |
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By Lemmas 43(c) and 49(d),
for every so that
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Corollary 1: Write
(a) |
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(b) |
with given by Lemma 49, and on the right there is uniqueness of expression.
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Remark: This normal form in has all components in
whereas the usual one obtained by imbedding in doesn't.
Corollary 2: is generated by the groups
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Proof. |
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By Lemma 49 and Cor. 1.
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Corollary 3: If is a Euclidean domain, then is generated by
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Proof. |
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Since the corresponding result holds for
this follows from Lemma 49(d) and Cor. 2.
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Example: Assume
We get that is generated by
The normal form in Cor. 1 can be used to extend Nielsen's theorem (see (1) on p. 96) from
to whenever has
is indecomposable, and has all roots of equal length (W. Wardlaw, Thesis, U.C.L.A. 1966).
It would be nice if the form could be used to handle
itself since Nielsen's proof is quite involved. The case of unequal root lengths is at present in poor shape. In analogy with the fact that in the earlier
development is a simple compact group if is indecomposable, we have here:
Every normal subgroup of is finite or of finite index if
is indecomposable and has The proof isn't easy.
Exercise: Prove that
is finite, and is trivial if is indecomposable and not of type
or
Returning to the general set up, if is a prime in we write
for the
norm defined by and
if with and prime to
Theorem 19: (Approximation theorem): Let and be as above, a principal ideal domain and its quotient
field, a finite set of inequivalent primes in and for each
Then for any there exists such that
for all and
for all primes
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Proof. |
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We may assume every To see this write
as above.
By choosing and and so that
and replacing
by we may assume
If we then multiply by a sufficiently high power of the product of the
elements of we achieve for
all If we now choose so that
then
so that
and finally
we achieve the requirements of the theorem.
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Now given a matrix over
we define
The following properties are easily verified.
(1) |
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(2) |
|xy|p≤
|x|p|y|p.
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(3) |
If |xi|p=|yi|p
for i=1,2,…,n then
|∏xi-∏yi|p≤
maxi
|y1|p…
|yˆi|p…
|yn|p
|xi-yi|p.
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Theorem 20: (Approximation theorem for split groups): Let θ,k,S,ε
be as in Theorem 19, G a Chevalley group over k, and
xp∈G for each p∈S.
Then there exists x∈G so that
|x-xp|p<ε
for all p∈S and |x|q≤1
for all q∉S.
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Proof. |
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Assume first that all xp are contained in some 𝔛α,xp=xα(tp)
with tp∈k. If x=xα(t),t∈k,
then |x|q≤max |t|q,1
because xα(t) is an integral polynomial in t
and similarly |xxp-1-1|p≤|t-tp|p, so that
|x-xp|p≤
|xp|p
|t-tp|p
by (1) and (2) above. Thus our result follows from Theorem 19 in this case. In the general case we choose a sequence of roots
α1,α2,… so that
xp=xp1xp2…
with xpi∈𝔛αi
for all p∈S. By the first case there exists
xi∈𝔛αi so that
|xi-xpi|p
<|xpi|p
andε|xpi|p
/|xp1|p
|xp2|p…
if p∈S and |xi|q≤1
if q∉S. We set x=x1x2….
Then the conclusion of the theorem holds by (3) above.
□
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With Theorem 20 available we can now prove:
Theorem 21: (Elementary divisor theorem): Assume θ,k,G,K=Gθ
are as before. Let A+ be the subset of H defined by:
αˆ(h)∈θ for all positive roots
αˆ.
(a) |
G=KA+K (Cartan decomposition).
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(b) |
The A+ component in (a) is uniquely determined
mod H∩K, i.e. mod units (see Lemma 49); in other words, the set
of numbers {μˆ(h) | μˆ
weight of the representation defining G} is.
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Example: The classical case occurs when G=SLn(k),
K=SLn(θ),
and A+ consists of the diagonal elements
diag(a1,a2,…,an)
such that ai is a multiple of ai+1 for
i=1,2,….
