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重複度1の表現と複素多様体上の
可視的な作用
Multip
licity-fre
ere
pre
sentatio
ns
and
visib
leactio
ns
on
com
ple
xm
anifold
s
ToshiyukiK
obayashi
(RIM
S,K
yoto
Univers
ity)
Sym
posiu
mon
Repre
sentatio
nT
heory
2005
Kakegawa,Shizuoka
Novem
ber15
-18,2005
アブストラクト:
正則ベクトル束に群作用が与えられているときに、そ
の正則な大域切断の空間に実現された表現を考える。 この
表現は、
1)底空間が非コンパクトならば一般に無限次元表現
であり、
2)底空間における群軌道が無限個ならば一般に既約
とは程遠い表現になる。
しかし、ある種の幾何的な条件(『可視的な作用』)が
満たされていれば、重複度が1という性質がファイバーへの
表現⇒大域切断における表現に伝播することが証明できる。
この概説講演では、有限次元および無限次元の表現に
おける重複度1の表現のたくさんの例を紹介し、それが『可
視的な作用』という幾何的な条件からどのように理解できる
かを説明する予定です。
[1]
Mult
iplici
ty-fre
eth
eore
min
bra
nch
ing
pro
ble
ms
ofuni-
tary
hig
hes
tw
eigh
tm
odule
s,P
roc.
Sym
pos
ium
onR
ep-
rese
nta
tion
Theo
ryhel
dat
Sag
a,K
yush
u19
97(e
d.
K.
Mim
achi)
,(1
997)
,9–
17.
[2]
Mult
iplici
tyon
eth
eore
min
the
orbit
met
hod
(wit
hN
as-
rin),
inm
emor
yof
Kar
pel
evic
“Lie
Gro
upsan
dSym
met
-
ric
Spac
es”,
(eds.
S.
Gin
dik
in),
161–
169
Am
er.
Mat
h.
Soc.
2003
[3]
Geo
met
ryof
Mult
iplici
ty-fre
ere
pre
senta
tion
sof
GL(n
),
vis
ible
acti
ons
onflag
vari
etie
s,an
dtr
iunity,
Act
aA
ppl.
Mat
h.81
(200
4),12
9-14
6.
[4]
Mult
iplici
ty-fre
ere
pre
senta
tion
san
dvis
ible
acti
ons
on
com
ple
xm
anifol
ds,
Publ.
RIM
S41
(200
5),49
7-54
9.
表現論シンポジウム講演集,2005pp.33-63
- 33 -
Eg.1
(Eig
enspace
decom
positio
n)
H:Vectorsp./
C,d
im<
∞
A∈
End
C(H
)1©
s.t.
Ais
dia
gonalizable
,
all
eig
envalu
es
are
distin
ct.
⇒H
=Ce 1
⊕···⊕
Ce n
≃C
n(canonical)
1©&
detA
6=0
⇓
πA
:Z
−→
GL
C(H
)is
MF
∈
∈
n7−→
An
Eg.2
(Fourierseries
expansio
n)
L2(S
1)
≃∑
⊕
n∈
Z
Cei
nx
f(x
)=
∑
n∈
Z
an
einx
Tra
nsla
tio
n(⇒
rep.ofthe
gro
up
S1 )
f(·)
7→f(·−
c)(c
∈S
1≃
R/2π
Z)
S1
yL
2(S
1)
isM
F
- 34 -
π:
G→
GL
C(H
)
gro
up
Def.
(naive)
(π,H
)is
MF
multip
licity-fre
e
ifdim
Hom
G(τ,π)
≦1
(∀τ
:irre
d.re
p.of
G).
L2(S
1)
∃1
←ein
x
⇔dim
Hom
S1(τ,L
2(S
1))
=1
(∀τ
:irre
d.re
p.
of
S1)
⇒S
1y
L2(S
1)
isM
F
- 35 -
Eg.3
(Taylo
rexpansio
n,
Laure
nt
expansio
n)
f(z1,...,zn)=
∑
α=
(α1,...,α
n)a
αzα1
1···z
αn
n
Poin
t(too
obvio
us)
∃1
aα∈
Cfo
reach
α
⇑
dim
Hom
(S
1)n(τ
,O
(0)
≦1
(∀τ
:irre
d.re
p.of
(S1)n
)
i.e.
