屋 久
S c i . R e p . Kanazawa U n i v . , Vo l . 2 6 , N o . 2 , p p . 2 7 ‑ s Q D e c e m b e r 1 9 8 1
On Quadratic and Quartic Characters of Quadratic Units Y oshiomi Furuta and Pierre Kaplan
De ρ a r t m e n t 0 1 M a t h e m a t i c s , F a c u l t y 0 1 S c i e n c e , K a n a z a w a U n i v e r s 的 U . E . R S c i e n c e s M a t h e m a t 勾 u e s , U n i v e r s i t e d e N a n c y 1
( R e c e i v e d O c t o b e r 2 7 , 1 9 8 1 )
A b s t r a c t . P r i m e d e c o m p o s i t i o n c r i t e r i a i n n o n ‑ a b e l i a n n o r m a l e x t e n s i o n s L o f d e g r e e 8 a r e s t u d i e d , w h e r e L i s o b t a i n e d b y a d j o i n i n g a s q u a r e , r o o t o r a q u a r t i c r o o t o f t h e f u n d a m e n t a l u n i t o f a q u a d r a t i c f i e l d a c c o r d i n g a s t h e n o r m o f t h 巴 u n i ti s e q u a l t o ‑ 1 o r 1 .
~1. I n t r o d u c t i o n
L e t m be a p o s i t i v e s q u a r e f r e e r a t i o n a l i n t e g e r . I n t h i s p a p e r we g i v e an e x p r e s s i o n t o t h e 2
n( n = 1 o r 2 ) r e s i d u e symbol o f t h e fundamental u n i t
Smo f t h e q u a d r a t i c f i e l d k = Q ( . / i T I ) , r e l a t i v e t o c e r t a i n primes q .
I f t h e norm o f ε
mi s ‑1 we c o n s i d e r t h e q u a d r a t i c c h a r a c t e r o f 臼・ Thisc a s e h a s been a l r e a d y s t u d i e d , f o r example i n [ 1 J , [ 4 J . I f t h e norm o f
Smi s + 1 we c o n s i d e r i t s b i q u a d r a t i c c h a r a c t e r . This c a s e has been s t u d i e d i n [ 5 J , [ 6 ] , and l a t e r i n [ 3 J s o a s t o i n c l u d e a l s o t h e c a s e where t h e norm o f 臼 i s‑1. Here we a p p l y t h e methods and r e s u l t s o f [ 2 J 削 hec o n s t r u c t i o n o f [ 3 J ; t h i s w i l l g i v e f 伽
O町 … r 削 a i nq a 釘 … r
by c e r t a i n prime d e c o m p o s i t i o n symbols d e f i n e d i 加 n [ 2 勾 J.
We s e t η=R+s ; m , where ( R , S ) i s t h e minimum p o s i t i v e s o l u t i o n o f ( 1 . 1 ) R2̲mS2= 1 .
By [ 3 , Lemma 1 J t h e r e e x i s t s a u n i q u e d e c o m p o s i t i o n m=de , d , e>O , a u n i q u e p a i r o f p o s i t i v e i n t e g e r s V , W and a u n i q u e number t=l o r 2 s u c h t h a t t=1 i f m~l (mod 4 ) , ( t , d )
ヰ ( 1 , 1 ) and t h a t
( 1 . 2 ) t=dV2̲e W2 , η 二
S2, where
( 1 . 3 )
I f N ( ε m)= ‑1 , ε=ε J z o r ε m and i f N ( ε m ) = 十 1 , S2 ニ ε~ o r ε m a c c o r d i n g a s t h e
2 7 ー
2 8 Y o s h i o m i F u r u t a a n d P i e r r e K a p l a n
d i o p h a n t i n e e q u a t i o n r 2 ‑ms 2 =4 h a s o r h a s n o t odd s o l u t i o n s ( r , s ) .
The p r i m e d e c o m p o s i t i o n symbol we w i l l c o n s i d e r i s [ d t , ‑ e t , q ] , a s d e f i n e d i n [ 2 , D e f i n i t i o n 1 . 1 and D e f i n i t i o n 5 . 2 ] .
L e t m=Pl P 2 … P r t h e d e c o m p o s i t i o n o f m i n a p r o d u c t o f p r i m e n u m b e r s . We c o n s i d e r odd p r i m e s q s u c h t h a t
1 , It , I P
( 1 . 4 ) (一一)=(~) q ' q " = ( 一 : : ; q ' )=1 ( i = l , " ' , r ) Then εcar 山 i r 伽 附 e 吋 d 出 a r 山
w
附耐 e l l 仙 叫 刷 叩 a 山 l t 刷 t)w 袖 h 児 e e 悦 r 閃 eq 杓 i sa p r i m e i d e a l ( ば O ぱ fd 拘 e 昭 g 悶 1 )d i 討 V 巾 附 i s s s
叩 ub 凶 f 臼 i 凶 e e 副 l l d 0 ぱ fQ 似 (r 工 工 , . , r τ ,;P了,… , . f 日.
