TR工J Mathe皿atics 22=1 (1986〕
ASPACE
CURVE OF GENUS 7 AND DEGREE 8 WHOSE
NORMAL BUNDLE ’
hS SEMI−STABLE
T㎝oaki⑯and Yoshihisa AKIMOTO 、 (Received MaTch 15? 1986) . . 1. Introduction : . ’. P.E.. Newstead [2] gave an example of“alnon−singular curve of genus g and・d。gree 9。n。’ Mbi・・u・face麺th・p・・j・ctiv・・pace P3 wh・se’n・磁1・ bundle is‘stable. As examples of space cu士ves whose normal bmdles are (semiづ stab16ジwe list the results: 9 ’ ・ ‘genUS
degree
〔semi−〕 stability 1 . of nonnal I)undle 2﹁ 6stable
[6]. ’P 3 6 ..唐狽≠b撃 [3] 4 「 7 s㎝i−stableF ・}[5]. 5 7stable
9 9stable
[2] MOreover E.Ballico and Ph;Ellia[5]showed the『existence Of mmy cuエveS with stable(resp. s㎝i−stal)1e〕nor al bundle. 1。・hi, P。p。。 we。h。11.gi・。.・n・th…x・卿1・・f・四rv・垣P㌔ths㎝i−stable normal bundle. ・.
THEOREM. 五θカ O Z)θ α genθiαZ non−$ingu 1α? OZtrvθ of 9θ批s 7 and 飽鯉θθ 8 Zη玩90nαnon_singZt Zar已ubie swfaee 5仇劫θproゴeσtib’e Sραθ¢P3. IIVieh the nOlmzzZ hundZe of O in P3 is semi_stubZe. Notation. We work throughout over.an algebraically closed field of characteristic O. If C is a. curve oh a surface S. wさus’6 the same Sy㎡bo1 0for the corresponding divisor class. 3132 T.ONO and Y. AKIMOTO
㍍:隠霊蒜乱:f。1夢、13k.・
o : ・ ・. ご=the no皿nal bundle of C in 5. o/s 2.Curves on a cubic surface L。t・S・be。加n−sin四1ar㏄6i・S。・£acと麺』th・p・・jecti・・space P3. th・・・i・・b・・頑f・㎝・㍉b・・曲9−・p・iX剛…、.…,・,・hi・h…n・t㎝・
conic・and no. three of血ich.are collinearぺWe denote、 bτEi the exceptiona1 ㎝ve c・rr・spOndin9・t・pi ri−1・…・6ノ・,紐d L.th・t・tal t・a・・f・孤・f・1血・ . 2 ・L・t.ei・Pi・5∫ゲ1・…・のbe・.th・divi・・r・1・・s・f Ei・L・t Z‘PiC SlnP
be the divisoT clasS of z}. Then Pio 3 is the.fτee. a1)1ian、 group generqted by Z・・1・…・・6・and the、垣te「s㏄ti㎝pai「血9°n Pi・S is giyen by ・4−…i−一…『…、一…、・・ゴー・f・r・≠ゴ・ For any divisor class O=αZ一ΣカZ弓whereα。ゐ1._。カ6 are integers, we have − d= 3a一Σカi・ ραr功=rα三1戊rd−2ノ/2一Σ●〆ゐグ1戊/2 砲…di・th・d・gree・f P・P。・ω)i・the a・it㎞・ti・g・nu・・f D・L・t・パ・th・ class of q hyperplai}’e seCtion, thqn h=3Z_Σ%.、 D・finiti・n・A励;・・r・la・S D−・Z一Σ bi・i ・・ s i・・αZZ・吻τ触9 ra⊃ゐ1♪… 3わ6九 Let、・b・an・n−singu….i・・ed・・ib1・curv・.pf ggn・・、7飢d d・gree 8.血P3. We have an exact Sequence . 、 . ・ 0−一一一◆ lcr3) 一一一一9p3 r3ノー→ Ocr3ノー一一一→ 0. This fOllgws the long e?.ract sequgnce of Cohomologies ・ ・−H°・rp3。%r・〃−i7°・rp3.%・r・〃−H°rら・cr・〃一一…rりASPACE.〔XJRVE OF GENUS 7 AND DEGREE 8
33
・ince・de・・,r−・)⑧%・・砲…q。 i・th・⑳nica・・heaf;・・ha・・’hirらδ〆・〃一・.・h・n・h°r…〆・〃三・8晒…enann−R・ch・h…㎝・酔)…g・t
h『〈p3診㌔ri「〃1 0rp339P3 r3,ノ ー hO rら0σr3∼∼ =.≧O−18三 2・tt th・ref・t6S・h・te『aT・’…qi・ti・・t i亘e血・ib・…biと・U・face・c・n七・i晦・・『 L・t3〃。β”b←.i廿・血cib1・chi6ic:surface・e6・t・ihi㎎0. th・t・t・i i・t・r・eとtid・.‘ of 5・and’3パis a’浮堰D・. Th㎝Z’is a lihざfbr degree feas6n. From』now・on IWe ass㎝・S”i・an㎝一singul・τ㏄bキr皿「face孤d・・place S by S”. The divisor万 on● is one of the fbllowihg types: rO.−1,0.03030♪の3 r1,0,0,0,0,1,1)3 r230」1,1,1,1,1」.「. ’.㌔.:。t蒜「。ご鷲。芸7;㌶蕊ぷぷ九誕ぷ・.