MF
Eg.4
(Peter-W
eyl)
¡¡
¡
irre
d.re
p.of
G×
G
G:com
pact
(Lie
)gro
up
L2(G)≃
∑⊕
τ∈
G
τ⊠
τ∗
Tra
nsla
tio
n(⇒
rep.of
G×
G)
f(·)
7→f(g
−1
1·g2)
⇒G
×G
yL
2(G
)is
MF
- 36 -
Eg.5
(Sphericalharm
onics)
Hl:=
f∈
C∞
(S
n−1):
∆S
n−1f
=−
l(l+
n−
2)f
L2(S
n−1)≃
∞ ∑
⊕
l=0
Hl
O(n
)irre
d.
⇓
O(n)
yL
2(S
n−1)
isM
F
⊗-p
roduct
rep.
SL
2(C)
πk
y
irre
d.S
k(C2)
(k=
0,1,2,...)
Eg.6
(Cle
bsch-G
ord
an)
πk⊗
πl≃
πk+
l⊕
πk+
l−2⊕
···⊕
π|k−
l|
↑
MF
- 37 -
Notatio
n
Hig
htest
weig
ht
λ=
(λ1
...,λ
n)∈
Zn,λ1≥
λ2≥
···≥
λn
⇓
πG
Ln
λ≡
πλ
:irred.
rep.
of
GL
n(C)
Eg.
λ=
(k,0,...,0)
↔G
Ln(C)
yS
k(C
n)
λ=
(1,...,1
︸︷︷
︸
k
,0,...,0)
↔G
Ln(C)
yΛ
k(C
n)
⊗-p
roduct
rep.
(G
Ln-c
ase)
Eg.7
(Pie
ri’s
law)
π(λ
1,...,λ
n)⊗
π(k,0
,...,0
)
≃⊕
µ1≥
λ1≥···≥
µn≥
λn
∑(µ
i−λ
i)=
k
π(µ1,...,µ
n)
↑
MF
as
aG
Ln-m
odule
.
- 38 -
⊗-p
roduct
rep.
(contin
ued)
Eg.
(countere
xam
ple
)π(2
,1,0
)⊗
π(2
,1,0
)is
NO
TM
F
as
aG
L3(C)-m
odule
.
⊗-p
roduct
rep.
(contin
ued)
λ=
(a,···,a
︸︷︷
︸
p,
b,···,b
︸︷︷
︸
q)∈
Zn,
a≥
b
Eg.
8.
(Stem
bridge
2001,K
–)
πλ
⊗π
νis
MF
as
aG
Ln(C
)-m
odule
if
1)
min
(a−
b,p,q)=
1(a
nd
νis
any),
or 2)
min
(a−
b,p,q)=
2and
νis
ofth
efo
rmν
=(x···x
︸︷︷
︸
n1
y···y
︸︷︷
︸
n2
z···z
︸︷︷
︸
n3
)(x≥
y≥
z),
or 3)
min
(a−
b,p,q)≥
3,
&m
in(x−
y,
y−
z,n1,n2,n3)=
1.
⋆
⋆
- 39 -
Eg.9
(G
Ln↓
GL
n−1)
πG
Ln
(λ1,...,λ
n)| G
Ln−1
≃⊕
λ1≥
µ1≥···≥
µn−1≥
λn
πG
Ln−1
(µ1,...,µ
n−1)
↑M
Fas
aG
Ln−1(C
)-m
odule
⇓Applicatio
n
Gelfand-T
setlin
basis
GL
n
πλ
yV
Fin
da
‘good’basis
of
V
Rem
ark.
Vis
not
necessarily
MF
as
a(C
×)n-m
odule
.
Idea
ofG
elfand-T
setlin
basis
GL
n(C)
∪
y
MF
C×
×G
Ln−1(C)
∪
y
MF
(C×)2
×G
Ln−2(C)
∪
y
MF
. . .. . .
∪
y
MF
(C×)n
- 40 -
Infinite
dim
ensio
nalcase
(cf.
Eg.9
)
Eg.1
0.
