VOL. 17 NO. 3 (1994) 457-462
PRODUCT INVOLUTIONS WITH
0OR 1-DIMENSIONAL
FIXED POINT
SETS
ON ORIENTABLE
HANDLEBODIES
ROGER B. NELSON
Department
of Mathematical Sciences BallState
University, Muncie,IN
47306-0590(Received August 20, 1992 and in revised form July 19, 1993)
ABSTRACT.
Let hbeaninvolutionwitha0or1-dimensionalfixed pointseton anorientablehan-dlebody
M. We
show that obvious necessary conditions forfiberingM
asA
xI
sothat h r x"
with r an involutionofA
and r reflection about themidpoint of l also turn out to be sufficienl.We
alsoshow that sucha "product" involution isdeterminedbyitsfixedpoint set.KEY
WORDS AND
PHRASES.
Orientablehandlebody, involution,invariant disk,fixed point set. 1992AMS
SUBJECT CLASSIFICATION CODES.
Primary57S17, 57S25.1.
INTRODUCTION.
Kim and Tollefson
[1]
showedthat ifA
isacompactsurfaceand hisaPL
involution ofA
xI
where
h(a
x01)=
A
xOI
(I
[-1,
1]),
then thereexists aninvolution9of surfacea
such that his equivalentto the involutionh’ ofa
xI
given byh’(x,t)
(9(z),
(t))
for(z,t)
Ea
xI
and(t)
or -t. Thatis, hisaproduct
involution. WhenA
isaclosedsurface the conditionalwaysholds;
allinvolutionsarethen productinvolutions. WhenA
has boundary,A
xI
is ahandlebody.For
ahandlebodyM,
onenaturally asks forwhich involutionsafiberingexistssothatthe conditionholds,
or equivalently,which involutionsareproducts.The question is answered by Nelson
[3]
for involutions of orientable handlebodies with 2-dimensional fixed point sets.In
thatcase, obvious necessary conditionsare shown to be sufficientas well.
In
this paper, we show that such is also the casefor orientable handlebodies carrying involutions with 0 or 1-dimensional fixed point sets.In
this setting,weshow that clearnecessary conditions are also sufficienttoguarantee that aninvolution h ofan orientablehandlebody M
is equivalenttorxronA
xI,
whererisaninvolutionofA
andr(t)
-t.We
alsoshow that these involutionsarecompletely
determinedby
their fixedpoint sets.In
this paperallspacesandmapsarepiecewiselinear(PL)
inthesenseof Rourke and Sanderson[5].
Notation generally follows[5, 3].
An
involution h onX
is a period two homeomorphism h"X
X.
The orbit space of involutionhisX,
X/
x wherex c yifand only ify x orh(x);
theprojectionmap isdenoted byph. Thefixed point setof hisFzx(h)
{x
c= Xlh(x
x}.
existsahomeomorphism
f
X
.’
suchthat hf-1
oh’of.
Recallthat ahandlebodyofnisaspace obtainedby choosing2ndisjoint disks in theboundary ofa3-ball andidentify,g in pairsby n
PL
homeomorphisms.Two
handlebodies are homeomorphicif and onlyiflheythesamegenus andareboth orientableorboth nonorientable.
2.
TECHNICAL LEMMA.
Ifhistobeaproductinvolutionof formrx r, then certainly
Fzx(h)
iscontained inA
x{0}.
Accordingly,wesay that asubset
S
ofM
isA-horizontal inM
ifM
A
xI
andS
canbe isotope(tinto
A
x{0}.
Ofcourse, not all systems of pointsor arcs and loops canbe the fixed point s(’! ofsomeinvolution ronA. Ifasubset of
M
isisomorphic to the fixed point set ofsomeinvolutio1A,
we say it is A-fixable.Note
thatallinvolutionsofacampact borderedsurfaceA (and
hence all A-fixablesets)
areeasilylisted[3].