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Proof of theorem. |
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First we reduce the theorem to the local case, in which θ has a single prime, modulo units. Assume the result ture in this case. Assume
x∈G. Let S be the finite set of primes at which x
fails to be integral. For p∈S, we write θp for
the local ring at p in θ, and define Kp and
Ap+ in terms of θp as K and
A+ are defined for θ. By the local case of the theorem we may
write x=cpapcp′
with cp,cp′∈Kp
and a∈Ap+, for all
p∈S. Since we may choose ap so that
μˆ(ap) is always a power of
p and then replace all ap by their product, adjusting the
c's accordingly, we may assume that ap is independent of
p, is in A+, and is integral outside of
S. We have cpacp′x-1=1
with a=ap for p∈S.
By Theorem 20 there exist c,c′∈G so that
|c-cp|p<|cp|p
for p∈S and |c|q≤1
for q∉S, the same equations hold for c′
and cp′, and
|cac′x-1-1|p≤1
for all p∈S. By properties (1), (2), (3) of
| |p, it is now easily verified that
|c|p≤1,
|cp′|≤1 and
|cac′x-1-1|p≤1,
whether p is in S or not. Thus c∈K,c′∈K
and cac′x-1∈K,
so that x∈KA+K as required. The uniqueness in Theorem 21 clearly also
follows from that in the local case.
□
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We now consider the local case, p being the unique prime in θ. The proof to follow
is quite close to that of Theorem 17. Let A be the subgroup of all h∈H such that all
μˆ(h) are powers of p,
and redefine A+, casting out units, so that in addition all
αˆ(h)
(αˆ>0) are nonnegative powers of
p.
Lemma 50: For each a∈A there exists a unique
H∈ℋℤ, the
ℤ-module generated by the elements Hα
of the Lie algebra ℒ, such that
μˆ(a)=pμ(H)
for all weights μ.
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Proof. |
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Write a=∏hα(cαpnα)
with cα∈θ*,nα∈ℤ.
Then μˆ(a)=∏(cαpnα)μ(Hα).
Since μˆ(a) is a power of p the
cα, being units, may be omitted, so that
μˆ(a)=Pμ(H) with
H=ΣnαHα.
If H′ is a second possibility for H, then
μ(H′)=μ(H)
for all μ, so that H′=H.
□
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If a and H are as above, we write
H=logpa,a=pH,
and introduce a norm: |a|=|H|,
the Killing norm. This norm is invariant under the Weyl group. Now assume x∈G. We want to show
x∈KA+K. From the definitions if
T=H∩K then H=AT. Thus by
Theorem 18 there exists y=ua∈KxK with
u∈U,a∈A. There is only a finite number of
possibilities for a: if a=pH,
then {μ(H) | μ a
weight in the given representation} is bounded below (by -n if
n is chosen so that the matrix of pnx is integral, because
{pμ(H)} are the
diagonal entries of y), and also above since the sum of the weights is
0, so that H is confined to a bounded region of the lattice
ℋℤ. We choose y=ua
above so as to maximize |a|. If
u=∏uα
(uα∈𝔛α),
we set supp u={α | uα≠1}
and then minimize supp u subject to a lexicographic ordering of the supports based on an ordering
of the roots consistent with addition (thus supp u<supp u′
means that the first α in one but not in the other lies in the second). We claim
u=1. Suppose not. We claim (*)
uα∉K and a-1uαa∉K
for α∈supp u. If uα
were not in K, we could move it to the extreme left in the expression for y and then remove it. The
new terms introduced by this shift would, by the relations (B), correspond to roots higher than α, so that
supp u would be diminished, a contradiction. Similarly a shift to the right yields the second part of
(*). Now as in the proof of Theorem 17 we may conjugate y by a
product of wβ(1)'s (all in
K) to get uα≠1 for some simple
α, as well as (*). We write
a=pH, choose c so that
H′=H-cHα is
orthogonal to Hα, set aα=pcHα,
a′=pH′,
a=aαa′. We only know that
2c=〈H,Hα〉∈ℤ,
so that this may involve an adjunction of p1/2 which must eventually be removed. If we
bear this in mind, then after reducing (*) to the rank 1 case, exactly as in the proof of Theorem 17,
what remains to be proved is this:
Lemma 51: Assume
y=ua=
[1t01]
[pcp-c]
with 2c∈ℤ, t∈k,
t∉θ and tp-2c∉θ.
Then c can be increased by an integer by multiplications by elements of K.
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Proof. |
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Let t=ep-n with
e∈θ*. Then
n∈ℤ, n>0 and
n+2c>0 by the assumptions, so that
c+n>c,
c+n>-c and
|c+n|>|c|.