(U(p,q)↓
U(p−
1,q))
∀π:
irre
d.
unitary
rep.
of
U(p,q)
with
hig
est
weig
ht
⇒re
strictio
nπ| U
(p−1,q)is
MF
as
aU(p
−1,q)-
module
Eg.1
1.
(G
L−
GL
duality
)
N=
mn
⇒G
Lm
×G
Lny
S(C
N)
≃
S(M
(m
,n;C))
This
rep.
isM
F
•Explicit
form
ula
···
GL
m−
GL
nduality
•G
enera
lizatio
n
(Bra
nchin
glaw
ofholo
.disc.
w.r.t.
sym
metric
pair)
Hua-K
ostant-S
chm
id-K
-¢ ¢ ¢ ¢
£ £ £ £
each
com
ponent···
finite
dim
∞−
dim
- 41 -
Anothergenera
lizatio
n
Eg.1
2(K
ac’s
MF
space)
S(C
N)
isstill
MF
as
aG
Lm−1×
GL
nm
odule
We
recall:
π:
Ggro
up→
GL
C(H
)
Def.
(naive)
(π,H
)is
MF
multip
licity-fre
eif
dim
Hom
G(τ,π)≤
1(∀τ:
irre
d.
rep.
of
G).
- 42 -
Observ
atio
n
n≤
1⇔
End(C
n)
iscom
mutative.
(π,H
):
unitary
rep.of
G
Def.
(π,H
)is
MF
if
End
G(H
)is
com
mutative.
More
genera
lly,(
,W
):top.re
p.
Def.
(,W
)is
MF
ifany
unitary
subre
p.
(π,H
)is
MF
(∃G-inj.
cont.hom
.H
→W
)
Eg.2
(Fourierseries
expansio
n)
L2(S
1)
≃∑
⊕
n∈
Z
Cei
nx
f(x
)=
∑
n∈
Z
an
einx
einx
(cf.
centrifu
galsepara
tor)
Tra
nsla
tio
n(⇒
rep.ofthe
gro
up
S1 )
f(·)
7→f(·−
c)(c
∈S
1≃
R/2π
Z)
S1
yL
2(S
1)
isM
F
- 43 -
Eg.1
3(Fouriertra
nsfo
rm)
L2(R)≃
∫ ⊕ RC
eiζ
xdζ
(direct
integra
lofHilbert
sp.)
f(x
)=∫ R
f(ζ
)eiζ
xdζ
Tra
nsla
tio
n(⇒
regularre
pre
sentatio
non
L2(R))
f(·)
7→f(·−
c)
Ry
L2(R
)is
“M
F”
. . .contin
uous
spectru
m
Sym
metric
Space
Eg.1
4.
G/K
:Rie
mannia
nSym
m.Space
⇒G
yL
2(G
/K
)is
MF.
Eg.1
5.
(counterexam
ple
)
G/H
:Sem
isim
ple
Sym
m.Space
⇒G
yL
2(G
/H
)
isNO
Talw
ays
MF.
- 44 -
Eg.1
4-A.
G/K
=SL(n,R
)/SO
(n)
L2(G
/K
)≃
∫⊕ λ
1≥···≥
λn
Σλ
i=0
Hλ
dλ
cont.spec.
MF
Hλ:∞
-dim
,irred.rep.of
G
Eg.1
5-A.
G/H
=SL(n,R
)/SO
(p,q
)
Multip
licity
ofm
ost
cont.spec.
inL
2(G
/H
)
=n!
p!q
!>
1if
p,q
>0.
⇒NO
TM
F
Vectorbundle
case
Eg.1
5-B.
G/K
=G
L(n
,R)/O
(n)
Vτ:=
G× K
τ→
G/K
τ:
unitary
rep.of
K
⇒G
yL
2(V
τ)
unitary
Gy
L2(V
τ)
isNO
Talw
ays
MF.
but
itis
MF
ifτ≃
Λk(C
n)
(0≤
k≤
n)
- 45 -
Exam
ple
sofM
ultip
licity-F
ree
reps
•Peter-W
eyltheore
m
•Cart
an-H
elg
ason
theore
m
•Bra
nchin
glaws:
GL
n↓
GL
n−1,
On↓
On−1
•Cle
bsch-G
ord
an
form
ula
•Pie
ri’s
law
•G
Lm-G
Ln
duality
•Pla
nchere
lfo
rmula
for
L2(G
/K
)(G
/K
:Rie
mannia
nsy
mm
etr
icsp
aces)
•(G
elfand-G
raev-V
ers
hik
)canonical
repre
sentatio
ns
•Hua-K
ostant-S
chm
idK
-type
form
ula
•(K
ac)
linearm
ultip
licity-fre
espaces
•(Panyushev)
sphericalnilpotent
orb
its
•(Stem
bridge)
multip
licity-fre
etensor
pro
duct
repre
sentatio
ns
of
GL
n
etc.
