The S c i e n c e R e p o r t s o f t h e Kanazawa U n i v e r s i t y , Vo 1 . . n N o . 2 . P P . 1‑6 March 1 9 5
生00 the Remark of La asonen's Theorem
ByTδru AKAZA
く
R e c i v e d J a u n a l ' Y 23 , 1 9 5 4 )
The p u r p o s e o f t h i s p a p e r i s t o i n v e s t i g a t e t h e a b s o l u t e c o n v e r g e n c e ' o f t h e f o l l o w i n g e x p a n s i o n by s e r i e s o f t h e fuchsoid f u n c t i o n g i v e n by L α α 3 o n e n . J )
~ 1 . L e t G n be a Fuchsian g r o u p o f g e n u s z e r o formed by n p a r a b o 1 i c , l i n e a r g e n e ‑ r a t i n g t r a n s f o r m a t i o n s ;
ノ
( 1 ) 7 ' . . : _~
一1 1 r /
守糾ν/ =1 ,
守ν f 記捌 e 伺 a l
ν v ' ‑ 1 τ す 窃 ご 弓 E 忌 ν
一一7τ‑( 芯 γ
一可7j;‑ し1 ν=1 , 2 丸 η f
and y e t be p r o p e r l y d i s c o n t i n u o u s on t h e p r i n c i p a l c i r c l e H ( / z / = 1) e x c e p t t h e s i n g u l a r p o i n t s o f G n . I f we c o n s i d e r t h e poinc α 必 ' st h e t a s e r i e s o f ‑ 2 d i m e n s i o n with r e s p e c t t o G n
(2 )
M 7
2 ω
ぬ リ
i
抗
ti s a b s o l u t e l y c ∞ o n v e r g e r 此 1 詑 tby t h e w e l l Next l e t u s c o n s i d e r t h e f u n c t i o n
ん
(h‑jMZ)dz+Cn=‑Ff z
おyd 山 +C n
α n un α n
(3)
ア
r~一一 -m--L一一ー 1+C n
‑1 ;:,
l . .
sy( z ) 中 n ) . . J ' ~ n
where α n i s any p o i n t i n t h e f u n d a m e n t a l r e g i o n Bn w i t h r e s p e c t t o
Gn. Bn i s s u r
‑ r o u n d e d by t h e b o u n d a r i e s which c o n s i s t o f t h e a r c s o f H and n p a i r s o f c i r c u l a r a r
侶which a r e o r t h o g o n a l t o H and t a n g e n t a t t h e p a r a b o l i c v e r t i c e s . (Fig 1 ) I f we c h o o s e C n s o t h a i t h e Lα u r a n t s e r i
田o f fn( z) may have n o t c o n s t a n t t e r m a t t h e o r i g i n . where fn(z) h a s a s i m p l e p o l e , ( 3 ) becomes
1 ~'r 1 1 , . . . . . (叫 ん ( z )
=一+三; z ~n l.. 8ν (z) I 一一一一一一一一 8 ν(O)J l , where : : : 8 ' d e n o t 田 t h esummation e x t e n d e d o v e r t h e a l l t r a n s ‑ f o r m a t i Gn o n s e x c e p t S o ・ ( 4 ) d e f i n e s a meromorphic f u n c t i o n which 4 a s p o l
,邸a tt h e e q u i v a l e n t p o i n t s t o t h e o r i g i n , i n t h e o u t s i d e o f s i n g u l a r p o i n t s o f G n •
Then fn(Z) h a s t h e f o l l o w i n g p r o p e r t i e s ;
( i ) fn(Z) i s
抑 制t o m o r p h i cfu
ηc t i o n w i t h r e s p e c t t o 1~4 G n , t h a t i s ,
fn ( 7 ' k ( Z ) ) In ( z ) (V
l'k e G
n )F i g . 1
< }
4 一
T AKAZA
( i i ) Since fn(Z)
'Is r
・e g u la r α st h e f u n c t i o n of l o c a l parameter t a t t h c p a r a b o l i c v c r t i c c 8
,i t i s a s i r n p l e α u t o r n o r p h i c f u n c t i o
九,( ii i ) Since fn( z ) h αs o n l y O n C p o l c a t t h e o r i g i n 向 B" ,
ltt a k e s ω ny
刊l u e o n l y o n c e i n Bn α nd t h e r e f o r e i t i s t h e p r i n c i p a l f u n c t i o
。向f On.
