Title On the Galois cohomology groups of algebraic tori and Hasse's norm theorem
Author(s) AMANO, Kazuo
Citation [岐阜大学教養部研究報告] vol.[13] p.[185]-[190]
Issue Date 1977
Rights
Version 岐阜大学教養部 (Dep. of Math., Fac. of Gen. Educ., Gifu Univ.)
URL http://hdl.handle.net/20.500.12099/47402
※この資料の著作権は、各資料の著者・学協会・出版社等に帰属します。
K azuo A m ano
D叩 , of M ath・, F ae. j Gen. E dl, e. , G ㈲ U nil, .
On the Galois cohomology groups of algeblʻaic tori and H asseʼs norm theor em
algebraic torus T defined over ん which splits over K .
§2. GALoIs CoHoM01.0GY oF ToR1
2. 1 L et 苔 be the group oI K - rational points of T and 7¯y栓 the adele group of 7 0ver 瓦 W e denote by X = HO・ ( G 。, T ) the set of morphisms of G ointo T defined over 尺 and which are also group homomorphisms, where G o is the multiplicative group of universal domain. W e let (M operate on χ by the rule ( 舵 /) ( 8d) = 式/( わ) for
s E @ , y E X, and l E 7¯ I Thenitiswenknown that 几 弓 X ⑧ 尺̀ and 不帰ミ X ⑧ 収
as (S5-modules, where み is the idele group of 瓦 Denoting by Cg= 収ZK̀ the idele class group of 瓦 we have the eχact sequence of (S5- modules since χ is Z -free; o → χ ⑧ 尺̀ → χ ⑧ み → χ ⑧ Cχ→ o and hence we can identify χ ⑧ Cχ with 八 / 几 as (S5-modules. Putting G ( ア) = T≒Z TX・we ca11Cバ T) the adele class group of Toごr瓦 The nthec upm ultiplic atio nbythecano nical c lassof 万 2(@, (λ) induc e s an
isomorphism か 明, C) ⊇か ・2((55, G ( 勁) for every integers 7z〔5〕.
A nalogous result in the local field is the following, L et p be a place in ん and 雫 a place over p in 瓦 W e denote bj h and K、1, the completions of ん and K by the places, respectively, and by (55叩the Galois group of A74,/ 馳, T hen the group 栴 of 尺1- rational points of 7¯ʼ is isomorphic to X ⑧ 八万ヽ; as (55r module and 少 e cup mu1リplication by the canonical class り of か ((MI , χj ) induces an isomorphism 拓 明 ,l。 即 ⊇ ? ゛ 明 い
/
Tり) for every integersI・71 〔3〕゜ / ・ʼ
2.2 J. T ate show ed the f0110wing results in て9 〕 . L et y be the fr ee abelian group genel̀ated by the places 畢 of 瓦 A n element s E (55 0perates on y by the rule
j(ミ|%雫)=回り(邱). yed e n o teb yWtりeke rn e lo fs u rje c tive叫h o m o m o rp h is m
y → Z defined 卜y :ミi ≒ 叩 → 2; 恥 . T hell the cup multiplication by the canoni- cal classes α1 ∈ か ( 勁, Hom ( Z , C g) ) , Q2 ∈ か ( (翫 Hom(Y , み ) ) and α3 ∈ 會 価 ,
(Received Oct. 15, 1977)
§|. INTRoDucTIoN
L et 几 be an ʻalgebraic number field of finite degree and K a finite Galois eχten- sion or だ w ith G alois group 既 lt is w ellknow n as the generalization of H asseʼs norm
˜
theorem that, if w e denote by N (k) the subgroup of the multiplicative group ん̀ of ん consisting of elements w hich are local norm from 尺 at every places of ん, then the
group凧ん )/倣y ぶ帽sisomorphictoafactor groupof 万一 3凹, Z).
T he purpose of the present paper is to generalize this theorem to the case of an
ΣM 妬 gi(召)
μ ⑧ /1
戸“((S 5, X (8) .Z) ≧HO¯2( (M・ X (8) Cg), か (@ , X ⑧ 杓⊇刄o-2(@ , X ⑧ み), か(@ , X ⑧ W) ⊇が 2(@ , X ⑧ 7ぐ勺;
moreover there exists a commutative diagram with eχact rows:
・ ・ ・→ か (@ , X ⑧ W) → か (肌 X ⑧ y) → か (吼 X ⑧ Z) → ‥
↓ ↓ ↓
・ ・ ・→ 方o¯2((55, X ⑧ 尺勺→ H“ 2((S 5, X ⑧ み)→ HO2((S 5, X ⑧ Cχ )→ ‥
よで二二言ご一 二鸞ゴ 言二九謡言二二三で
yl be a G-module, 召 an 有一module and j 谷 召 a pair oI H- homomorphism s such thl t yM is the identity on 召 and y1° Σ が ( ム) , direct sum. T hen for any G-module 皿
we have an isomorphism g ・
K azuo A mano
186˜ - -
the canonical (MI -injection which maps a non zero element of 尺,l, onto the idele having that element as 畢 -component and having l as components at all places other than 畢 .