If we multiply y on the left by
[pne-e-10],
on the right by [10-e-1pn+2c1],
both in K, we get [pc+n00p-c-n],
which proves the lemma, hence that u=1. Thus
y=a∈A, so that x∈KAK.
Thus G=KAK. Finally every element of A is conjugate to an
element of A under the Weyl group, which is fully represented in K (every
wα(1)∈K). Thus
G=KA+K. It remains to prove the uniqueness of the
A+ component. If G′ is the universal group of the same type
as G and π is the natural homomorphism, it follows from Lemma 49(d) and Theorem 19, Cor. 2 that
πK′=K and from Lemma 49 that π maps
A′+ isomorphically onto
A+. Thus we may assume that G is universal. Then
G is a direct product of its indecomposable factors so that we may also assume that G is indecomposable. Let
λi be the ith fundamental weight,
Vi an ℒ-module with λi
as highest weight, Gi the corresponding Chevalley group,
πi:G→Gi the corresponding homomorphism,
and μi the corresponding lowest weight. Assume now that
x=cac′∈G, with
c,c′∈K and
a∈A+. Set
μˆi(a)=p-ni.
Each weight on Vi is μi increased by a sum of positive roots, Thus
ni is the smallest integer such that pniπia
is integral, i.e. such that pniπix is since
πic and πic′
are integral, thus is uniquely determined by x. Since {μi}
is a basis of the lattice of weights (μi=w0λi),
this yields the uniqueness in the local case and completes the proof of Theorem 21.
□
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Corollary 1: If θ is not a field, the group K is maximal in its commensurability class.
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Proof. |
|
Assume K′ is a subgroup of G containing K properly. By
the theorem there exists a∈A+∩K′,
a∉K. Some entry of the diagonal matrix a is nonintegral so that by
unique factorization |K′/K| is infinite.
□
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Remark: The case θ=ℤ is of some importance here.
Corollary 2: If θ=ℤp and
k=ℚp (p-adic integers and
numbers) and the p-adic topology is used, then K is a maximal compact subgroup of
G.
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Proof. |
|
We will use the fact that ℤp is compact. (The proof is a good exercise.) We may assume that G
is universal. Let k‾ be the algebraic closure of k and
G‾ the corresponding Chevalley group. Then
G=G‾∩SL(V,k)
(Theorem 7, Cor. 3), so that K=G‾∩SL(V,θ).
Since θ is compact, so is End(V,θ),
hence also is K, the set of solutions of a system of polynomial equations since G‾
is an algebraic group, by Theorem 6. If K′ is a subgroup of G containing
K properly, there exists a∈A+∩K′,
a∉K, by the theorem. Then
{|an|p | n∈ℤ]}
is not bounded so that K′ is not compact.
□
|
Remark: We observe that in this case the decompositions G=BK and
G=KA+K are relative to a maximal compact subgroup just
as in Theorems 16 and 17. Also in this case the closure formula of Theorem 16, Cor. 7 holds.
Exercise (optional): Assume that G is a Chevalley group over ℂ,ℝ or
ℚp and that K is the corresponding maximal compact subgroup discussed above. Prove the
commutativity under convolution of the algebra of functions on G which are complex-valued, continuous, with compact support, and invariant
under left and right multiplications by elements of K. (Such functions are sometimes called zonal functions and
are of importance in the harmonic analysis of G.) Hint: prove that there exists an antiautomorphism
φ of G such that
φxα(t)=x-α(t)
for all α and t, that φ preserves every double coset relative to
K, and that φ preserves Haar measure. A much harder exercise is to determine the exact structure
of the algebra.
Next we consider a double coset decomposition of K=Gθ itself in the local case. We will use the
following result, the first step in the proof of Theorem 7.
Lemma 52: Let ℒ be the Lie algebra of G (the original Lie algebra of §1 with its coefficients
transferred to k), N the number of positive roots, and
{Y1,Y2,…,Yr}
a basis of ⋀Nℒ made up of products of
Xα's and Hi's with
Y1=⋀α>0Xα.
For x∈G write xY1=Σcj(x)Yj.
Then x∈U-HU if and only if
c1(x)≠0.
Theorem 22: Assume that θ is a local principal ideal domain, that p is its unique prime, and that
k and G are as before.