( i v )
'1'h e p r i n c i p a l d o r n α 向 。 f f
〆z ) , t h a t i s t h e i r n α g e 0 f B" , c o i n c i d e s t h e O u t ‑ s i d e of α c i r c l e .
( v )
'1'h e 悦 f n ( z ) G
nc O n v e r g c s t o t h e f u n c t i o n of t h e fuchsoid group G which i . s a p p r o x i r n a t e d b y t h e a b o
凶Fuehsi α n8 u b g r o u p s {G
'1'h e pri n c i p α 1 ' d o r n α印
',of 六 z )
ねt f t eo u t s i d e 0 f t h e c i r c l e wi I h ~~aditl8 とO.
号 2 . Under what c o n d i t i o n i s t h e s e r i e s o f fn(Z) a b s o I u t e l y c o n v e r g e n t f o r n
→∞? To
島r e s o l v et h i s problem we need t h e f o l l o w i n g Iemma and ・ b e r g ' s consequences.~)
L e m r n a . B e t t v e e n ( z ) α nd t h e B l a s c h 1 c e
幽Product ( δ
〕H n ( z ) = I J G'!
Jt ,~空手」一~U},i~
t h e r e e x i s i s t h e r e l 窃 t
i‑on
ニ
│ α ν I( i a ν = 8 ν ( 0 ) )
ニ
( α ν l e ‑ i d y
(6) =九十 竺竺「
l I n
i s t h e s i m p l e a u t o m o r p h i c f u n c t i o n which maps Bn o n t o t h e u n i t d r c l e . S i n c e H n ( z ) and fn have t h e same Bn and b e l o n g t o t h e same 0" , t h e r e e x i s t s a r a t i o n a l r e l a t i o n between Hn ( z ) and f ' n And s i n c e t h e b o u n d a r i e s o f t h i s p r i n c i p a l domains a r e c i r c l e s , t h e r e l a t i o n must be 1 i nea
じh e n c ewe o b t a i n q . E . D
倍Now t h e p r i n c i p a l domain
'1'"o n t o which Hn maps Bn 1 S o b t a i n e d by removing n p o i n t s from I H" I く 1 . These p o i n t s i n
'1'n c l u s t e r t o t h e singularεndpoint s e t 3 ) on J 1 ln I = 1 f o r
n→∞, which c o r r e s p o n d s t o t h a t on I z I = 1 , t h a t i s , t h e c l u s t e r s e t o f p a r a b o J i c v e r t i c e s .
Myrberg め p r o v e dt h a t t h e l i n e a r measuτe o f t h i s s i n g u l a r e n d p o i n t . s e t depends upon t h e c u t s which make
'1'n s i m p l y ‑ c o n n e c t e d . and t h e d i s t r i b u t i o n o f t h e s e i n n e r p o i n t s .
I f each o f t h e s e n p o i n t s i n
'1'n i s c o n n e c t e d with t h e p e r i p h e r i e by a s t r a i g h t l i n e a l o n g r a d i u s s o t h a t any two o f them a r e n o t on t h e same r a d i u s , t h e n we o b t a i n a f i g u r e c a l l e d R α d i a l 8tar S" . (Fig 2 )
I f t h e i m a g e s ' o r n c u t s a r e d e f o r ‑ med i n t o c i r c u l a r a r c s o r t h o g o n a l t o H w i t h o u t movIng t h e e n d p o i n t s 00
Fig.2
H, t h e image o f Bn i s c a l l e d Nonnal Radial 8t α
1"S ' " . (Fig
$~
F i
耳切3
1 n Fig 1 d e n o t e by ( b n) t h e s e t o f s i d e s and (sn) t h e s e t o f c i r c u l a r a r c s o f H
complementary t o ( b n ) i n t h e boundary o f Bn. Now we c o n s i d e r t h e harmonic measure
ω n
(z)= ω
(z, (s" , ) , ) which i s hannonic i o Bn , 1 on (sn) and 0 0
ロ(bn) , 5 ) Of
On t h e ' R e r n a r k 0] L α
削o n e n ' 8 7 ' h e o r e r n ‑ 3 ‑ c o u r s e 0< ωn ( z ) 手 1 , and t h e s e q u e n c e { ωn ( z ) } i s monotone d e c r e a s i n g and converges u n i f o r m l y t o a harmonic f u n c t i o n ω ( z , (s) , B) i n . a wide s e n s e by Harna 、 側 theorem ,
where B i s t h e fundamental r e g i o n o f t h e f ' U C h 8 0 i d group G 1 i m Gn ・ The l i m i t
n
→田
f u n c t i o n ω ( z , (s) ,
B)i s c a l l e d t h e harmonic . measure o f t h e fundamental r e g i o n B , and ω
(II)=ω
(II‑l(Z))t h e harmonic measure o f t h e 8t α r s 8=lim 8n and 8 1
=lims ' n .
n~ ∞ n→∞
By u s i n g t h i s harmonic measure , M y r b e r g g o t t h e f o l l o w i n g c o n s e q u e n c e s .
