"

PR■TFTBUNGSOFTHE
A凶F回uC▲HMAnBEMATuCALsoC1ETv
vblmtelZO・NumbET2.F■bm□Jy1994
NON-COHEN-MACAULAYSYMBOLICBLOW-UPS
FORSPACEMONOMIALCUIWES
ANDCOUNTERExAMPIESTOCOwSIKSQuESrlON
SHIROGOTO,KOJINISHIDA,ANDKEI-ICHIWATANABE
(CommunicaledbyEricFnedlandeT)
AnsTRAcT・LetA=AIIJr,γ、Z]]andkⅡ、]befbnnalpowersenesnngs
ovemneldと,andletn>4beanime8ermchlhat〃差Omod3・陸t
o:八一A【[nidenotethehomomomhismo「昨algebmsde6nedbyp(X)=
77n-コ,。(y)=TW-z)河,and。(Z)=781,-3.WepuIp=KCT。、Then
Rn(p)=①l三op(i)isaNoetheriannngi「andonlyircM>0.Henceon
COw3ikosquestionthcに画℃counIeにxamplesamongthepnmeidcalsdenning
spacemonomialcurves,too.
LINTRoDucTIoN
I心t』=k[[X,Y,Z]]andS=k[[刀]befbrmalpowersenesrmgsovera
he1.k,andlet〃,,〃2,and〃3bepositiveintegc応withGCD("I,〃1,〃〕)=1.
Wedenotebyp(",,〃2,〃3)thekemelofthehomomolphismp:八一sof応
algebmsdefinedbyO(X)=T"1,。(Y)=T"2,and。(Z)=か,、Hence
p=p("肋〃1,〃3)istheextendedidealintheringAofthedehningidealfbr
themonomialcurvex=!"1,J=ln2,andz=rm,inAii・
AUttlemorcgeneraUy,letpbeaprimeidealofheight2ina3-dimensional
r巳gularlocalringA・WeputR`(p)=E">op(")T〃(herETdenotesanin‐
determinateoverd)andcaUitthesymb6iicReesalgebraofp・Then,as
isweUlmown,R`(p)isaKrullringwiththedivisorclassgroupZ,andif
RS(p)isaNoetherianringitscanonicalmoduleisgivenby[R,(p)](-1)(Cf,
e,8.,[12,CoToUary3、4]).ConsequenUyR,(p)isaGorensteinring,onceitis
COhen-Macaulay、TheTeadeTsmayconsult[2,3]fbracriterionofRs(p)being
COhen-Macaulay,wheにsevemlexamplesofprimeidealspwithGorenstein
symboUcReesalgebmsaJEexplomed,too・
Nevertheless,aswashrstshownbyMorimotoandthefirstauthor[8],R`(p)
arenotnecessalilyCohen-Macaulayevenfbrthespacemonomialcuwesp=
p(",,〃2,〃]).ThisrBseaJchisasuccessionto[8]andtheaimistoprovidethe
fbllowingnewexamples.
Receivedbytheedjto届January22,l9Z2and,hMUvisedfbrm,May28,1912.
1991A血KA2mpUicsSbubjepCⅢエfsi〃mIio"・PrimaIyl3HO5,l3Hl3,l3H15;Secondmyl4H20,
14J17.
Theauthorsa定paniallysupporledbyCmnt-in-AjdlbrCO-opcmliveRescar℃h、
、I994AmmcmMmhemmcnlSo句eoy
OOOZ・”W94SLOO+3.29perp■GE
383
牛ⅡローIrtJ1■■・・・]④エヨ0口-・早足凸い七勺・・□rr0■砂1㎡殆・・Tチー引汁Ⅲ岬’‐(▼が西
s,GOT0,K.NISHIDA,ANDlLwATANABE
384
Theorem1.1.Cho“eα〃i"(…「〃z4solhar〃姜Omod3q"。”lP=
p(7"-3,(5"-2)皿8"-3).Lelc=Y3-X"Znα"。(zsFⅢmelAalchA=P>0.