(a) |
BI=Up-HθUθ
is a subgroup of Gθ.
|
(b) |
Gθ=⋃w∈WBIwBI
(disjoint), if the representatives for W in G are chosen in Gθ
|
(c) |
BIwBI=
BIwUw,θ
with the last component of the right uniquely determined mod Uw,p.
|
|
|
Proof. |
|
Let θ‾ denote the residue class field
θ/pθ, Gθ‾
the Chevalley group of the same type as G over θ‾,
and Bθ‾,Hθ‾,…
the usual subgroups. By Theorem 18, Cor. 3 reduction mod p yields a homomorphism π of
Gθ onto Gθ‾.
(1) π-1(Uθ‾-Hθ‾Uθ‾)⊆U-HU.
We consider G acting on ⋀Nℒ as in Lemma 52. As is easily seen
Gθ acts integrally relative to the basis of Y's. Now assume
πx∈Uθ‾-Hθ‾Uθ‾.
Then c1(πx)≠0 by the lemma applied
to Gθ‾, whence
c1(x)≠0 and
x∈U-HU again by the lemma.
(2) Corollary: ker π⊆U-HU.
(3) BI=π-1Bθ‾.
Assume x∈π-1Bθ‾.
Then x∈U-BU by (1). From this and
x∈Gθ it follows as in the proof of Theorem 7(b) that
x∈Uθ-HθUθ,
and then that x∈BI.
(4) Completion of proof: By (3) we have (a). To get (b) we simply apply π-1 to the
decomposition in Gθ‾ relative to
Bθ‾. We need only remark that a choice
as indicated is always possible since each wα(1)∈Gθ.
From (b) the equation in (c) easily follows. (Check this.) Assume
b1wu1=
b2wu2
with bi∈BI,
ui∈Uw,θ. Then
b1-1b2=
wu1u2-1w-1
∈BI∩Uθ-=
Up-,
whence u1u2-1∈Uw,p
and (c) follows.
□
|
Remark: The subgroup BI above is called an Iwahori subgroup. It was introduced in an interesting paper by
Iwahori and Matsumoto (Pabl. Math. I.H.E.S. No. 25 (1965)). There a decomposition which combines those of Theorems 21(a) and 22(b) can be found. The present
development is completely different from theirs.
There is an interesting connection between the decomposition
Gθ=⋃BIwBI
above and the one, Gθ=⋃(BwB)θ,
that Gθ inherits as a subgroup of G, namely:
Corollary: Assume w∈W, that S(w)
is as in Theorem 16, Cor. 9, and that π:Gθ→Gθ‾
is, as above, the natural projection. Then π(BIwBI)=Bθ‾wBθ‾,
and π(BwB)θ=⋃w′∈S(w)Bθ‾w′Bθ‾.
Hence if θ‾ is a topological field, e.g.
ℂ,ℝ or ℚp, then
π(BwB)θ is the topological closure of
π(BIwBI).
|
|
Proof. |
|
The first equation follows from π-1Bθ‾=BI,
proved above. Write w=wαwβ… as in
Lemma 25, Cor. Then (*)
(BwB)θ=
(BwαB)θ
(BwβB)θ…
by Theorem 18, Cor. 1. Now (BwαB)θ⊇x-α(p)
and wα(1) and is a union of Bθ
double cosets. Thus
π(BwαB)θ⊇
Bθ‾∪
Bθ‾wα
Bθ‾
=BθGα,θ‾.
The reverse inequality also holds since
(BwαB)θ⊆
BθGα,θ
by Theorem 18, Cor. 1. From this, (*), the definition of
S(w), and Lemma 25, the required expression for
π(BwB)θ now follows.
□
|
Appendix. Our purpose is to prove Theorem 23 below which gives the closure of BwB under very general
conditions. We will write w′≤w if
w′∈S(w) with
S(w) as in Theorem 16, Cor. 9, i.e. if w′
is a subexpression (i.e. the product of a subsequence) of some minimal expression of w as a product of simple reflections.
Lemma 53: The following are true.