Since(55, (55わ H λ y 47(resp. H lh), and 眉 (resp. 販 ) satisfy theconditions of semi-
local theory, we have
路(町 X ② H 肩) ⊇か((も , X ⑧ 肩), か ((55, X ⑧ H 叫 )⊇か ((S 5わ X ⑧ 叫 )
for any fixed prime 雫 dividing p . Since Uい s cohomologically trivial if 心 is unram-
ified over んp, X ⑧ 1h is also cohomologically trivial by the theory of local fields.
T herefore, if our set (B contains all places p of ん which ramify in 瓦 we have
≒リa(JX⑧肩) ×口 IJ錨丿(8)U皿 Fo re a c h雫o f瓦wed e n o teb y沁:鰐→お
⊇Σg(1(8) j) (M 区) £)
and the two maps
。 (1 ⑧ j)res 。
亙” (G, M ⑧ j ) 二 二 ご 堵 (H, M 妬 B) cor( 1 ⑧ j)
are mutually inverse isomorphisms, where ” γeyʼ denotes restriction map and ”cor ” denotes corestriction map.
2.4 1f S is a finite set of places p in ん, w e also denote by the same symbo1 ら the set of the places 畢 of 尺 which divide some place p E ら . P utting
ベニEITり X HTり, theadelegroupTAxof7¯ ʼisdefirledastheirlduct沁eli°itof 几 relative
平Es 平 唾 s
to G , w here 几 ぶ= χ 必 U 、j s the unit gr oup of 尺 平. S ince t H = χ ⑧ 尺喬 , w e hav e 7¯毎
T herefore we have the following
P ROPOsITlON L か (町 7¯ʼ≒ ) ≧ Σ か 回 礼 几 い 力 r 凹 eひ む洽 即 rs ・・.
2. 5 P utting yヽ= y Z 畢, we have y = Σ n ( direct) as (S5-module. A ccordingly
心ゆ 11
we have X ⑧ y = Σ ( χ ⑧ n ) as (S5-module. F or each place ̀μ oI K , we define a
(S 5ヽ l;-homomorphism ら : Z → y by 沁 0 ) = 71雫. Since (S 5, (55、 r, 妁 , and, Z43 satisfythe
the conditions of semi-local theory, we have
か (@ , X ⑧ y) ⊇Σ か (@ , X ⑧ 瓦 ) 二Σ か (叫 , 7≒ )
T herefore we have the foUowing
⊇Σか (叫 , X).
二肌 かl(賜, 7¯ ʼり ).
pE ご
P assing to the inductive limit over sufficiently large S , we have
堵 ((聚 T愧) ⊇ lim μ”(眠 7笑)
で
か(眠 7瓦) ⊇H か (り, r17≒)
l・∈ 2 劉 p
が 2(町 7臨) Σ
HO¯2( @ , 几g) か 2回礼 几l )
知 明 , X ⑧ y)
2。6 U sing these propositions, w e have the follow ing P RoPOsITIoN 3. The fotLom維g diagm m 18 commut(ltiue:
PRoposm ox 2. か ( @ , X ⑧ y) = Σ か ((恥 , X ) foT 四 e巧 integeT8 n.
p
耳か(吼, X)
LVUα ,
・X
Z
˜
方O¯2(叫 , 几い 恚
P RoOR T he top horizontal isomorphism iʼ is induced by the maps
か回礼蜀 犬 か(町 X ⑧ 杓
and the bottom horizontal isomorphism i is induced by the m aps
COバ1 ⑧ ふ)
By the fundamental relation between corestriction and cup product, we have
Q U(C OT(1 ⑧ バ) ぐ) = c o代Te8 a2U( 1⑧ に) ぐ)
= cor G ヽ1, ・ Tes Q 2 U ぐ) = co ぺ 沁 叫 U ぐ )
= coべi ⑧ 晦 ) (偽 U ぐ),
whereぐ ∈ か 凹も 萄, andバ‰ theprojectionHO ・ (y, χ) → X defined by八( 九)
= μ雫) for 雫 ∈ (B. Therefore we have our proposition.