711I2nlhe花2xjslsq〃eleme"【AEp(3p)saljqb'i"glAee9Ⅸα"ly
lengnhU/(x,c,A))=3p・lengthW(x)+p)
1F・・八・千・・・.叩小咄排朴r・LJⅡかく、命f】1Jか‐P設いJIL、嵌喉凹肝血幸一㈹肛別仏器篭眠Ⅲ、皿睡攝払、Ⅲ‐隅脚fu咄蝿即懸山畑砥Ⅲ吋聰鰯幡ⅡⅢ鋤
ノbrα'、,xE(X,Y,Z)Alp.】"pα"たwlmR`(p)isαノVb2lhemz〃rmg6m〃α
のhe汁Mzczzl(lbzy・
Heにletusnotethattheidealpintheabovelheorcmisgenemtedbythe
maximalmino応ofthematrix
(’V郷:二|);
hencepisaselfL1inkedidealinthesenseofHerzogandUlrich[6,CoroUaly
1.10】,thatis,p=(ノ,g):pfbrsomeelemems/,gEp・Roughlyspeakins
seMLlinkedidealsenjoymoreexceUentnaturesthanthoseoftheidealswhich
aICnotselfLlinked、FOrmstance,thesubringsA[p7,p(2)T2】ofR`(p)are
alwaysGorenstein,ifthecoITespondingidealspa泥selfLlinked(see[6,Proof
ofTheorCm2.1]).Fromthispointofviewitseemsmthernaturaltoexpectthat
thewholeringsR`(p)areGorensteinatleastfbrselfLlinkedidealsp;however,
theanswerisnegativeasweclaiminTheo応、1.1.
。■ロロⅡ■■11|ケト■■■PIBL2■UⅡ日日p■・FUI■■IⅡ.Iir
1,[8]Morimotoandthehrstauthorconstmcted,fbreachprimenumberP,
spacemonomialcurvespwhosesymbolicReesalgebrasR`(p)areNoetherian
butnotCohen-Macaulay,ifthecharacteristicchAofthegroundheldlcis
equaltop、Neverthelesstheirexamplesare〃olselfLlinkedand,fmrthelmore,it
isnotclearfbrtheirexampleswhetherRs(p)areNoetherianornotinthecase
whe妃thecharacteristicisdiHbremfiomthegivenprimenumberp,whileour
examplesareNoetheriana、dnon-Cohen-Macaulaywheneverchk>0.
ThisadvantagenatumUyenablesus,passingtothereductionmoduloprime
numbe応,toexplo定thecaseofchamcteristicO,too,MoreoveT,somewhat
surprisingly,asanimmediateconsequenceofTheorem1.1wegetthefbllowing
counterexamplestoCowsik,squestionI1],thataskedwhetherthesymbolicRees
algebmR`(p)beaNoetherianringfbranyprimeidealpina定gularloca1ring
A:
COroUmy1.2.L21p62aprim2iUlbzzノslaleu!』〃、CO花、1.1.7111mR`(p)jS
nolqノVbaAmα〃mUgbヴchk=O
WhenCowsikraisedthequestion,heaimedaIsoapossiblenewapproach
towardtheproblemposedbyKronecker,whoaskedwhetheranyirrCducible
allinealgebmiccurveinAecouldbedehnedbyn-1equations、I、fact,
Ⅶ波.‐研玲・如叩・I.n㎡.
‐DL0000■Ⅱ□らJ■仏叫Eu昨Pu・b9IⅢIDIⅢ
:
:
Cowsikpointedoutin[1]thatpisaset-theo正ticcompleteinte唾ction,if
R`(p)isaNoetherianringandifdimA/p=1;however,asisweUknown,
whileKroneckeT、sproblemremamedopenonCowsick,squestiontherewas
al肥adygivenacounterexamp1ebyRoberts[,].Becausehislirstexampledid
notrCmainpnmewhenthelingwascomp1eted,herecentlyconstmctedthe
secondcounterexamples[10]・Theyareactuallyheight3primeidealsinafbrmal
powerseriesringwithsevenvariablesoveraIield;now,providingnewand
simplercounterexamplesamongtheplimeidealsinthefbnnalpowerseries
nngQ[Pr,y,Z]],ourCoroUaryLZseltlesCowsik,squestion,thoughitsays
nothingaboutKronecker,sproblemitself
 ̄
NON厄OHEN・MACAUlAYSYMBOUCBLOW・UPS
。85
TbesimplestexamplegivenbyTheoremL1istheprimeidealp=
p(25,72,29)=(XIl-YZ7,y3-X4Z4,ZI1-X7Y2)in』=IC[[X,Y,Z]]、
ThesymbolicReesalgebraRs(p)isaNoetherianTingbutnotCohen-Macaulay,
ifcM>0,andRs(p)isnotaNoetherianring,ifcM=0.Therefbre,the
symbolicblow-upR,(P)=Z">oPい)T〃isnotahnitelygeneratedQ-algebm
fbrtheprimeidealP=(XlI-YZ7,y3-X4Z4,ZlI-X7】'2)inthepoly‐
nomialnngO[X,y,Z],too
NowletusbneOyexplainhowthispapeTisorganized・TheproorofTheorem
1.1andCoroUaryl,Zwillbegivenin§4.Section3isdevotedtoareduction
techniquemoduloprimenumber5.1,§ZweshaUsummanzesomepldiminary
stepslhatweneedtoproveTheorem1.1andCorollary1.2.