r9:4二3二3S 3⊃『3」3戊 if 1}is 6£’typ6 ro,._1303030. o∫o) : ’ 「・ ‘: ヅ 「8・3・3・㌶・2・2,.if五i・・f typ・「1・°・°・°・°・1・1戊 「7・3・2・2ぷ2!if□手r°f type (.2・°・1・1・1・1・1ノ・、 、 Since孤y°f t}’e°thrτ.classes in the list c孤be.t「an・}f・・m・d t・、the class r7・3・2・2・2・2」2両aご㎞9・i・th・’・加ice・f El・…・%・鳩」s}ia11・tak・・t・ bel°ng t°the class「7・3・2・そ・そ・2・.2ノ・We haye °S(のi・veワ⑭1・(・ee[4P・ deg(C. Z}ノ=4. By M..Watanabe・s・results,σis proj㏄tive工y fidrna1(see.’o]「〕’.;‘…he⊇・6({・・6・, we・融・㎞曲t th6⊇・W叫・f・sudh・
genera1 (:urve is semi−stable. 3・S㎝i−・t・bility’ Ef%.;亘.』竺・e澱rt sr㎝rr.. ・ 、.
o−→弓r3ノー一→rcr3)一一一ぴ3)一一一→o’__ω
34
T。ONO and Y. AKIMOTO This gives rise to a.homomorT)hism、 . 王・・°rp3。・,r・〃一一一e°rら・ξr・〃……rβノ Thus any homogeneous polyn㎝ial of degree 3 which vanishes on O defines a ・ecti・n・・f Nξr3ノ・・t f・11・w・f・㎝(・)th・t・エS ・r・・precisely・t th・ sipgular points of the corresponding cubiC surface 5,.which lie on(7. 凛,・e・ha・1・h・W th・t the』・gm・珈皿了・f(B−」 i・i・・rP卿hi・m. ㎜・げ・e°r・3。・C剛一三一.・°rc. ・b .r・〃『』・ve・. h°r叫r・ノ∼一・・. 、
PROOF. No cubic can be singUlar qt every point of.σ. Hence・°rp3,弓r・〃一・, and・h・h…m・頑i・・アi・血」…i・・. .−..
