、
ON AN APPLICATION OF THE GROUPRING
THEORY TO A COMPLETE NON.SINGULAR
ALGEBRAIC CURVE OF GENUS g≧2
BYS別SHI WADA
㎞tmduc60n. Let C be a complete non−singular algebraic curve of genus g≧2with it,$ Jacobian variety J. Let k be a◎ommon field of definition of C and J. In§1 we consider a special part of the endomorphism ring A(」)of 」, in order to get oerta血criterions for simplicity or non−simplicity of J, and show that if(x,夕)satis丘es k(C)=k(k, y)an《張 ∫()cl, vi)=Owhere∫(X, y)is in k[X,γ]irreduCible in k[X, y]and l is any餌me rationa1. integer other than the characteristic of k, then J is non−simple. In§2, we・treat・an血volution of the group血g R=2(G) by the 9roup血9 theoretic’ manner, different to the general theory of algebra【司. h1§3, we 9ive 1inearly equivalent relations of divisors of C, when R”of Corollary 2 of’ Proposition l is a quarternion diVision ring of the丘rst kind of the sense of Albert[5]. §10n a group血g 2(G) Let Po=(xl, x2,....,xn)be a representative ofPill an a缶ne representative of C and put P6=(κf, xS,....,x紘)fbr an elementσof G. Then・P8 is a・ sp㏄ialization of Po over k, and over this there is an unique extended specialization Pσof P.Forσandτof G, we have(Pσ)τ=Pστ. We take a subgroup H of G such that H == (σ∈GlPσ=P), and take a set(S of diVisors of C which are linearly equivalent to some た一rational divisors of C. We takeαof Z(G), that is,α=Σ侮σwith侮∈Zandσ∈G. And we put Pα=Σ aσPσ. 1£tR, be a subset of R such that R,=(α∈RlPma∈(5 for some positive m of Z). Then、 we get 9 >LEMMA 1. R’肋吻加永鋤1邨R励α励丑」〃η吻品・
PRooF. We easily see that R, is a right ideal of R since Pt「τ is a specialization of Pσ over Io and over k, and by Lemma 10 of【2.2】. For the latter assertion, we t丞e a醐so品of’ C×C which has a generic pOint P×Pσ over k. Then by NO 24 of[2.1], we have Xa(戸)= P7(σ)’「,〃』Xe(Xr(P)=P7《σ)’〆τ)where no is the odrer of H. For a of R, andβof R, we take positive integers m, n such that〃叱r and nβare in Z(G). If〃7α=Xaσσ十27 bττandi〃β=Σα5σ十Σb:τwhereσare llot血五r andτare血刀∼we take a divisor抑=Σaa
*Received June 10, 1975 9’ 董10
S.WADA
Xa十πo(Σゐ:)∠1 where∠is a diagonal of C×C. Then we get Xp(P)=PnP’H. As獅(Pρ) tis a specialiZation of Xβ(P)over the spedalization血om P to Pρover克, we get Xp(P抗α)= 」助ψ互癩.If 1)is a鳶一rational divisor of C such that P功α∼1), then by Theoreme 40f【2.1], ’there is aた一rational numerica1 function q on C such that Xp(D十div(g))is defined and り 一1「β(D十(五マ(切)∼Xβ(pmα). Hence we get the proof.(ρ.E.D.) Next, we a§Sume that k is algCbraically closed. Let C be a complete non−singular alge− Lbraic curve de丘ned overたsuch that L=丘(P)f()r a generic po血t P of∂over友. Let @ be .a set of divisors of e which are 1inearly eqUivalent to some k−rational divisors’ of e. Then ’we get LEMMA 2. Me assume k=k. Let R,=(α∈Rl.Pmα∈(きノ∂r some positiye mげZ). :77ien・R’is a〃ideal of R,αη∂α耳∈R’lff HaH∈R’. Proo£As∬=(σ∈GlPσ=P)=1, we㎞ow that R/is an ideal of R. We take a nat一 エ 噸ral nlap∫丘om C to C such thaげ(P)=Pand with it’s graph 1}. Then we get‡乃(P)= .!−1(P)=ΣPτ=PH and f−1(Pσ)=PHa.lfαH∈R’, then fbr a suitable positive integer τε亙 Jn and a k−rational divisor 1)of C, we get、PmaH∼1). By Theoreme 40f[2.1】, there is a k− ratiOnal numerical function《g on C such thatプー1(1)十div(g))is de丘ned and linearly equiv_ ・alellt to∫−1(pmaH)・Henoe pHmaH∈㊦. SoαH∈R, means HaH∈・颪,. Conversely, if HαH ・∈Rノ,then there is a suitable positive血teger刀and丘一rational divisor万such that戸砿αH∼ コ .D. And there is a瓦一rationa1 numerical血nctionψ on C such that 1}(万十divω)is defined .and 1祖early equivalent to rf(PnHαH)=no、PnαH. So we complete the proof.(ρ. E. D) R=0(G) is a regulaロing of the sense of Neumann and a semi−simple ring. We加t N パ .R=HLRL[. As(H/no)2=珂〃o, R is alsO a regular and simple血g, genetated by 7(σ)over 2. If we de◎ompose R into a d丘㏄t sum R, and R”, and decompose into a direct sum of 」∼ノ・孤d互”th・t iS, R=R’θR”=R’(D互”, w・g・t∈丑哀’丑Φ頭・五=艮・e互〃.