Otherwisespecified,inwhatfbllowsletA=k[[X,y,Z]]denoteafbrmal
powersenesrmgoverafixedfieldk・止tm=(X,Y,ZMbethemaximal
idealofA・Wedenoteby2AW),fbranA-moduleノV,thelengthofノV、FOr
agivenprimeidealpinAweput
R`(p)=ZP1jD7mcA[刀,
、>O
Rj(,)=Zp⑩17m(=R`(p)[丁-1])こ」[7,T-IL
jHEZ
Gs(p)=R;(p)/丁~IRj(p)
whereTisanindeterminateoverA.
2.PRELIMINARIES
FirstofallletusrecallHuneke,scriterion[7]fbrR`(p)tobeaNoetherian
rin8FbrawhUeletい,、)denotearegularlocalringofdim』=3andlet
pbeaprimeidealofAwithdim』/p=1.
Theo肥、2.1[71.〃rhaで2xjs『雄me"砥/Ep(I)α"dgEp(、)wilhp“irjve
j"legF7Tノα"dms84垣hlノ、【
2A(』/(x,/,g))=ノ、.cAu/(x)+p)
ノbrmmeXEmlp,肋enR`(p)jSdzノVDah2rm〃〃"gリゲリiemAeノieItfA/mjS
m/i"ilelrhecD"ve応ejSdMsolme、
ThenextpropositionallowsustoarbitrarilychoosetheelementJrinTheo‐
rem2・LTheresultisimpliciUyfbundin[7,ProofofTheo1℃、3.1】andisdue
toHuneke
Pmposition2.2.Lα/Ep(ノ)α"‘gep(、)(ノ,m>O)α"‘αmswmerAm
2A(』/(x,/,g))=ノ、・2」W(x)+p)
ノbrsomexEmlp.〃enlhe”ovee984zz"〃holtisノb「α"yxEmlp・
PrUqfWemayassume/=、、Then(/,g)Aisareductionofp(、);actuaUy,
[p(、)]2=(ノ,g).p(、)(Cf[Z,PToofofProposition(3.1)]).laR=Apand
n=pAp・Thenastheideal(/,g)Risareductionofmmandas(R,、)isa
2-dimensionalregularlocalring,weget
2尺(R/(八g)R)=ど鵬)R(R)=elh(R)=、2
ポニ
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四項廷Z式房迂参一出口z定ぐ口冨圏z望.CPC、.、
H・・一
霧蕊
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;11艸可=」七二=話邑~里由
NON・COHEN・MACAULAYSYMBOUCBLOW・UPS
387
NowrCcaUthatc=γβ+β,mod(X)andweget
2Au/(X,c,9192))=(β+β').g`(』/(x,y,9,82))
=(β+β')・2AU/(X,y,go))=m0.2Au/(X)+p).