Suppose t h a t 8 i s t h e R α 出 α 1 8t α
ro f t h e i n n e r boundary p o i n t s (α) whose c o o r d i n a t e s a r e r v t i 9 v ( ν=1 , ス ‑ … . .) .
( i ) 0 1] t h e s e r i e s
(7) : L ( 1
一円〕i s ] i n i t e , t h e l w r r n o n i c r n e α s u r e 0] S i 8 p 0 8 i t i v e .
( i i ) O L e t q n b e t h e r n o n o t O n e i n c r e α s i n g f ' u n c t i o n 0] n , t l ! a t i s , q n →∞ ]or n →∞ ι nd ( a ) i s d i s t r i b u t e d s o r e g u l a r α8
、 2 π i k
αnくの=
(1‑+‑) 、
'1n I e
‑‑‑P;;‑(o<k くれ),
ω h e r e Pn i 8 t h e d i s t r i b u t i n g ] u n c t i o n 0] ( α ) . 1]
(8) Urn 手 ι z ∞ ,
n
→∞ '1n
t h e h a r r n o n i c me α 8 u r e 0] α ny Radial 8t α r i 8 z e r o . ( i i i ) O 1]
q 晶
、J
(9) p
h一 く
∞2
4
t h e Radial 8tar 8 h α s p o s i t i v e "
凶r m o n i cm e a s u r e . (9) ] o l l o
例] r o r n ( i ) o , ' t ' f
附p u tr 1 t
= (l‑‑.!ー).
, 令' i n
( i v ) 0 7 ' h e l i n e a r m e a s u r e 0] (
β)i n t h e r α d i a l ]und α r n e n t α r e g i o n i s z e r O u n d e r t h e c o n d i t i o n (8)αη d positi~)6 under t h e c O n d i t i o n ( 9 ) .
! 3
3 . Now l e t
山p r o v et h e a b s o l u t e c o n v e r g e n c e o f t h e s e r i e s ( 4 ) for n →∞, which i s t h e e x p a n s i o n o f ]n ( z ) , by t h e lemma and Myr
和r g ' s c o n s e q u e n c e s ( i ) 。 ー ( i v O).
From t h e p r o p e r t y o f t h e group G , two c a s e s o c c u r with r
邸p e c tt o t h e behav i . o r , of ( 4 ) f o r n →∞.
(A) The c a s e where G has no l i m i t c i r c 1 e , t h a t i s , G i s p r o p e r I y d i s c o n t i n u o u s on E ! .
In
出i sca 田 , s i n c e t h e line~r measure o f ( の i so b v i o u s l y p o s i t i v e , we ob 匂 i n ( 1 0 ) : L (l‑J z v ] ) く ∞
by t h e well‑known B u r n s i d e ' s theorem の , where z ν=8ν(0) ( 8 ν e G ) . I t leads t o
、the
convergence o f t h e B l a s c h k e ‑ P r o d u c t .
IIo l 8 ν ( z ) J a t o n c e . From t h e c o n d i t i o n (10)and
t h e form
‑ 4 ‑ T AK
A.ZA(11)
1 U ) l I 1 ‑ l M Z〕I B
‑a;z‑
一 二 一 I 示子言j) ‑ ‑ ‑ 1 二 戸 , where
α ν Z 斗ん
Sν1 / ¥ (z) . . . ‑ ‑ . / =
一r J ) Z 十九 ( α J ν ‑s νγν= 1 )
i s any t r a n s f o r m a t i o n o f G , we o b t a i n t h e a b s o l u t e c o h v e r ε e n c e o { t h e POinc(
,1r e ' s t h e t a s a r i e s o f ‑2 d i m e n s i o n with
1'e s p e c t t o G 。
T h e r e f o r e
(12)
1 J
1
一 川町
﹁ L
Vω G
ム
ー 一
z wハ
which canbe o b t a i n e d from f n by n →=, i s a l s o a b s o l u t e l y c o n v e
1'gen t . S i n c e t h e c e n t e r b n =
f" (泊) o f t h e boundary c i r c l e o f Dn coav
合 唱 明t o b an
ヨt h el i n
閃rmeasu
問。
ft h e image o f (向 i sa l s o p o s i t i v e , Dn c o n v e r 宮 田 t ot h e p r i n c i p a l domain D o f
f( 吟 p
t h a t i s , t h e o u t s i d e o f t h e bound ョ r y c i r c l e with r a d i u s 1
Ct1> 0 and c e n t e r b . Then by t h e lemma t h e r e l a t i o n
f(z)=b →‑一一生‑
H(z)
記
x i s t sbetween t and H f o r n →∞.