Accord
ingly,various
techniq
ues
can
be
applied
ineach
MF
case.
Forexam
ple
,one
can
1)
look
foran
open
orb
it
ofa
Bore
lsubgro
up.
2)
apply
Little
wood-R
ichard
son
rule
sand
variants.
3)
use
com
putat’l
com
bin
atorics.
4)
em
plo
ythe
com
mutativity
ofthe
Hecke
alg
ebra
.
5)
apply
Schur-W
eylduality
and
Howe
duality
.
- 46 -
Aim
···
To
give
asim
ple
prin
cip
le
that
expla
ins
the
property
MF
ofall
these
exam
ple
s,
and
more.
§2M
Ftheorem
multip
licity
free
H,K
:Lie
groups
D:
com
ple
xm
fd.
P→
D:
H-equiv.
prin
cip
alK
-b’d
le
µ:
K→
GL
C(V
)
⇓ Settin
g1
H-equiv.
holo
.vectorb’d
le:
V:=
P×
KV
→D
⇓ HyO
(D,V)=
holo
.sectio
ns
- 47 -
Settin
g2
σ1
yP
diff
eo.
σσ2
yK
auto.
σ3
yH
auto.
s.t.
σ(h
pk)=
σ(h)σ
(p)σ(k)
∋∋∋
HP
K
σy
D≃
P/K
anti-holo
morphic
Recall
Hy
Px
K
↓ D
B B B B B B B B B B
BM
∩∩
Hy
Px
Kµ
−→
GL
C(V
)
↓ D
B⊂
Pσ
:=p∈
P:σ(p)=
p
subset
⇓
M≡
MB
:=k∈
K:bk
∈H
b(∀b∈
B)
Assum
ptio
n
(a)
HB
K=
P
(b)
µ| M
isM
F
say,
µ| M
≃l ⊕
i=1
νi
(c)
µ
σ≃
µ∗
as
K-m
odule
s
νi
σ≃
ν∗ i
as
M-m
odule
s(∀i)
- 48 -
Assum
ptio
n
(a)
HB
Kcontain
san
interior
poin
tof
P
(b)
µ| M
isM
F
say,
µ| M
≃l ⊕
i=1
νi
(c)
µ
σ≃
µ∗
as
K-m
odule
s
νi
σ≃
ν∗ i
as
M-m
odule
s(∀i)
Poin
tofAssum
ptio
ns
(a)
···
base
sp.
(b)
···
fiber
(c)
···
oft
en
autom
atic
Theore
m(M
Ftheore
m)
Assum
e∃σ
and
∃B
⊂P
σ
satisfy
ing
(a)∼(c).
⇒H
yO
(D
,V)
isM
F.
- 49 -
Poin
t
•propagatio
nofM
Fproperty
fiberM
F⇒
sectio
ns
MF
•geom
etry
ofbase
space
···
‘visib
leactio
n’
yV
H↓
y
D
⇒H
y
O(D
,V)
Assum
ptio
n(c)
µ
σ≃
µ∗
as
K-m
odule
s
hold
sif
σis
aW
eylin
volu
tio
n
i.e. σ∈
Aut(
K),
σ2
=id
s.t.
σ(g)=
g−1
on
som
em
ax
toru
sof
K
e.g
.K
=U(n),
σ(g)=
g
- 50 -
§3Visib
leactio
n
holo
morp
hic
Hy
Dcom
ple
xm
fd,connected
Def.
The
actio
nis
(stro
ngly)visib
le
∃D
′⊂
Dopen
subset
if∃σ
yD
anti-holo
morp
hic
∃N
⊂D
totally
real
s.t.