(a) |
If w′ is a subexpression of some minimal expression for
w, it is a subexpression of all of them.
|
(b) |
In (a) the subexpressions for w′ can all be taken to be minimal.
|
(c) |
The relation ≤ is transitive.
|
(d) |
If w∈W and α is a simple root such that
wα>0 (resp.
w-1α>0), then
wwα>w (resp.
wαw>w).
|
(e) |
w0≥w for all w∈W.
|
|
|
Proof. |
|
(a) This was proved in Theorem 16, Cor. 7 and 9 in a rather roundabout way. It is a direct consequence of the following fact, which will be proved in a later
section: the equality of two minimal expressions for w (as a product of simple reflections) is a consequence of the relations
w1w2…=w2w1…
(w1,w2 distinct simple reflections,
n terms on each side, n= order w1w2).
(b) If w′=w1w2…wr
is an expression as in (b) and it is not minimal, then two of the terms on the right can be cancelled by Appendix II 21.
(c) By (a) and (b).
(d) If wα>0 and
w1w2…ws is a minimal
expression for w, then w1…wswα
is one for wwα by Appendix II 19, so that
wwα>w, and similarly for the other case.
(e) This is proved in Lemma 46.
□
|
Now we come to our main result.
Theorem 23: Let G be a Chevalley group. Assume that k is a nondiscrete topological field and that the
topology inherited by G as a matric group over k is used. Then the following conditions on
w,w′ are equivalent.
(a) |
Bw′B⊆
BwB‾.
|
(b) |
w′≤w.
|
|
|
Proof. |
|
Let Y1 be as in Lemma 52 and more generally
Yw=⋀α>0Xwα
for w∈W. For
x∈G let cw(x)
denote the
coefficient of Yw in
xY1. We will show that (a) and (b) are equivalent to:
(c) |
cw′ is not identically 0 on
BwB.
|
(a) ⇒ (c). We have xβ(t)Xα=Xα+ΣtjXj
with Xj of weight (0 or a root)
α+jβ, and
nwXα=cXwα
(c≠0) if nw represents w in
W in N/H. Thus (*)
BwBY1⊆k*Yw+
higher terms in the ordering given by sums of positive roots. Thus cw′ is
not identically 0 on Bw′B, hence also not on
BwB, by (a).
(c) ⇒ (b). We use downward induction on N(w′).
If this is maximal then w′=w0,
the element of W making all positive roots negative, and then w=w0 by (c) and
(*) above. Assume w′≠w0.
Choose α simple so that w′-1>0,
hence N(wαw′)>N(w′).
Since cw′(BwB)≠0
and BwαbwB⊆BwB∪BwαwB,
we see that cwαw′(BwB)≠0
or cwαw′(BwαwB)≠0,
so that wαw′≤w or
wαw′≤wαw.
In the first case w′<w by Lemma 53(c) and (d). In the second case if
w-1α<0 then
wαw<w by Lemma 53(d) which puts us back in the first case, while if not
we may choose a minimal expression for w starting with wα and conclude that
w′≤w.
(b) ⇒ (a). By the definitions and the usual calculus of double cosets, this is equivalent to: if α is simple,
then BwαB‾=B∪BwαB.
The left side is contained in the right, an algebraic group, hence a closed subset of G. Since
BwαB contains 𝔛α-1
and the topology on k is not discrete, its closure contains 1, hence also
B, proving the reverse inequality and completing the proof of the theorem.
□
|
Remark: In case k above is ℂ,ℝ or
ℚp, the theorem reduces to results obtained earlier. In case k
is infinite and the Zariski topology on k and G are used it becomes a result of Chevalley (unpublished).
Our proof is quite different from his.
Exercise: (a) If w∈W and α is a positive root such that
wα>0, prove that
wwα>w (compare this with Lemma 53(d)), and conversely if
w′≤w then (*)
there exists a sequence of positive roots α1,α2,…,αr
such that if wi=wαi then
w′w1…wi-1αi>0
for all i and
w′w1…wr=w.
Thus w′≤w and (*) are equivalent.
(b) It seems to us likely that w′≤w is also equivalent to: there exists a
permutation π of the positive roots such that w′πα-wα
is a sum of positive roots for every α>0; or even to:
Σα>0(w′α-wα)
is a sum of positive roots.
Notes and References
This is a typed excerpt of Lectures on Chevalley groups by Robert Steinberg, Yale University, 1967. Notes prepared by John Faulkner and Robert Wilson.
This work was partially supported by Contract ARO-D-336-8230-31-43033.
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