(B) The c a s e where G has l i m i t c i r c l e .
By M y
1'b e r g ' s c o n s e q u e n c e s t h e l i n e a r measure o f t h e s i n g u l 旦 r end コ , o i o t s e t o f t h e Star o f I 1 ( z ) depends upon t h e d i s t r i b u t i o n o f t h e i n f i n t e i n n e r i s o l a t e d boundary p o i n t s ( α ) which a r e images o f t h e p a r a b o l i c v e
1't i c e s .
(lB) I f t h e d i s t r i b u t i o n o f (必) s a t i s f i e s t h e c o n d i t i o n o r
,when t h e d i s t r i b u t i o n i s s o i r r e g u l a
1't h a t t h e s e t ( α ) may not s a i i s f y t h e c o n d i t i o n ( 9 ) , i f i t s a t i s f i e s t h e c o n d i t i o n ( 7 ) , we o b t a i n ( 1 0 ) j u s t t h e same a s ( A ) . ( 1 0 ) i s t h e n e c e s s a r y and s u f f i
国c i e n t c o n d i t i o n f o r t h e e x i s t e n c e o f t h e Green f u n c t i o n on D and a 1 1 3 0 f o r t h e a b s o l u t e c o n v e r g e n c e o f t h e t h e t a s e r i e s o f ‑2 dimension o r G 7 ) . I n t h i s c a s e t h e Green f u n c t i o n w i t h a l o g a r i t h m i c p o l e a t t h e o r i g i n i s g i v e n by ‑ l o g I I G 1 8 ν 。 ) 1 8 )
T h e r e f o r e t h e s e r i e s ( 4 ) i s a b s o l u t e l y c o n v e r g e n t f o r
n ‑.= , and t h e r e l a t i o n ( 1 3 ) e x i s t s between
f(z) and 1 I ( z ) , and t h e p r i n c i p a l domain o f
fc o n s i s t s o E t h e o u t .
s i d e o f t h e c i r c l e with r a d i u s J a 1 > 0 and t h e c e n t e r b .
The c a s e (1s) c o n t a i n s (A) a s t h e s p e c i a l c ョ s e
,Becaus
己(A)may be c o n s i d e r e d a s t h e c a s e where i n ( l t h e o r d e r o f Pn ( 1 ・ o rt h e g
1'ade o f d i s t r i b u t i o n o f ( α ) ( i r r e g u l α i s s o s m a l l i n c o m p a r i s o n with t h a t o f qn 0
1't h a t o f approach o f (ω) t o t h e p e r i p h e r i e t h a t t h e s e r i e s i n ( i ) 0 i s c o n v e g a n t a t O l l c e .
(2B ) When t h e c o n d i t i o 日明) o r : 8 (1‑1' ν)= ∞ f o r i r r e g u l a r d i s t r i b u t i o n i s s a t i s f i e d
w i t h r e s p e c t t o t h e s
色t( α ) , we need t h e f o l l o w i n g c o n s i d e r
而a t i o n
,Though t h e l i n e a r measure o f (s) i s z e r o under (8) by ( i i ) o , t h e c o n d i t i o l l ( 1 0 )
。
。
may C C C u r f o r some group , But t h e s e r i e s
2Jυ‑Izν1) i s d i v e r g e
ロti n t h i s c a s e .