σ| N
=id
Nm
eets
every
H-o
rbit
inD
′
σstabilizes
every
H-o
rbit.
Assum
ptio
n(a)
+(stro
ngly)
visib
le
actio
n
Assum
ptio
n(a):
HB
K=
P
forsom
eB
⊂P
σ
⇒N
:=
PσK
/K
meets
every
H-o
rbit
on
D:=
P/K
⇒H
yD
visib
le
(σ:
involu
sio
n⇒
N:
totally
real)
- 51 -
Exam
ple
ofVisib
leactio
ns
T=
a∈
C:|a|=
1
(≃
S1)
Eg.
Ty
C⊃
R
∈
az7→
az
Rm
eets
every
T-orbit
R
⇒T-actio
non
Cis
visib
le.
holo
morphic
Hy
(D
,J)com
ple
xm
fd,connected
Def.
Actio
nis
visib
leif
∃D
′⊂
D,
open
∃N
⊂D
s.t.
totally
real
Nm
eets
every
H-orbit
Jx(T
xN
)⊂
Tx(H
·x)
(x∈
N)
- 52 -
sym
ple
ctic
Hy
(D
,ω)
sym
ple
ctic
mfd
Def.
(G
uille
min
-Stern
berg
,
Huckle
berry-W
urz
bacher)
Actio
nis
coisotro
pic
(orm
ultip
licity-fre
e)
if
Tx(H
·x)⊥
ω⊂
Tx(H
·x)
(x∈
D)
isom
etric
Hy
(D,g)
Rie
mannia
nm
fd
Def.
(Podesta-T
horb
erg
sson)
Actio
nis
polarif
∃N
⊂D
s.t.
Nm
eets
every
H-o
rbit.
TxN
⊥T
x(H
·x)
(x∈
N)
- 53 -
Hy
D
com
pact
com
pact,K
ahle
r
Rie
mannia
n
Polar
Visib
le&
dim
S=
dim
M
Coisotro
pic
(m
ultip
licity-
free)
Com
ple
xSym
ple
ctic
Stro
ngly
Visib
le
Com
ple
x
Exam
ple
sofVisib
leactio
ns
Eg.
Ty
Cis
visib
le.
∪ R⇓
Eg.
Tn
yC
nis
visib
le.
∪ Rn
⇓
Eg.
Tn
yPn−1
Cis
visib
le.
∪
Pn−1
R⇓
Eg.
U(1
)×
U(n−
1)
yB
n
(fu
llflag
variety
)is
visib
le.
Eg.
U(n)
yPn−1
C×
Bn
isvisib
le.
- 54 -
Unders
tandin
gofvisib
leactions
HL
⊃
⊃G ∪ Gσ
:=
Tn
U(1
)×
U(n−
1)
⊃
⊃U(n)
∪O
(n)
Geom
etr
y² ±
¯ °P
n−1R
meets
every
Tn-o
rbit
on
Pn−1C
‖G
σ/G
σ∩
L‖ H
‖G
/L
(visib
leaction)
←→
Gro
up
®
© ªG
=H
GσL
⇒Eg.3
l
Gro
up
®
© ªG
=L
GσH
⇒Eg.9
l
Gro
up
² ±
¯ °(G
×G)=
dia
g(G)(
Gσ×
Gσ)(H
×L)
⇒Eg.7
⇓T
heore
m
Various
kin
ds
ofM
Fre
sult
inclu
din
g
•(Fourierseries)
Ty
L2(T)
Eg.2
•(Taylo
rseries)
Tn
yO
(C
n)
Eg.3
•(G
Ln↓
GL
n−1)Restrictio
nπ| G
Ln−1Eg.9
•(Pie
ri)
π⊗
Sk(C
n)
Eg.7
•(K
ac)
GL
m−1×
GL
ny
S(C
mn)
Eg.1
2
- 55 -
Eg.
G/K
com
pact
sym
m.sp.
⇒G
yG
C/K
Cis
visib
le.
⇓T
heore
m
Eg.1
6.
Gy
L2(G
/K
)is
MF.
Eg.1
7.(vectorbundle
case)
Vk=
U(n)
×O
(n)Λ
k(C
n)→
U(n
)/O
(n)
U(n
)y
L2(V
k)
isM
F.