Forαof R and a sU table positive血teger m.such that uα∈Z(G), we putγ(mcr)=XρεHmaHρ=HmcrH qnd take a divisor Xma of C×Csuch that為α(P)=P抗αH. Then we know
that we may rewrite為αas掲伽めsince it depends only onγ伽の, and〃』Xア伽α)αγ(πβ)(P))= p,(悟α).7(犯β)=noXγ(mα)γ(nβ)(p), since γ(吻)●γ(nβ)==γ(〃叱どH・、Hnβ). By correspondin9 .ζγ伽のof A(」)tσ,17(mの, we getζア伽α)・ζア(πβ)=ζ7(mの.γ(πβ). If we putεア(の=(1加)ζγ伽α), then we getε7(のεγ(β)=ε7(d).γ(β)whereγ(α)=HαH=(1/mγ(微). Moreover we easily パ :getεγ(の・γ(β♪=ε7(α)十εア(β). Hence we have a homomorPhism from R into Ao(の. If N ロ ,εア(α》=0,we get aH∈R’apd so 7(∋∈R,(since R’=R’for the case k=た). Thus we get N N PRoposrHo)cf 1・ If we assu〃te k t k‘mdput R’・=HR/Htnid Rノ’=HR”H/br R, cmd .反,f of Lenznza 2(R=R’∈DR”), then there is an iniectiohノ}ro〃1五”into Ao(」). − A■ COROLLARY 1. Ifk=kand R〃is not a読?砧,o〃ring, theπ」 is n.onrsimple. PRooF. Under these conditiolls, there are two non−zero eleme亘tsξandηof、4(」)such 電hatξ・η=0. Hellce J is non−shnple.(ρ.E.1).)ON APPHCATION OF THE GROUP RING THEORY
1蟄・ CoRoLLARy 2. 」ゲ五=1, then R”of R=R,∈D R,’〃tay be considered to be containeグinA。(」). .…
PRooF. ff H=1, then we have L=k(P). So we may consider C=C and so we nc ee(1{ nOt to aSsUlne k==k. (e.E.1).) LEMMA 3. 〃’k(C)=k(p)=k(x,ア)and Lo= kCソ),・then R, eontains an element− G=Σσ. 一 「 PRooF. As Pσ=Σ Pσ is kO,)−rational since it is not changed by any T of G, Sψ(PG)=ΣqA σεθ (、Pσ)is also a k(夕)−rationa1 point of J, where pa is a canonical map from C into 」. Hence we− get a k−rationa1 map f from a rationa1 curve r defined over k into/such thatノ’(p)=Sg・ (Pσ).By Theorprn 8 of【2.2],f(y)is a fixed point of.. 」. By tak口ig a specialization from(._,. Pσ,....)to(....,ρσ,....)over丘where all Cσ are k−rational points of C, we see・ that f(ン)is a k−rational point of J. Hence R’contains G. ‘ (e.E.D.)・ LEMMA 4. If the genus 9≧2and if there are伽o elementsσ,τq〆Gぷ〃ch’肋’στ一1奄…、ぼ 功θ〃σ一τ庄R’. ● PRooF. If there is a k−rational divisor 1)of C such that Pσ一Pτ∼1), then Pρ一P∼Ef()r ρ == aT−1 and some k−rational divisor E of C, by Lernrna IQ of[2.2】. We take a generic point J P/over k(P)of C, then as、P is a generic point over k of C, by taking a specialization(P’, P”)of(P,Pp)over k, we get P”−Pノ∼E∼Pp 一一 P. Hence we get P’十Pp∼P十P”..This means l(P十PP)≧2. But we have l(P〆十P)=2−g十1十1(x−P,−PP). But.
