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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
form1,−λ,tover(D,τ).Thenwehavethefollowing:
LemmaThehermitianformhisanisotropicover(D,τ).
ProofLet∆=
a∈D:Nrd
D|L(a)∈l[[t]]
betheuniquemaximalℓ[[t]]-
orderinD.Everyelementaof∆canbewrittenasa=πnb,wherebisa