Eg.
G/K
non-c
om
pact
sym
m.sp.
⇒G
yΩ
⊂cro
wn
GC/K
Cis
visib
le.
⇓T
heore
m
Eg.1
6′ .
Gy
L2(G
/K
)is
MF.
Eg.1
7′ .
(vectorbundle
case)
Vk=
GL(n,R
)×
O(n
)Λk(C
n)→
U(n
)/O
(n)
GL(n
,R)
yL
2(V
k)
isM
F.
- 56 -
G/K
Herm
itia
nsym
m.space
Eg.
G=
SL(2
,R)
K=
SO
(2)
H=
a0
0a−1
:a
>0
G/K
≃z∈
C:|z|<
1
Ky
G/K
visib
leH
yG
/K
visib
le
K-orbits
H-orbits
Theorem
H⊂
G⊃
K
Assum
e
G/K
Herm
itia
nsym
m.sp.
(G
,H)
sym
metric
pair
⇒H
yG
/K
isvisib
le
⇓T
heorem
Eg.18
πλ,π
µ:
hig
hest
wt.m
odule
s
ofscalartype
⇒π
λ⊗
πµ
isM
F
Eg.19
πλ
:hig
hest
wt.m
odule
ofscalartype
(G,H
):
sym
metric
pair
⇒π
λ| H
isM
F
- 57 -
Also,fo
rfinite
dim
ensio
nalcase
⇓T
heore
m
Eg.2
0(O
kada,1998)
gC
=gl(
n,C)
λ=
(a,···,a
︸︷︷
︸
p
,b,···,b
︸︷︷
︸
n−
p
)∈
Zn,a≥
b
πλ| h
Cis
MF
if
hC
=
gl(
k,C)+
gl(
n−
k,C)
(1≤
k≤
n)
o(n,C)
sp(n 2,C)
(n:even)
G:
non-com
pact,sim
ple
Lie
gp.,
G/K
Herm
itia
n
Eg.
SU(p,q
),S
O(n,2
),S
p(n,R
),
SO
∗(2
n),E
6(−14),E
7(−25)
gC
=k C
+p+
+p−
Def.
(π,V
)∈
Gunitary
hig
hest
wt
rep
⇔υ∈
V∞
:dπ(X
)υ
=0
(∀X
∈p+)
6=0
µ KW
rite
π=
πG(µ)
(µ∈
K)
Def.
π:
holo
morphic
discrete
serie
s
⇔Hom
G(π,L
2(G))
6=0
π:
scalartype
⇔dim
µ=
1
- 58 -
Def.
(G,H
)sym
metric
pair,
holo
morphic
type
⇔∃τ∈
Aut(
G),
τ2
=id
s.t.
(G
τ) 0
⊂H
⊂G
τ
τy
G/K
holo
morphic
gC
=k C
+p++
p−
∪∪
∪∪
hC
=gτ C
=kτ C
+pτ ++
pτ −
tτ⊂
kτ
Cartan
∩∩
Cartan
t⊂
k
ν1,.
..,ν
k
maxim
alset
of
strongly
orth.roots
in∆
(p−
τ+
,tτ)
Note:
k=
R-rank
G/H
Def.
(G,H
)sym
metric
pair,
holo
morphic
type
⇔∃τ∈
Aut(
G),
τ2
=id
s.t.
(G
τ) 0
⊂H
⊂G
τ
τy
G/K
holo
morphic
Eg.
τ=
θ,
H=
K
Eg.
G=
Sp(n,R
)(n=
p+
q)
H=
Sp(p,R
)×
Sp(q
,R)
H=
U(p,q
)
H=
U(n)
(=K
)
cf.
H=
GL(n
,R)
not
holo
.type
- 59 -
Theore
m
πG(µ)∈
G:holo
.disc.series,
scalarty
pe
(G
,H
):sym
metric
pair,
holo
morp
hic
type
⇒
πG(µ
)H
≃∑
⊕
a1≥···≥
ak≥0
aj∈
N
πH
µ
∣ ∣ ∣
tτ−
k∑ j=
1a
jνj
H=
K···
Schm
id(1969)
GL(n
,C)
yN
⊂nilpotent
orb
itM
(n,C)
Eg.2
1(Panyushev
1994)
O(N
)is
MF
as
GL(n
,C)-m
odule
⇔N
=N
(2p,1
n−2p)
(0≤
∃p≤
n 2)
N(2
p,1
n−2p)∋
01
00
p
···︷
︸︸
︷0
10
0
0···n−
2p
0
︷
︸︸
︷
- 60 -
Eg.