Clearly,ifH
isequivalentto rx r, thenFx(h)
must be both A-horizontalandA-fixable.Constructinga
fibering
forhandlebody M
asA
xI,
with respectto which which involutionh isaproduct,isaninductive process.We
decomposeM
into"smaller" h-invariant pieces whichmaybeassumed,by induction hypothesis,to carrysuchastructure. Thefiberingof
M
isreconstructed from thefiberingson thepieces. The following lemma enables us to carry out the crucial stepofbreaking
M
intoh-invariant pieces.Fixedpointsetsofinvolutionsoncompactsurfaces have theproperty thatonecanalwaysraise
theEulercharacteristicofthesurfaceby cutting
along
an arc which either misses the fixed point. setorcrosses afixed point loopinexactlyonepoint. If hisaninvolutiononhandlebody
M,
thenFix(h)
being both A-horizontal and A-fixableclearly implies that the genus ofM
canbelowered by cuttingalong
aproperly
embeddeddiskmissingFix(h)
orcuttingafixedpoint loopinexactly one point. The following lemma shows that this can be accomplished equivariantly. Following Theorem 1,it will alsobeclear that thiscutting property implies thatFix(h)
isbothA-horizontal and A-fixable.LEMMA
1.Let
M
be an orientable handlebody with involution h having either 0 or 1-dimensional fixed point setFix(h).
Suppose
also thatFix(h)
is bothA-parallel
and A-fixable.Then thereexistsa
properly
embeddednon-boundary-parallel
diskD
suchthat eitherD
orelse
D
is h-invariantand cutsafixed pointloop
inasingle
point.PROOF:
A
diskD
inM
is inh-general
position moduloFix(h)
ifD
and hD are both ingeneral
position withrespecttoFix(h)
andifD- Fix(h)
andhD-Fix(h)
areingeneral
position.We
alwayspresume that a diskD
is inh-general
position and alsothat(D- Fix(h))V
OM
and(hD
F,x(h))
V10Marein generalposition.We
may,asnotedabove,
assumethat thereexists properly embeddednon-boundary-parallel
diskD
inh-general
position.Although D
f3hD isformally
asubgraph
ofthe 1-skeleton of thetriangulationofM,
wewillnotneed to consider
D
f3hDwith thisdegree
of detail.Instead,
weregard D
f3hD has consistingofA-objectswhere
(a)
aA-loop g
isacycle
ofedges
formingasimpleclosedcurve componentor suchWe partition theA-objectsinto
(i)
loops,(ii)
arcs withboth endpoints inFix(h)- OM,
(iii) arcswith neitherendpointinFiz(h)UOM,
(iv)
arcs withboth endpointsinOM
and(v)
arcswithoneendpoint in
OM
andthe other inFix(h).
Note
thatarcs of type(ii)
do not actuallyoccur.A
A-object is called extremalon
D (or hD)
if it either bounds or cobounds with other A-objects adisk
E
onD
(or
hD)
theinteriorofwhich intersectsnoother A-objects.This proof requires the
repeated
"simplification" of a diskD
by removal of A-objects fromD
f3hD.Although
removal ofaA-object canalways
be accomplishedwithoutaddingany pointstoDVlhD,
thenatureofthecomponentsofDnhD maybechanged
sothataA-loop iscreated by the removalofaA-arc. Accordingly,
define the complexity ofadiskD
by
c(D)
(a, l),
wherea isthe number ofA-arcs
and the number ofA-loops
inD
flhD. The ordered pairsarelexicographically orderedsothat removal ofaA-arc
always lowers thecomplexity.
Let E
be the collectionof non-boundary-parallel disksproperly
embeddedinM
and missing fixed pointarcs. ChooseD
EE
such thatDfqFiz(h)
is minimal. Thehypotheses
regardingFi:r(h)
make this intersection contain at mostonepoint. ThereplacementsofD
bya "simplified"D’
EE
will not addto this intersection.
