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Publié par | profil-zyak-2012 |
Nombre de lectures | 24 |
Langue | English |
Extrait
RepresentationsoflinearLie
groupsandapplications
SalemBenSaid
(i)
ThiscourserequiresknowledgeofthecoursesdonebyJ.-P.AnkerandL.
Kamoun.
(ii)Theselecturenotesarewritteninenglishforthestudentstogetuseto
thislanguage.
AsonecanseefromtheworksofEulerandGauss,mathematiciansofprevious
centuriesdidnotonlypublishgeneraltheoremsbutalsoexampleswhichthey
mightenjoyedandwhichmighthaveeducatedthereader.
FromSiegel[
GesammelteAbhandlungenIV,1979]
PContents
art1.BasicRepresentationTheoryofLinearL
1.LiealgebrasoflinearLiegroups:Abriefreview
2.RepresentationsoflinearLiegroups
3.AnapplicationofNelson’stheorem
Part2.Representationsof
SL
(2
,
R
)
,
andBeyond
4.Basicpropertiesof
SL
(2
,
R
)and
SU
(1
,
1)
5.Finitedimensionalrepresentationof
SL
(2
,
R
)
6.Anapplication:Thewaveequation
7.Thediscreteseriesrepresentationof
SL
(2
,
R
)
Bibliography
3
eirGuosp561022
920383248475
cisaB
traP
1
Representation
Linear
eiL
Theory
Groups
fo
1.LiealgebrasoflinearLiegroups:Abriefreview
Definition
1.1
.
Atopologicalgroup
G
isasetthatissimultaneouslya
groupandatopologicalspacesuchthat:
(i)
themap
(
a,b
)
!"
ab
from
G
#
G
to
G
iscontinuous,
(ii)
themap
a
!"
a
!
1
from
G
to
G
iscontinuous.
Let
H
beasubgroupof
G.
Byinheritingthetopologyfrom
G,H
is
atopologicalgroup.If
H
isanopenin
G,
then
H
isclosed.Indeed,the
complementof
H
in
G
istheunionoftheorbits
xH,
with
x
$
G
and
x
%$
H.
Thisisduetothefactthatif
x
%$
H
then
xH
&
H
=
!
and,therefore
G
\
H
=
'
x
"#
H
xH.
Ontheotherhand,thetranslation
L
x
:
y
!"
xy
is
continuousfrom
G
to
G.
Thus
xH
isanopensetin
G,
andtheunionof
opensetsisanopen.Hencethecomplementof
H
in
G
isanopenand,
therefore
H
isaclosedsubgroupin
G.
On
M
(
n,
R
)weconsiderthenorm
(
A
(
:=
)
A,A
*
,
)
A,B
*
:=tr(
t
AB
)
.
!Clearlywehave
(
AB
(+(
A
((
B
(
.
Thegroup
GL
(
n,
R
):=
A
$
M
(
n,
R
)
|
det(
A
)
%
=0
,
#"inheritingthetopologyfrom
M
(
n,
R
)
,
isatopologicalgroup.Moreover,
GL
(
n,
R
)isanopensetin
M
(
n,
R
)asdet:
M
(
n,
R
)
,"
R
isacontinuous
mapand
GL
(
n,
R
)isthecomplementoftheinverseimageoftheclosed
subset
{
0
}
-
R
.
Definition
1.2
.
Agroup
G
iscalledalinearLiegroupifthereisan
n
$
N
suchthat
G
isisomorphismtoaclosedsubgroupof
GL
(
n,
R
)
.
Example
1.3
.
(i)
Themap
det:
GL
(
n,
R
)
,"
R
$
iscontinuous.
Itskernel
SL
(
n,
R
)=
g
$
GL
(
n,
R
)
|
det(
g
)=1
#"isaclosedsubgroupof
GL
(
n,
R
)
.
Itisanoncompactgroupunless
0Inn
=1
.
$%
(ii)
Let
J
=
,
I
n
0
.
Thesymplecticgroupisbydefinition
Sp
(2
n,
R
)=
g
$
GL
(2
n,
R
)
|
t
gJg
=
J.
#"Inparticular
Sp
(2
,
R
)=
SL
(2
,
R
)
.
Wecanshowthatforevery
g
$
Sp
(2
n,
R
)
wehave
det(
g
)=1
.
Thus
Sp
(2
n,
R
)
isaclosed
subgroupof
GL
(2
n,
R
)
.
(iii)
Weset
GL
(
n,
C
)“=”
g
$
GL
(2
n,
R
)
|
g
!
1
Jg
=
J.