0≤
2p≤
n
1)
U(n)
yN
(2p,1
n−2p)is
visib
le.
2)(P
anyushev)
O(N
2p,1
n−2p)
isM
F
N(2
p,1
n−2p)∋
01
00
p
···
︷
︸︸
︷0
10
0
0···n−
2p
0
︷︸︸
︷
Sketch
ofpro
of
G=
U(n)
H=
U(p)×
U(q)
(p+
q=
n)
G× H
M(p,q;
C)→
N(2
p,1
n−2p)
Hy
M(p
,q;
C)
visib
le
⇒G
yN
(2p,1
n−2p)
visib
le
More
exam
ple
sofvisib
leactio
ns
Eg.
n1+
n2+
n3
=p+
q=
n
Grp(R
n)
meets
every
orb
itof
U(n
1)×
U(n2)×
U(n3)y
Grp(C
n)
(⇒
visib
leactio
n)
⇔(U(n1)×
U(n2)×
U(n3))×
O(n)×(U(p)×
U(q))
։U(n)
⇔m
in(n
1+
1,n2+
1,n3+
1,p,q)
≦2
- 61 -
⇓T
heorem
MF
property
ofthe
followin
g
•G
Lm
×G
Ln
yS(C
mn)
Eg.1
1
•G
Lm−1×
GL
ny
S(C
mn)
Eg.1
2
•the
Stem
brid
ge
list
of
πλ⊗
πν
Eg.8
•G
Ln↓(G
Lp×
GL
q)
Eg.2
2
•G
Ln↓(G
Ln1×
GL
n2×
GL
n3)
Eg.2
3
•∞
-dim
ensio
nalversio
ns
...
⊗-p
roduct
rep.
(contin
ued)
λ=
(a,···,a
︸︷︷
︸
p,
b,···,b
︸︷︷
︸
q)∈
Zn,
a≥
b
Eg.
8.
(Stem
bridge
2001,K
–)
πλ
⊗π
νis
MF
as
aG
Ln(C
)-m
odule
if
1)
min
(a−
b,p,q)=
1(a
nd
νis
any),
or 2)
min
(a−
b,p,q)=
2and
νis
ofth
efo
rmν
=(x···x
︸︷︷
︸
n1
y···y
︸︷︷
︸
n2
z···z
︸︷︷
︸
n3
)(x≥
y≥
z),
or 3)
min
(a−
b,p,q)≥
3,
&m
in(x−
y,
y−
z,n1,n2,n3)=
1.
⋆
⋆
- 62 -
Eg.2
2.
(G
Ln↓(G
Lp×
GL
q))
n=
p+
q
πG
Ln
(x,...,x
,y,...,y
,z,...,z
)| G
Lp×
GL
qis
MF
(
n1
(
n2
(
n3
ifm
in(p,q)≤
2or
ifm
in(n1,n2,n3,x−
y,y−
z)≤
1(K
ostant
n3
=0;K
rattenthale
r1998)
Eg.2
3.
(G
Ln↓(G
Ln1×
GL
n2×
GL
n3))
n=
n1+
n2+
n3
πG
Ln
(a,...,a
,b,...,b)| G
Ln1×
GL
n2×
GL
n3
isM
F
(
p
(
q
ifm
in(n1,n2,n3)≤
1or
ifm
in(p,q,a−
b)≤
2
Orb
itm
ethod
GAd
yg∗
⊃O
G
∪↓pr
HAd∗
yh∗
⊃O
H
H⊂
G⊃
Ksym
m.
pair
Herm
itia
nsym
m.pair
g⊃
k⊃
center
⇔R·z
⊂g∗
6=
0
Theore
m(Nasrin
&K
-)
IfO
G∩
R·z6=
∅,then
#(O
G∩
pr−
1(O
H))/H
≦1
forany
OH.
- 63 -