o → 几 二→ T4 二 → cバ勁 → 0
P assing to cohomology groups, we have the eχact sequence
¨ ʼ→ 万¯1(凧 几) 二 戸¯1(勁, T柚)二 万¯1(眠 Cバ7¯ ʼ))
→ 分〇(既 几) 二 方o(眠 八い二 か (町 Cバ7つ) → ¨ t
P RoPosITlON 配 T he foLLom錐g diagm m is commut(l tiue
叫
Σ 叩
188
K azuo A mano
§ 3. HAssEʼsNoR,y I Ti-{EoRE,v l of ToRI
W e consider the following eχact sequence of (S5-modules
Σ か (叫 , X)
p レ ・
か (吼 萄
↓ 万 卜 ↓ Uα I
Σ
耳が¯2((551, 711)二 →HO2((55・八g)
レ
かO (町 CバT) )
Uα1
か+2((55゛・几いこ →が 2(@・八g) 二 か¯ 1 ʼ2((S 5・Cバフ))
T his proves the proposltlon
Denote by yV げ ) 二 几 ら ( O yV‰ Z臨T ≒ ) the subF oup of 几 collsistillg of elellle趾 s which are local norm from 几 at every places ol k, where 几 is the group of ん- ra- tional points of 7¯I T hen, as our nlain result, we have the generalization of H asseʼs norm theorem to the case of. an algebraic torus.
THEon回. Ld F 酸 X加 s晶gro叩 げ 方¯3(町 X) geneT(1ted by こヽ 駱(鉦3((Mも X) ) μ『
eveび p. 7¯ʼhe71 uXe h(1tJe (1n 180・ oTphis・
斤げ )/y V卜 l哉Tχ三H- 3((9, 萄/R
P RooF. By virtue of proposition 4, we have
yV( 7¯ ʼ)ZNい TX= KeT( f *)
≧か 1(@, CバT) ) /j *(か I (眠 八い )
レ・
P RooR By virtue of T ateʼs commutative diagram 2.2 and proposition 3, we have the following commutative diagram
Σ ら : Σ か (叫 , X) → か ((M, X (8) y) → か ((55, X)
T herefore we have Ke八 i *) = O by proposition 1
7¯ 八)
こ H-3 ((55, X) /E
ConoLL八nY( 〔 5〕 )・U K Zk 18 cydic eχten810n, lj e hE e l¥y( 7) = y Vχ 71TIい
P ROOF. By virtue of K neserʼs paper 〔3〕 , there is, for every integers 71, a ca nonical injection
か (町 7¯ y) → Σ か (叫 ,
Hiコj Haip︹ 巴 回〇く乱 9の{ 〇 } } 〇だFnroヨ〇r phi s ms せ ( 夕 肆)㈲tぺ 芦こ﹃ ゛ せ ( 夕 jし gJjyD JO
where E and E are the idele grI ps of i and L respectivelyo p ooaatoコaFn {〇rus 7 which thの∃oa巴の
̀ 心 r
○
1 卜 3ン レ ト X レ ミ 1
0
レ ミ 1
0Now we consider the exact sequence ofQ5 m odules2
94 ArrΓ一〇yH一〇z Ho z oz GAΓo一㎝ mxHmz∽一z
? 51s sectionswe give Hassets norm thのo9日 {〇 9の QSの o{ コ○コー(い巴or のx芯 コー
器〇品 回F叫 p roくのの〇mmut a t i vedi agram f orTasakag ss peci alt orH2 8︺゜
Ho 5e Z jee 6 modulej4 there ls
the ch aracter m odule H om ( リ Q し ʼ
sr諮 N ︹ ( ぶへ む)=M NF Q=り? Q 乱 心=M P
4j ﹂et ﹂ be a separabl e ext ensi on 〇f i and K a fi ni t e ( い巴 〇i s extensi on 〇f 1
containing L W e denote b y 6 a乱 か 9 の 0 巴 or 9 0up of K 4 and K ZL respectiv e ly ﹂ et Q5 u g g b e a left coset decom p 〇sition W e consider the follow ing left 05 m 04已 a
y=N︹ 9ふ︺ ゛哨=ンヘ NF
}⁚)assing to coh om 〇10gy groupS w e obtain th e toUow lng ex act sequence2
where Rxzj Gし cIonotes the algebraic group detined over A70btained by restricting the
11eld ot delln ltIon A to t B y ︹吝 ︺o w e h av e j M R J O o a乱 hence w e obtain }にlassets n orm th eo rem fo r ﹂ Z k
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G al oi s extensi on X Zk W e consider a
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{ so︲Eヨの コ巴〇コ巴 { 〇回Q
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moqMsm
司( ″ ) へ 芦︑ y ︒旧H MR Xj Z E
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K azuo A mano
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