Ifanextremal A-objecton hDisof
type
(i)
or(iii),
the the objecteitherbounds,
orcoboundswithother
A-arcs,
adiskE
inthe interiorofhD. Thesecases arehandledinKwun
and Tolefson[2].
D
is isotopic to diskD"
E
wherec(D")
<
c(D)
by the procedureof[2,
Lemma
1].
Theonly remaining A-objects,oftypes(iv)
and(v),
aretreatedbelow.Iftheextremal A-objecton hDisatype
(iv)
arca,then aU/’,
fl’
anarcinhDV10M,bounds
adisk
E’
on hD.Also,
aU,
anarconDVIOM,
boundsadiskE
onD,
whereE
hE’.Suppose
U
/’
boundsadiskE"
onOM.
ThenE
UE’U
E"
bounds a3-cellC
withfaceE"
inOM
suchthatN(C)
f3hN(C)
,
whereN
denote asmall regular neighborhood.Let
Gt
bean isoptoy ofM,
withsupport inN(C),
whichmovesE through
C’,
/across
E"
andE
offofE’. Let
D’
GI(D)
andthen
c(D’)
<
c(D).
If
/U/’
does not bound a disk onOM,
thenD’
E
UE’
is a nonboundary-parallel
diskproperly embeddedin
M
suchthatD’
ClhD’
t.
To
this pointwehavedemonstrated that we canfindD
E
such thateitherD
tqhDI
or elseD
ClhD consistsentirely oftype
(v)
arcs.We
needonly
show that from the lattercasewe can constructan h-invariant disk in Pcontaining exactlyonefixed point.Suppose
DfqhDconsistsonlyoftype(v)
arcs. SinceD
wasoriginally chosensothatDf3Fiz(h)
wasminimal, andsincethe above simplifications ofD
addno newpoints toD
f’lhD,
allofthe type(v)
arcsemitfromasingle
fixedpoint.D
istherefore partitioned into"wedge shaped"
regionsE,
boundedby type
(v)
arcsa,_ and a, andarcfl,
onOD.
Similarly, regionsE
on hDarebounded bya,_, a,, andfl,
wherefl
COhD.Definean equivalencerelationon
{E,}
byE,
E
iffE
hE,. Eachequivalenceclass has twomembers.For
eachclass,
replaceeitherE,
byE
orE
byE.
The resultisanh-invariant diskD
containingexactly
onefixed point.We
neednowonlynotethatD
isnotboundary-parallel.
Ifitwas
boundary-paralled,
itwould coboundaninvariantballwithanh-invariantdiskonOM.
This disk contains afixedpoint.Bu
this isclearly
not possible unlessD
cuts afixedpointarcinstead3.
MAIN THEOREM.
THEOREM
1.Let
M
beanorientablehandlebody
with involutionh having0or1-dimensional fixedpoint setFix(h). Suppose
alsothatFix(h)
is B-horizontaland B-fixable whereM
%B
1.Then thereexistacompact, bordered surface
A
such thatM
A
I
and h isequivalent tor r.wherer isanonidentityinvolutionof
A
andr(t)
-tforall EI
[-1, 1].
PROOF: Let
D
be the disk providedby Lemma
1. EitherD
hDorD
3hD.
In
either case, letM’
M-
N(D)- N(hD)
whereN(D)
andN(hD)
aresmall regular neighborhoods ofD
and hDchosen to coincide(be h-invariant)
whenD
hDand tobedisjointwhenD
3hD.
Either
M’
is a connectedhandlebody
with induced involution h’ or elseM’
M_I
t3M1
whereM,
is invariant with induced involution h,orh(M,)
M_,.
In
the case whereM’
isconnected,g(M’) < g(M),
otherwiseg(M,) < g(M),
-1,1,N(D)
’
D
[-1,
1]
andwedenoteby
D,
thedisk
corresponding
toD
{z}
onOM,,
OM’,
orON(D).