St・p・・T・・(M・p…h°rp3,・。r・〃. w・c・n・id…h・f・…亘・g・xactsequence
O−一 IC「5ノー一→%3「3ノー−00「3ノー→0・
Since C i・p・・jectiv・1y・・㎝・1, w・h・ve a・h・・t・xact sequ・ncC ・一一.・°r・5,・,r3〃一一U°r・3.%・r・〃−rH°rら・。r・〃一一・・ th…C・re h°r・3。・,r・〃−hl° (・3。㌔・r・〃−h°rら・。r・戊ノ三・・−r24・・一・ノー・・ S・・p2…c…npPe・h°rらN3r・〃.・・c・n・ider・・h・・f・…㎞・・xact sequence .・一一一P・c/s−一%一一一Ns/p31・一一一・・
Dualising th・ab・v・ se・lu・nce and t・n・・ring by OCr3)gives ・n・exact se・岬ce°一一→.°・一一一π;r3戊一一在r・)一一・・
ASPA〔E CURVE OF GENUS 7 AND DEGREE 8
35
FT㎝the above sequence, we have. h°・らゾr・〃・h°rら・。ノ・h°rらiVE/。r・〃……r・戊dn th・・ther h孤d・悔r5ノ・・。rLICハ也…五i・3h−°・
Next, we consider the fbllowi皿g exact sequence0−一一%「乙一の』一一%r乙ノー一→0〆乙1の一一一〇・
This gives rise a cohomology sequence ・一一・°r・.・・S・(L−・刀一一・°r・.・。r・〃一一・°r・.・。r・1のノ一・卍r・。%口〃一一・…
一r五一σノ is of t)理)e r5,331,1,1,1,1)・ Since −rZ}_0) is an a町)1e divisor (see [4]〕, hi r・. os rD−・〃一・r・一・。・脚・h・舳i・a v皿・・h垣・・h・・r㎝,・h・輌・w・油・・ ・°r・.・。r・〃一ニー・°rら・〆・1・刀……r・) Since五 is a line, we get an exact sequence0−一一b%一一→.%ω一一一←()P・「−1ノー一一一〇・
竃1竃i鷺遷総籠三ll撫1㌫
w・・h・v・h°rら・6r・〃一・・聴c・mP・・tes th・p…f・Q・E・D・ LEMMA 2・ Z}et O Z)θ α geneヱu Z non−8ingZt 1αr cnt?ve Oアgenus 7 and deθree 8 Zying on●. Then irr;educib Ze・ubde SZt?faees contαining o have distinet 8ingu 1αr p(>ints at mosカ 2 0n O一 PROOF. Letぷ「be an irreducible cubic surface containing O.s(st=cuL.
36 ・T.ONO and Y. AKIMOTO Here, singular points of 5’which 1手e on o㎜st lie on O(五〇1.et.・日. be a general plane, such that『5’∩ H=1}u丑f」 where M is a conic. Then we have two cases: ・ . ” , 1). Caseヱ}≦t丑f. In fact, it is easy to show that: : 一 ,−三 ・ ・. ..:・:一・; . ・ the singiilar points of’5’which lie on O would then also・be singular points ofヱ}∪∬イ, and .「・ ・. . F ’ the singular points of 3「which lie on C would then a1三50 be points. of・Z}・∩M. .:・. 「 ’ .〉 ・ . ・.’ … Therefbre, we have the next inequality .・ . ・ ・.#{the singU1・r・POint・・f 3’・.砲i・h』1i・・n・ρ・}Sdeg・rL.M戊一2.. . 2). Case Z} ⊆M◆ In this case五 is the singular locus ofβ’. Therefore 5∩5’should contain五doub1γ, which is麺ossible fdT degree reas㎝. Q.E.D・ 一・: IHEORHN正. Let Cゐ¢・四ene”aτn・n−sZ⑭Zのcux}ve・f genus 7 Ctnd degree 8
ZW。ηα。。・一吻。Z□翔力Z・蹴恒・5£・カ㌦・・ゴ・・tiv・spaee P3. n・⊇・
。。r・m。Z加磁Z・・ア・蹴P㌔・・mZ.・鋤・.. ,、.・ .
PRCOF・1…d・・t・・h・⊇・t巫a i・・㎝i−st・b1…ince d・9噛3戊・4・i・’ i・suffi・i・nt t・・加w t地t旭3ノーha・n・ユine、 subb・nd1・・f d・gree at leaSt 3・ L・切be a 1血e subbund1・・f∬ξr3/and E’b・∬ξr3〃E・脆・・n・id・・the exact sequence, . . ・ ・ . :・一一一E−一一∬6倒一一E「一一一… \
麗、1蕊。二:ζ霊n6:1:eC麗;鵠罐、㌃蓋認。’ご
have followillg two cases. ’二ASPACE CURVE OF GENUS 7 AND DEGR旺i 8 37・ Case(1). S卿。,e・th・t・h°rC.・E,−0.1・this ca・e,・血ce・h°rらEり≧2 and O is not rationa1, we have∂reg E t >1. Eherefore we have〈海g E= . d・g Nc“r5」−d・g・E’ 一・4−d・σ・E’・3・・ C。,e(2〕. S・pP・se th・t hO rC,E,=1. Fi・・t,『鳩・・n・id・・th・di・g・am・ ・x・・E°・c・.・E)・L→E°rら哨・〃 ・ll㊥一1) E°・(P3.J,r・〃・ S・pP・・e・E H°rらE)i・an・n−ze・。 secti・・, th・n・d・t・㎝m・・acubi・・u曲・・ 5’(≠5/containing O and it is zeTo precisely at the singular pOints of tlle