Hencetheelememscandglg2satisfyHuneke,scondition(2.1)sothatc
andglsatisfythisconditionasweUlnfact,because2A(A/(X,c,9,唖))=
2A(A/(X,c,8,))+2A(』/(X,c,92))=m0.2列(A/(X)+p),wegelbythein‐
equalities2A(」/(X,c,91))三i・[’1W(X)+p)and2A(A/(X,c,唖))三
(耐o-i)・gAU/(X)+p)thatノ`,(A/(X,c,9,))=j・山(A/(X)+p).Thuswe
havejE夕,whichcontmdictstheminimalityofmo・
ThenextresultisthekeyinouTproofofCorollary1.2
ComUmy2、6.ASSWmelhqlβ+β'=3α"dlAaj31〃,、Ldpbeap"m2mJ碗be「
ノbrMijdiwe(zsswmelhα〃h罪axmanekme"lgEppp)sα【幼mglAe珂凹α"〃
2A(」/(X,c,g))=3p・(`,(』/(X)+P).、“ヴgT3ped[{p(")T''},≦,,<卯],we
hqvemo=3口"。R`(p)=A[p7,p(Z)72,p(3)T3l
PmqfAs2AU/(X)+p)=〃,andasc=Y3mod(X),wehavethat
2A(A/(X,c,go))=、on,=3.2A“/(X,Y,go)).
Hence31mo,because31〃Ibyourassumption・Iamo=3m,、Thenas
mol3pby止mma24,weseemI=1oTp・Ifmlwercequaltop,thcnmo=
3PandsowemusthavebyProposition2.5thatgT3PEAI{p(")T"},≦"<3pL
whichcontmdictsourstandardassumption,Thusmo=3andwegetby[2,
Proposition(3.7)(4)]thatRs(p)=A[p丁,p(2)T2,gOT3].
3.REDucTIoNTomEcAsEwHEREchA>O
Ia〃山〃2,and〃3bepositiveintege応withGCD(",,〃2,〃3)=1.Let
C=Z[X,Y,Z]andZ[nbepolynomialringsoverZ・Wedenoteby9D:C一
Z[刀thehomomorphismofrmgssuchthatp(X)=T"',。(Y)=T"ユ,and
。(Z)=T"3.匹tI=KerpandconsidertheringCtobeagradedringwhose
graduationisdehnedbyCb=Z,qIヨX,Q2ヨァ,andChjヨZ・Then
thehomomonphismppreservesthegraduationsothatJisnatumllyagmded
pIimeidealinC、
FOragivenheldk,4k=k[[X,Y,Z]]denotesafbrmalpowersenesrmg・
WeputBk=kIjr,Y,Z],thatweshaUidentifywithA②zC、止tpjtdenote
theidealp(",,〃1,〃〕)conside冗dinAA;hence,pJt=“〃WeputBt=Jα・
ThusPhisthedenningpnmeidealofthemonomialcuwex=r"',y=『"2,
andz=["jinAi,
Thepurposeofthissectjonistoprovethefbllowing
Theorem3.1.A"ⅨmeZharR`(pq)jSqノV02lherWz"rj"8,11“ピルe庵exな【p“"jv2
j"【率晒ノV,、α"。e佗me"us/,gq/J(、)swcMiat〃p曲aprjme”m62r
α"。〃pzjv,wEaノMuys他泥
(1)p『)=J(噸)。&。"‘
(2)2Ak(`4k/(x,/,g)Aと)=、2.2A‘(八k/(x)+pk)
ノbr1heノi2IWEZ/pZ、
PmqfWeputB=80,P=化,and〃=(X,γ,Z)β、ThenasA=
AQisafmhfUUynatextensionofBM,thesymbolicReesalgebraR`(PBM
li1
388
!
S・GOT0.KNIgHUDA・ANDXWATANABE
isaNoethelianlingtoo;therCfbre,byTheorem2・landPToposition2・Zwe
maychoosetwoelements/,gE(PB」v)(、)withmapositiveintegersothat
2B"(BM/(X,/,g)B")=伽2.2恥(β"/xBM+PBM).LetⅣ=(X,γ,Z)C、
ThenasCAr=B〃andJqv=PBAf,wecantake/andgtobeinノ(、).
陸tsECVbeanelementsuchthatsI(、)CJm・WeexpandO(s)=
ELsjTjinZ[T]withsb≠OandchooseapnmenumbcrpsothatPl3..
Thentheelement了=1②sofBk=k②zCisnotcontainedinPi=ノBAand
5沖)BjEJ碗必=JW0・HenccJ(凧)BkEB(碗).