Thedisks(hD),
aresimilarly identified. The proof now follows by inductionon thegenusg(M). We
first handle independently the caseswhere the desiredproduct structure cannotbeassumedbyinductionhypothesis. Thisis thecasewhen
M
isa3-ballB
with involutionh such thatFix(h)
#
orwheng(M) >
andhisfree.When
M
-
B
z,
itis well known that hisequivalenttoeitherreflectionthrough
the center point orrevolutionabout acentralaxis.In
eithercaseM
B
I
and h3
r wherefl
is reflectionthrough
the center point or acentralaxisonB
2.
Przytycki
[4]
classifiesallfree Z,-actionson handlebodies.By
Theorem of that paperthereisat mostoneorientationpreservingandoneorientationreversing freeinvolution up toequivalence on anyhandlebody
M
with gg(M)
>
1.Furthermore,
theseoccur onorientablehandlebodiesonly when thegenus isodd.
Hence,
we needonly presentan example of eachin factored form tocompletethis first step.
In
bothcases we viewthehandlebodyofgenusg asA
I
whereA
isthe2-sphereS
less29
disjoint disks and h 7" r.
In
the orientationpreservingcase,r isinducedby theantipodalmap onS
when g invariantpairsof disksareremoved toformA.
In
the orientationreversing case,-
is inducedbythe involutiononS
with two fixedpoints(i.e.
revolutionabout acentralaxis piercing thefixedpoints)
whenaninvariant diskabout eachfixedpoint andg- invariant pairsofdiskare removedtoformA.
The inductionstep consists in cutting
along
invariantD
oralong
D
t3hD,
that is,deleting
N(D)
UN(hD)
fromM,
assuming a product structure on each piece by induction hypothesis, modifying theproduct
structures tobeconsistentwitheach other and then reassembling the pieces.We
consider twocases.Case
1: ThediskD
providedby Lemma
ish-invariant.By
inductionhypothesis,N(D)
andM’
orM_I
andM1
areeach endowed with aproduct
structuresuch thathIN(D
and h’ or h_l andhi
areeach equal tosome r r. The difficultyisthatthe"scars",
i.e. the images of theD,
on these pieces, arenot necessarilyconsistentwith thefiberings. It
isnot immediately thecasethatD,
aI
whereaissome arc onOA
andthe pieceofinterestfibers asA
I. We
show thatthefibering
maybesochosenby
deforming the original fiberingto meet this condition.Since
D
is h-invariant,we may assume thatFix(h)
is 1-dimensional. Then eachD,
is alsopt,,M’
M". Note
thatph,(D,)
isadiskonOM"
containingpointp,,(z,)
initsinterior.Now M’
factors asA’
I
with h’ havingafixed point arcterminatingin z,.Let
abean invariantarc onOA’
containingx,. Thereis anisotopyonO(M’*)
with support inasmallregular
neighborhood of diskp,,(a
I)
Up,(D,)
movingp,,(a
I)
ontop,(D,)
and constant onpt,,(x,).
Thisextends toan isotopy
K’
ofM’*
with support in acollar(Rourke
and Sanderson[5,
Theorem2.26])
of theboundary. This isotopy may beassumedconstanton
pt,,(Fix(h’)).
Hence,
it lifts to anisotopyKt
of
M’
movingaI
ontoD,
while fixing all points ofFix(h’).
We
deform thefibering ofM’
byK
-1 sothatwemayassumethat
D,
aI.
Thesamedeformationcanalso be carried outonM,.
This same argument is now applied in a neighborhood of
D,
inN(D)
with the additional requirement that ifN(D)
isfiberedas asB
I
and13
isaninvariantarc onOB
containingx,, then(plN(D))(3)
is initially isotopedintoapositioncorrespondingwiththeadjusted position ofp,(a)
on
p,,(M’).