ν~l
0 1 1 t h e Remarlc 0 f L ω
C!. " o n e n ' . g
'1'h c r
!"t1 3 m
一九一B e c a u s e t h e boundary o f D i s e i t h e r t h e l i m i t i n g circl~ with r a d i u s
>0 o f t h e b O U l l d
同ary c i r
cJE s o f {Dn} o r t h e l i m i t i n g p o i n t o f t h e i n f i n i t e i s o l a t e d p o i n t s from the c o n s t i ・
i u i i o n o f Gn , and i f t h e boundaτy i s {orm
:ef , t b e l i n e a r measure o f (s) i s p o s i t i v e and i t i s i n c o n s i s t e n t w i t h t h e f a c t t h a t t h e measure o f (的 i sz e r o . S i
ロc e t h e boundary o f D must be t h e o n l y o n e l i m i t p o i n t and hence t h e G
l'I J e n f u n c t i o n on D c a n n ' t e x i s t , and t h e s e r i e s : : 8 ( 1 ‑ 1 z ν 1 ) i s d i v e r g e n t . T h e r e f o r e t h e p O
'IllCc t r e ' S t h e t a s e r i e s o f ‑2
ν~-l
dime
口s : o nand t h e s e r i e s ( 4 ) c a n n ' t be a b s o l u t e l y c o n v e r g e n t f o r n
→∞,a n d the r e l a t i o n ( 1 3) d o e s n o t e x i s t between
f( の and I I ( z ) .
From t h e a b o v e c o n s e q u e n c e s (A) and (B) , we o b t a i n t h e E o l l o w i n g t h e o r e m .
'1'h e o r c r n . Lct ( ) b c t h e f l l C h s o i d grou p
lUh i c h i B (t
Pp
l'oumated b y t h e Fuehsian s u
かg
l'oup α n c l Pll , q η b t ; t h e function
りl't 1
w8 c t ( a )
=( 1 二 c i O ν) UI , t h e p
l'i n c i p a l dorn α in o n t o
11 (z ) 1 ) l aps B.
'1'
hen ザ t h ec o n c l 似 0
'11(9)
or t h e cona μ i 0 7
ιz : ぞ
Lく ∞
, ~l ヨ n
ヱ
(l‑r ν ) く ∞
ν 一 1
( r c g u l a r ) ,
( i r r e g u l ω r )
ぬs a f i s f i e c l , t h e p r i n c ' l paZ function f ( z ) 7 v i t h r e s p e c t t o G h α s t h e e x l ) ( m s i o n
( 1 2 ) 削 Z4 十 ヲ 〔 七 f l
万一五;!寸P
'W!d
c h
ねα b s o l u t e l yc o n o e r g c n t α nd b e t
'We e
比 f( z ) un c Z Blas
cJd c e ‑ P
l'o c l u d t I I ( z ) t h e r e l a t i o n ( 1 3 )
e , a s t s
1f tM c o n c l 似 o n (8)
0 1 ' t h c c O n c l i t i o n
f
( z )
=b
十一一豆一l l ( z )
ヲ ' j ) "
lpn 一三ーニ工 00
n
ー qn
2 : ( l ‑ r ) ' ) ∞
ν~l
( r c g u l a r ) ,
( ' I 1 ' r e g u l 叫 r ) i s s a t i : ザ i e c l , ( 1 2 ) i 8 n o t a b s o l
'llt e l y c o n v c
l'g e n t , ( t n c l t h e ? ' e l a t i o
η( 1 3 ) c l O C 8 n o t e : a s t .
R e r n a r l c . 1 n t h e i n v e s t i g a t i o l l o f t h e a b s o l u t e c o n v e r g e n c e o f t h e s e r i e s ( 1 2 ) , t o l l s e t h e p r i n c i p a l domain o f I I ( の , which i s t h e w e l l ‑ k n o w n , g e n e r a l , and standard a l l t o . . morphic f u n c t i o n with r e s p e c t t o 0 , i s more c o n v e n i e n t t h a n t h a t o f
f( z ) . For the r a d i u s and t h e c e n t e r o f t h e boun c 1 ary c i r c l e o f
f,(,z ) vary and c o n v e r g e t o t h o s e o E t h e 1 i m i t i n g boundary c i r c 1 e o f
f(z)
1 n t h e f u e h s o i c l group , t h e r e l a 丘 0 1 1 which e x i s t s i
l1t h e Ftlchsum subgroup , a s the lemma i n t h e s e c t i o n 2 shows , 1 c 0 e s n o t always I e a d t o t h e same r e l a t i o n f o r 均 一 歩 ∞ .
‑>
‑ 6 ‑ AKAZA
References
1)