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Homogeneousvarieties-zerocyclesof

degreeoneversusrationalpoints

R.Parimala

AbstractExamplesofprojectivehomogeneousvarietiesoverthefieldof

Laurentseriesoverp-adicfieldswhichadmitzerocyclesofdegreeoneand

whichdonothaverationalpointsareconstructed.

LetkbeafieldandXaquasi-projectivevarietyoverk.LetZ

0(X)

denotethegroupofzerocyclesonXanddeg:Z

0(X)→Zthedegree

homomorphismwhichassociatestoaclosedpointxofX,thedegree[k(x):k]

ofitsresiduefield.

Westudythequestion:forwhatclassesXofvarieties(respectively,what

classesoffieldsk)isittruethatforX∈X,ifXadmitsa0-cycleofdegree

one,thenXhasarationalpoint.

IfXisacurveofgenuszerooroneoveranyfieldk,thenXhasarational

pointonceitadmitsazerocyclesofdegreeone.However,thequestionismore

reasonabletoaskforclassesofrationalvarietiesorhomogeneousvarieties.In

thesettingofrationalvarieties,thereareexamples,duetoColliot-Th´el`ene

andCoray[CTC]ofconicbundlesovertheprojectivelineoverap-adicfield

withazerocycleofdegreeone,whichhavenorationalpoints.Weshalllist

fromliteraturesomequestionsinthisdirectionforhomogeneousvarieties.

Q(HP

r)(Veisfeiler)[V]LetXbeaprojectivehomogeneousvarietyunder

aconnectedlinearalgebraicgroupdefinedoverafieldk.IfXhasazero

cycleofdegreeone,doesXhavearationalpoint?

1Q(PHS)[Se]LetGbeaconnectedlinearalgebraicgroupdefinedoverafield

k.LetXbeaprincipalhomogeneousspaceforGoverk.IfXhasazero

cycleofdegreeone,doesXhavearationalpoint?

Thefollowingquestionscombinetheabovetwoinamoregeneralsetting.

Q(H)(Colliot-Th´el`ene)[To]LetXbeaquasi-projectivehomogeneousva-

rietyunderaconnectedlinearalgebraicgroupdefinedoverk.IfXhasa

zerocycleofdegreeone,doesXhavearationalpoint?

Q(Hd)(Totaro)[To]LetXbeaquasi-projectivevarietyunderaconnected

linearalgebraicgroupdefinedoverk.IfXhasazerocycleofdegreed>0,

doesXhaveaclosedpointofdegreedividingd?

Totaromentionsthatthemostreasonablecasesofhisquestionarewhere

XisaprincipalhomogeneousspaceorifXisprojective.

ThefirstexamplewhereQ(H)hasanegativeanswerisdueto[F];the

stabilizerofarationalpointoverthealgebraicclosureforthesehomogeneous

spacesisafinitegroup.

Inthispaper,wegiveexamplestoshowthatQ(HPr)hasanegative

answeringeneral.

§1Connectedness

Weremarkthatconnectednessisessentialwithrespecttothesequestions.

Thereexistautomorphisms(Colemanautomorphisms[HK])offinitegroups

withtheproperties:

1.fisnotinner.

2.foreverysylowsubgroupHofG,f|

H:H→Gisgivenbyf(x)=

y

Hxy−1

Hforsomey

H∈G.

2Theclassof[f]inH1(G,G)forthetrivialactionofGonGisnon-trivial,

butrestrictedtoeachp-sylowsubgroup,itistrivial.WethankP.Gillefor

bringingtoourattentiontheseexamples.

§2Principalhomogeneousspaces

ThecaseofQ(PHS)iswideopen,andinspecialcasesQ(PHS)isproved

tohaveanaffirmativeanswer.ThecasesofPGL

nandO

nareclassical;for

O

ntheresultgoesbacktoatheoremofSpringer[Sp].Thecaseofunitary

groupsissettledintheaffirmativebyEva-BayerandLenstra[BL].Apos-

itiveanswertoQ(PHS)whenkisanumberfieldisduetoSansuc[Sa].

AmainingredientintheproofistheHasseprincipleforprincipalhomo-

geneousspacesundersemisimplesimplyconnectedlinearalgebraicgroups

definedovernumberfields.ThereareconjecturesconcerningHasseprinciple

forfieldsofvirtualcohomologicaldimension2,duetoColliot-Th´el`enewhich

havebeenprovedforclassicalgroups[BP].OnecanshowthatQ(PHS)has

anaffirmativeanswerforprincipalhomogeneousspacesunderconnectedlin-

earalgebraicgroupsdefinedoverafieldofvirtualcohomologicaldimension2,

providedthecorrespondingsimplyconnectedgroupsatisfiesHassePrinciple

conjecture[P1]

§3Projectivehomogeneousvarieties

ThequestionQ(HPr)alsoadmitsapositiveanswerfornumberfieldsand

thiscanbededucedfromthetheoremofHarderonHasseprincipleforprojec-

tivehomogeneousvarietiesdefinedovernumberfields[H].Ifonefollowsthe

proofofBorovoiofHarder’stheorem,usingthenon-abelianH2ofSpringerto

studyhomogeneousvarieties,onecanshow,usingtheresultsof[CGP]that

ifkisa2-dimensionalstricthenselianfield,Q(HPr)hasapositiveanswer.

3ItisgoodtostudyQ(HPr)inthecaseof2-dimensionalfields.Apositive

answercanbederivedforclassicalgroupsoverC

2fields[P2].

§4Example

Inthissection,weconstructanexampletoshowthatQ(HPr)hasanega-

tiveansweringeneral.Thisexampleisarefinementofanexamplegivenin

[PSS].

Letkbeap-adicfieldcontainingaprimitivep-throotofunityξwith

p≥5.LetK=k((t)).Letℓ|kbeadegreetwoextensionwhichistotally

ramifiedoverk.Then|ℓ∗/ℓ∗p|>|k∗/k∗p|.LetL=ℓ((t)).Letµ∈ℓ∗besuch

that[µ]∈ker(N

ℓ/k:ℓ∗/ℓ∗p→k∗/k∗p)and[µ]=1inℓ∗/ℓ∗p.LetDbethe

cyclicalgebraofdegreepoverLdefinedby:

Xp=µ,Yp=t,XY=ξYX.

Itisrepresentedby(µ)∪(t)∈H2(L,µ

p).Wehave,bychoice,cores

L/K(D)=

1sothatDsupportsaninvolutionofsecondkind.LetτbeanL|Kinvolution

onD.Letλ∈k∗besuchthatλ/∈N

ℓ/k(ℓ∗).Lethbetherank3hermitian

form󰀍1,−λ,t󰀋over(D,τ).Thenwehavethefollowing:

LemmaThehermitianformhisanisotropicover(D,τ).

ProofLet∆=󰀅

a∈D:Nrd

D|L(a)∈l[[t]]󰀆

betheuniquemaximalℓ[[t]]-

orderinD.Everyelementaof∆canbewrittenasa=πnb,wherebisa