~2. C a l c u l a t i o n o f [ d t , ‑et , q ]
We a p p l y [ 2 , Theorem 5 . 1 ] w i t h d
1二 d tand d2 二 e t .As t 2 =dtV 2 ‑etW 2 , t h e number m o f [ 2 , Theorem 5 . 1 ] i s e q u a l t o 1 . A l s o t h e number d o f [ 2 , Theorem 5 . 1 ] i s e q u a l t o o u r number t . Thus we o b t a i n :
PROPOSITION. L e t q b e a ρ r i m e number s a t i s f y i n g ( 1 . 4 ) , and c o n g r u e n t t o 1 m o d u l o 8 i f t i s e v e n o r i f m i s e v e n and d o r ‑ e 三 1(mod 4 ) . Then
d t ,
1‑et [ d t , ‑ e t , q ] = ( ー τ )=( ‑ ‑ i ー ) ,
ωh e r e b , X , Y i s any s o l u t i o n o f t h e f o l l o w i n g d i o ρ h a n t i n e e q u a t i o n s u c h t h a t ( b , x , y)=l and ( b , 2m) =1:
a ) qb 2 =X2 十 XY+‑‑z‑Y2 m+1 b ) qb 2 =X 2 +mY2
c ) qb 2 =X2+4mY2 d ) qb 2 =X2+16mY2
e ) qb 2 =(b 十 8X+4Y)2+16mY2
~3. Determina
ifm 三 一 l(mod4 ) , d 三 一 e 三 1(mod 4 ) , t=l;
ずm 手 1(mod 4 ) , d o r ‑e 三 1(mod 8 ) , t=l ; (σm=: ‑l(mod 山 一e 二 刊 od4 ) 目
σ mj ‑1 (mod 4 ) , d o r ‑e 三 5(mod 8 ) ; ず m 三 2(mod 4 ) , d o r ‑e 三 1(mod 4 ) ; ず m 三 一 l(mod4 ) , t = 2 .
As i n [ 3 ] we s e t d ' 二 d t , e ' = e t , μ=t‑WI ご e , li=t+W; ご e ' , v=2t+2V; 司 r‑
v=2t‑2V./ 百 . , and d e f i n e t h e f o l l o w i n g f i e l d s :
( 3 . 1 ) k=Q ( . f τ 百 ) , K=Q ( . ; c r , . , r 二百), L=K ( . / ! i ) .
Then , a s μ 戸 = V2 d ' , L i s a d i h e d r a l e x t e n s i o n o f Q whose s u b f i e l d s t r u c t u r e i s a s
On Q u a d r a t i c a n d Qu a r t i c C h a r a c t e r s o f Q u a d r a t i c U n i t s 2 9
f o l l o w s :
¥ /
¥σ
ー 一
︑ Q
¥ Q /
7 ν
¥
/ 川
¥
¥
As ( d ' , e ' ) = l o r 2 and a s vand μare prime t o one a n o t h e r up t o a s q u a r e f a c t o r i n K , t h e o n l y i d e a l s which can r a m i f y i n L/k l i e above 2 . T h e r e f o r e t h e c o n d u c t o r f o f L/k i s a power o f 2 .
Let S be t h e ray c 1 a s s f i e l d modulo f above k , and l e t 立 andK* r e s p e c t i v e l y t h e c e n t r a l c 1 a s s f i e l d and t h e g e n u s f i e l d above K r e l a t i v e t o S / Q .
Then LcS , and , a s Gal(L/K) b e l o n g s t o t h e c e n t e r o f G a l ( L / Q ) , Lc 食. As L/Q i s non a b e l i a n , L a ; K へ s ot h a t [ 食 :K 勺=2
Let q be a r a t i o n a l prime c o n g r u e n t t o 1 modulo 4 and decomposed i n K* i n i d e a l s o f t h e f i r s t d e g r e e . From [ 2 J we have
K/K*
( 3 . 2 ) [ d ' , ‑ e ' , q J =(一一可ー)
L/K , , u , , v
=(一一一一一)=(~)ニ(ー), N K * / K q 'q' 'q
/食¥,
K ¥ K / L
特
where q and q a r e prime i d e a l f a c t o r o f q i n K* and K r e s p e c t i v e l y . According t o [ 2 , Theorem 4 . 3 J , t h e f i e l d K* i s g i v e n a s ( 3 . 3 ) K* ニ同 Q ( { r T ) ,
where ko i s t h e o r d i n a r y genus f i e l d o f k , and f i s a p o s i t i v e r a t i o n a l i n t e g e r , which can be ca 1 c u l a t e d from t h e c o n d u c t o r f o f t h e e x t e n s i o n L/ k . As f and f have t h e same prime f a c t o r s , f i s a power o f 2 . T h e r e f o r e t h e p r i m e s q= 1 (mod 4 ) c o m p l e t e l y decomposed i n K* a r e t h e p r i m e s s a t i s f y i n g ( 2 . 1 ) and , i n t h e c a s e where f>4
( 3 . 4 ) q 三 1(mod f ) .
From t h e v a l u e o f f g i v e n i n [ 3 J , we d e d u c e t h e v a l u e o f f . Both o f them a r e l i s t e d i n t h e
f o l l o w i n g t a b l e :
3 0 Yoshiomi F u r u t a and P i e r r e Kaplan
Tab!e 1
j 1 1 . t
ラd , e f
1 o r 2 1 o r 4 m 三 1(mod 4 )
E