#"
1.LIEALGEBRASOFLINEARLIEGROUPS:ABRIEFREVIEW7
Itistheinverseimageoftheclosedsubset
{
0
}
-
M
(2
n,
R
)
bythe
continuousmap
g
!"
g
!
1
Jg
,
J
from
GL
(2
n,
R
)
to
M
(2
n,
R
)
.
Thus
GL
(
n,
C
)
isaclosedsubgroupof
GL
(2
n,
R
)
.
(iv)
Theorthogonalgroup
O
(
n
)
isthesetof
g
$
GL
(
n,
R
)
suchthat
t
gg
=
g
t
g
=
I
n
.
Thatis
|
det(
g
)
|
=1
forall
g
$
O
(
n
)
.
Thisisan
exampleofacompactlineargroupsince
(
g
(
=
(
g
!
1
(
=1
.
The
subgroup
SO
(
n
)
of
O
(
n
)
definedby
SO
(
n
)=
SL
(
n,
R
)
&
O
(
n
)
isa
closedsubgroupof
GL
(
n,
R
)
.
Let
K
be
R
or
C
.
A
K
-vectorspace
g
providedwitha
K
-bilinearmap
g
#
g
,"
g
,
(
x,y
)
!"
[
x,y
]
iscalledaLiealgebraif
[
x,y
]=
,
[
y,x
]
,
.
x,y
$
g
,
andtheJacobiidentity
[
x,
[
y,z
]]+[
y,
[
z,x
]]+[
z,
[
x,y
]]=0
holds.Thedimensionof
g
asa
K
-vectorspaceiscalledthedimensionof
theLiealgebra
g
.
HenceforthwearemainlyinterestedinLiealgebrasover
=K.RExample
1.4
.
(i)
Everyassociativealgebra
g
isaLiealgebrawith
[
x,y
]:=
xy
,
yx
forall
x,y
$
g
.
(ii)
Theset
M
(
n,
K
)
isanassociativenon-commutativealgebrawith
composition
XY
oftwoelements
X,Y
$
M
(
n,
K
)
givenbyma-
trixmultiplication.Inviewof(i),
M
(
n,
K
)
isaLiealgebrawith
[
X,Y
]:=
XY
,
YX,
for
X,Y
$
M
(
n,
K
)
.
Henceforthwewillwrite
gl
(
n,
K
)=
M
(
n,
K
)
.
(iii)
If
g
isaLiealgebra,and
g
0
isasubspacewith
[
x,y
]
$
g
0
forall
x,y
$
g
0
,
then
g
0
isaLiealgebra.Forinstance
sl
(
n,
R
):=
X
$
gl
(
n,
R
)
|
tr
X
=0
,
#"so
(
n
):=
X
$
gl
(
n,
R
)
|
X
=
,
t
X
#"areexamplesofsuchLiealgebras.ThedimensionoftheLiealgebra
sl
(
n,
R
)
is
n
2
,
1
,
andthedimensionof
so
(
n
)
is
n
(
n
,
1)
/
2
.
Exercise
1.5
.
Verifythatthematrices
$
01
%$
00
%$
10
%
E
=00
,F
=10
,H
=0
,
1
formabasisof
sl
(2
,
R
)
withtherelations
[
E,H
]=
,
2
E,
[
F,H
]=2
F,
[
E,F
]=
H.
8Exercise
1.6
.
Showthat
g
=
R
3
providedwiththecomposition
((
p,q,r
)
,
(
p
%
,q
%
,r
%
))
!"
(0
,
0
,
(
pq
%
,
p
%
q
))
isaLiealgebra,calledtheHeisenbergalgebra
heis
(
R
)
,
withbasis
P
=(1
,
0
,
0)
,Q
=(0
,
1
,
0)
,R
=(0
,
0
,
1)
andrelations
[
P,R
]=[
Q,R
]=0
,
[
P,Q
]=
R.
Definition
1.7
.
Let
G
bealinearLiegroupcontainedin
GL
(
n,
R
)
.
Henceforthwewillwrite
g
=Lie(
G
):=
X
$
gl
(
n,
R
)
|
exp(
tX
)
$
G
forall
t
$
R
.
#"Theorem
1.8
.
Theset
g
providedwiththebilinearmap
[
∙
,
∙
]:
g
#
g
,"
g
,
(
X,Y
)
!"
[
X,Y
]=
XY
,
YX
isarealLiealgebra,calledtheLiealgebraof
G.
Remark
1.9
.
(i)
Clearlywehave
Lie(
GL
(
n,
R
))=
M
(
n,
R
)
,