as l(x−−P,一、PP)=’g −2. So we get 1(P十PP) =1.This is a contradiction. Hence weknow thatσ一τeC R’. ・ (ρ.E.1).〉
LEMMA 5. Le’Gb㊨吻12吻e l be a輌’』l integer other than’乃θ』c−一
’eristie ofk, and let G be genera彪d by two e∼ementぷσ,τぷ〃c力吻’♂=τ1=1αη4στ=τσ.. If we a∬醐e吻’五=1,功εη」‘ぷηoη一ぷ」〃rple. PRooF. As G is abelian and H=:1, R is a direct sum of fields, and R’and R”are ideals、 of R. If l is simple, R”must be a field F finite over C, by Coro皿aries of Proposition 1. We・ decomposeσandτasσ=eσ十ぷandτ=eτ十t Where eσ and er are in R’, andぷand’・ are in亙We get 5』tl=1’ for the unity 1 of F. ff vve take a p血ritive 1・th root e of unity 1, we getぷ=ζa and t=ζb for some integers a, b. lf a=0, then we haveσ一1∈R’and this、 contradicts to Lemma 4. Henee a is pOsitive and s顕皿er than 1. By也e s㎝e㎜er,か is also positive Smaller than 1. Hence there is a positive rational integer m such that s = tm. This means thatσ一Pm∈R,. But (n ’mキ1=耳『since G is generated byσandτalld is. of order 12. Hence by Lemma 4, we get a contradiction.’@ (e.肌㍉
PRoPosmoN 2. Lθ’Cゐθα’ω〃!plete non−sing〃lar atgebraic cur昭げ8iθ〃〃s g≧2. Lと’た一12
S.WADA
、 ㊨ε伯砲ωげ晒ゴtion・and(x,ア)ゐθαη〃αぴぷ励吻τ克(C)=克(x,夕).聯c励痂M餌〃z− itive 1−th roO’ζq〆〃η∫カノ}vhere l is a pri,〃te ra’io〃α1 Z〃teger other than功θcharacteri,ぷtic ofk, イZ〃d ifJacobia〃Varieり「」ρノ’CisぷZ〃rρle, then Wθhaye丘(X,ア)=丘(xZ,ア)or k(LAcl,ア)=k イ)cl,アリ. PRooF. Any isomorphismρof k(xz, yl)over k(xl, yl)sends(x,γ)to(ζax,ζby) for some血tegers a, b. Hence k(x, y)is a Galois extension’of k(ltl,アリ, since k containsζ. lf k(x,夕)早k(xz, y)2k(xz,アリ, k(x,ア)and k(xz, y)are cyclic extensions of degree 1 of k イx膓,ア)and k(xz,アリ, respectively. The Galois group G of k(x,γ)/k(lvl,ア‘)is generated byσ, andτsuch that(x,ア)σ=(ζax,ア)and(x,ア)τ=(ζbx,ζ⑲)with rational integersα, b and c. We see that(x,γ)στ=(ζa+bx,ζcy)=(x,γ)τσ, that is, dr=τσ. As♂=τ1=1, by hemma .5,we know that 」 is non−simple. This is a con仕adiction.且ence we get k(x,γ)=k(xz,ア)◇rk(xl, y)=た(xl,アり. 、.. ・ ‘ (ρ.E.D.)
PRoPosmoN 3. L¢’(]. J, k and l be the same aぷ仇Pr卿ぷZ’ion 2.ザ〃(C)=た(x,γ) .a〃4(x, y)ぷa’isfies an equationプ{x膓,アリ=O where f(Xiコη)垣〃丘【X, y],かrε吻c茄彪よ〃泥 ¶x.γ],then・J・is・no〃一ぷ伽ple. PRooF・If J is simple, then by Proposition 2, there are functions F and F/in k(X,、Y) ・such that x=:F(bel, y)orγ=」F”(xl,γi). But any pair(ζαx,ζbア)stay on the curve off(Xl, yt)=0. So we getζx=F(xl,ア)ixorζγ=F,(xz,夕り=γ. These are false. Hence J rmust be nOn−s血}ple・ (e.、E.1).)ExAMpLE. A curve】声…=F(X2)where F(X)is of degree>2and is not a square
・of any polinomial of k[X]corresponds to it’s birationally equivalent and complete non− s血gular curve C With non−simple Jacobian variety」」,proVided−that k has not the charac一 寸e]tlstic 2. t§2.On皿血vol岨on of a group ring、R飾r a curve We know that any group ring
・de丘ned over the rational number field has a positive hlvolution. But, fbr a group ring R, −we can constmct an祖volution by sp㏄ial calculations and deVices and can use it as we )use the positive one・1£t RI be a㎜血1al ideal of R. Let頁be a division』ring such that ・R1=;: Kczμ with a center 9. We takeα=Σa‘σi of R and put a=Σai 6i−1. W・猛・w丘・m。g。噸・h。。ry、in[5]・h。・皿i。。。1。ti。。0’。fK、面㏄, th。,e1。、i。n ,b−1〆b=臼br anyぷof」ζ and a fixed unit b of R1. Now, we can defme aρ一linear ahd(∼一・valued丘mctiona1σO on R byσ(α)=a1 forα=al十a1σ2十...,a,碗∈ρ,σi∈G.
Forαandβ=:Xbjσ」, we haveαβ=Xa吻σ吻=(Σ aibji)1十....,where刀satis丘es
・6t6it=1and is uniquely deten血ed by i. By this刀we can representβasβ=Σ函εσ宛. So ・we haveβα=Σゐゴ硫(萢σ元=(Σ「bdiai)1十.....Hence we have U(αβ)=σ(βα). Clearly σ(α十β)=σ(∋十σ(β).Forぷof」9, ss’== sbSb−1. if s∈Ω,∬’=sS, since S is again in the t・e・t…fR・廿・m・α一食・姦=・3飴・al1・・fR、.W・g・t良、一(alR、∋。)三R、,、in㏄・A。i、、i