WenowTecaUthat[(PB")(、)]z=(/,g)BJM・(PB刑)(、)(Cf[2,Proofof
Proposition(3.1)])andchooseanelement5=ご(X,Y,Z)ECVVsothat
§・[I(、)]2E(ノ,g)C・I(、).ThenifpisaprimenumbeTandifpl5(0,0,0),
theelement§=1②:ofBkisnotcontainedinMルー(X,y,Z)BAand
F・[”)Bk]ユニ(/,g)Bk・I(、)Bk・Thuswehave[I(、)。k]2=(/,g)」ルノ(''Mk
indk,
Letuschooseapnmenumberpsothatpisanon-zero-divisoronD=
C/XC+J(、)(thischoiceispossible,becauseAsszDisanniteset).Thenas
Xisanon-zem-divisoronC/I(、),wegetacommutativediagmm
O
I
O-C〃(、)当C/J(、)-D-O
pIP↓p1
O-C〃(、)当C〃(、)一D-O
withexactrows・ConsequentlyXisanon-ze「o-divisoronBk/I(、)Bk,while
Pl:=、/7T祠万万because〃二J(、)BにBk;theにfbrc,thegradedidcalJ(、)Bk
is且-primary(noticethatdimBk/J(、)Bk=1,asdimBk/PX=I)andsowe
havCthatPW")EJ(碗)Bk、
SummarizingtheaboveobservationsandchoosingapnmenumbeTpsothat
PISb,Pl《(0,0,0),andpEUqE…q,wegetthatpF1=”]Albandthat
[pFml]2=(/,g)AI`W1fbrthehcldk=Z〃Z・Th…condasscrtion(2)now
fbUowsfiommeequality[pF1]ユー(/,g)」ルpF1simUmlyasinthcpm・fof
Pmpositionユ2.ThiscompletestheproofofTheo肥、3.1.
4.PRooFoFTHEoREML1ANDCoRoLLARY1・Z
Letn≧4beanintegersuchthat〃差Omod3andletkbealield・In
thissectionweexploretheprimeidealp=p(7〃-3,(5〃-2)恥8〃-3)in
』=k[[X,Y,Z】]・ThepurposeistoproveTheoreml・landCoroUary1.2.
Firstofallr己callthatpisgeneratedbythemaximalminorsofthematrix
(顎二葱二|)
(Cf[5]).WeputQ=Z3"-1-Xz"-1YZ,6=X3n-l-YZz''-1,andc=
y3-X風Z〃(hencep=(α,6,c)).NoticethatanypairofzJ,6,andcfbrms
aregularsystemofparametersinAp,sincethereistheobviousrclation
(4.1)X"α+YZb+Z2"-IC=0.
NONCOHEN・MACAUしWSYMBOUCBLOW・UPS
389
Let
d2=X"-ly5Z"-l-3Xユ"-1YzZ2'1-1+X,"-2y+Z5"-2,
.3=_X]"-ZY7+2X"-lY5Z3"-1+X4"-ZY4Z〃
-5jr2Jv-lY2Z4"-1+3X5"-ZYZz'l-X8"-3Z+Z7"-2,
.』=Y8Z2"-2-4X"Y5Z3"-2+X4"-1Y4Z"~!+6Xz"y2Z4"~z
-4X5"-lYZz"-1+X8lu-2-XZ7"-3.
ThenadirCctcomputationeasilychecks
X"d2-y62+Z'w-lqc=0,
(4.2)Xn-Ib2c+qdz-Z"-ld3=0,
Xd3+Ybc2+ZdB=0.
HcncediEp(2)andd3,djEp(3).
ThenextlemmaisaspecialcaseofmuchmoTegeneralresults(Cf[3,CoTol‐
1ary(2.6)]and[4,Theo妃m(5.4)]),however,webrieflygiveadiJEctplDoffbr
thesakeofcomplcteness・
陸mma43.p(2)=p2+(。z)α"。p(3)=pp(2)+(。〕,。』).
PmqfAs(X)+p2+(。h)=(X)+(】'6,y4Z2"-1,y3Z3n-l,Y2Z4"-2,Z5"-2),
weget
2八(」/(X)+pZ+(の))=3(7〃-3).
Ontheotherhandbytheadditivefbrmulaofmultiplicityweget
[ん“/(X)+p(2))=24,(AP/p2Ap)・2A(A/(X)+p)=3.(7〃-3).
Hence2A“/(X)+p(z))=2Au/(X)+pz+(d2))sothatwehave(X)+p(2)=
(X)+p2+(ぬ),whichimpliesp(2)=p2+(。Z)+Xp(2).Thusp(2)=p2+(。z)
byNakayama,slemmaConsequently
(X)+pp(2)+(d3,.,=(X)+(Y,,Y8Zm-2,y7Zz"-1,Y6Z3"-1,
y5Z4"-Z,Y3Z5"-2,yZ7"-3,Z7"-2),
whence
2A似/(X)+pp(2)+(ぬ,。』))=6(7"-3).
BeCaUSe2A仏/(X)+P(3))=均,(AP/P3Ap)・(』仏/(X)+P),WehaVe
2`(」/(x)+p(3))=2`W(x)+pp(2)+(。〕,。』))
and,thus,p(3)=pp(2)+(。],`』)asrEquirCd
Pmposi回0,4.4.,2㎡"9A/(c)+p(3)心〃αCDAe"-MzcmUm'.
n℃q八匹teh“/(c)+p(3))dcnotethcmultiplicityofXAinA/(c)+p(3).
ThenasAp/cApisaDVR,wegetbytheadditivefbrmulaofmultiplicitythat
e3`W(c)+p(3))=3(7"-3).
Ontheotherhandwcgetbylだ、ma4.3anditsproofthat
(X)+に)+p(3)=(X)+(Y3,YZ7"-3,Z7"-2).
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NON-COHEN・MACAULヘYSYMBOLICBLOW-UPS
391
wechooseapnmenumberpsothatpZmax{Ⅳ’2m/3}andchoosean
clcmcmhofpF,)sothatf劇催いk/(X,c,力))=3p・(7"-3),whercc=
Y3-X"Z〃imdk(thesecondchoiceispossiblebyTheorem1.1).Thenboth
thepairsc,hand/,gsatisfyHuneke,scondition(2.1),Whencewegetby[Z,
Proposition(3.1)(2)]that
G+=
(cT,AT]P)G
(/Tm,gTm)G
whereG=q(p)andG+=Zj>oGi,Consequently/Tm,gTmisaG-regular
sequencebecausesoisthesequencec7,hT3pbyl2,Proposition(3.7)(3)]、
Hencewehave
(/,g)np11=(/,g)・p1-'''1
fbralliEZ,while(/,g)。pr碗-1)by[Z,Proposition(3.4)];thus,p1)=
(/,g)・p1-“)fbrallj〉Zm-LThisparticularlyimpliesRs(p)=
A[{p11Tj},≦…_ルノT碗,9丁碗]sothatwChaveAT3,E』[{p11Tj}!≦…].
ThusAT"EAI{p1)Ti}!≦i<3小because2m≦3pbyourchoiceofp・Sincc
317〃-3byourstandardassumption,wegetbyCoroUary2、6thatR3(p)=
』[p7,p(z)T2,p(3)T3].Consequentlyoneoftheconditionsstatedin[4,Theo‐
rem(6.1)(2)]mustbesatishedfbrthedataα=恥β=1,γ=恥。'=2"-1,
β'=2,a、。γ'=2"-1,whichisobviouslyimpossible・HenceR`(pk)cannot
beaNoetherianring,ifchk=0.ThiscompletestheproofofCoroUary(1.2).
Remark4、5.丁hesameproofworksfbrthcfbUowingexamples,too・Let〃Z
Sbeanintegersuchthat317〃-10and〃菫-7mod59・唾tp=
p(7〃-10,5"z-7〃+1,8〃-3).ThenRS(p)isnotaNoetherianring,if
cM=0.Thesimplestexampleofthiscascisthepmmeidealp=p(25,91,37)
=(Xl4-YZ7,Y3-X5Z4,Z11-X9Y2)inQ[[X,y,Z]]・ThecorTcsponding
matrixisgivenby
(李郷)
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DEPARTMEluToFMATHEMATucs,MEuJHUIwuvERsuTY,HIoAsHIMmh、TAMA-Ku,KJLwAsAnu-sHl,
JAPAN
GRADuATEScHooLoFScIEUwcEApIDTEcHNoLooY,CHuBAUNIvERsITY、YAYoI-cwo,CHIBA・sH1、
JAPAN
DEPARTMENToFMATnuEMATIcALScIENcEs,TOxJuUNuvERsITY,HlRATsuKA-sHu,KANAcAwA,
JAPAN
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