ReassemblynowgivesM
A
I
whereA
isconstructed from thecompact bordered surfacesB
andA’
orA,,
-1, 1, byidentificationalong arcs onthe boundary.Hence
A
is alsoa compactborderedsurface.Clearly,
h r v whererisconstructed froma’
ora,, -1, 1,andtheinduced involutionof
B.
Case
:
DC3hD whereD
isthediskprovidedby Lemma
1.In
thiscasewenotethatN(D)
and
N(hD)
aredisjoint formFix(h).
Theargement
isbasically thesame asfor case except thatone need not take special measures on
Fix(h).
Sincep,(U(D))
ph(U(hD))
in the orbit spaceM*,
the isotopyK’
withsupport inN(D,)
lifts to isotopyKt
with support inN(D,)tO N(hD,),
-1, and commutes with h. []
4.
UNIQUENESS.
Any
productinvolution with 0-dimensionalor 1-dimensional fixed point set on an orientablehandlebody may be routinely
generated
from an involution with identical fixed point set on an orientable,compact
bordered surface. Since classifying the involutions on compact surfaces isan easy
matter,
all product involutions with 0-dimensionalor 1-dimensioanl fixed point sets onorientable handlebodiescan nowbeeasily
generated.
Our
problem is that thegenerated
list might contain many redundancies.For
instance, ifM
is the orientablehandlebody
withgenusthree,
aninvolution withtwo
fixedpoint arcsmay begenerated
inat least the following threeways:(i)
T1 ronAx
I
whereA1
isthe2-sphere S
less fourdisks. Involution’x isconstructed fromthe involutionon
S
withasinglesimple closedcurve asfixed point setby removingtwoinvariantdisksintersectingthis fixed point
loop
andaninvariant pair of disksdisjoint from thefixed point set.(ii) r
r onA
I
whereA
is the torusT
less two disks. Involution r2 is constructedfrom s x
idsa
onT
$1 xSx,
where sis complex conjugation, by the removal ofinvariant disksintersectingeachof thetwo fixedpoint
loops.
(iii)
rax
ronAa
xI
whereAa
isagain the torusT
less twodisks. Involutionra
isconstructedform the involutionv on
T
$1Sx,
wherev((x,y))
(y,x),
byremovalof two invariant disksfrom the singlefixedpoint
loop
ofv.the 0-dimensionalcase.
In
either case,weseeksufficientconditionstoconcludethattwoinvoluti(,,s areequivalent. The most obvious necessary condition turns out tobesufficient.THEOREM
2.Let h
and9 beproduct
involutionswith 0-dimensionalor 1-dimensionalfixedpoint setsonanorientable
handlebody M.
IfFix(h)
-
Fiz(9),
then h 9.PROOF:
Thisproofisbyinduction the thegenusofM.
Again,wehandleindependently h()se cases which cannot be assumed by induction hypotheses.Note
that the involutions h and 9 are bothorientationpreservingwhen their fixed point sets are1-dimensionalandorientation reversing when that have 0-dimensional fixed point sets.Hence,
the aforementionedwork of Przytycki[4]
coversthecaseof free involutionsonhandlebodies withnonzerogenus. Theresult is wellknow for involutionsofa
3-ball.
In
general,
in ordertoshow h 9 wefind ahomeomorphismf
M,
M;
betweenorbitspaces which can be lifted toa homeomorphism
f"
M
M
such thatpgf
f*Ph.
Observingthat
ph(F,z(h))
andpg(Fiz(g))
arethebranchpoints of branched coverings,wenote thatf"
canbe lifted if
f*ph(Fiz(h))
pa(Fiz(9)).
We
nowconsider the inductionstep. Sincehand9areboth of the formr r wecan,withoutusing
Lemma
1, find invariantnonseparating disksDn
andDu,
intersectingcomponects
ofF,z(h)
and
Fiz(9)
of thesametype.
M[
M-
N(Dh)
andM
M-
N(D)
have induced involutionsh’ and
9’
and9(M)
9(M)
<
9(M).
By
induction hypothesis, thereexist homeomorphisms k’’M
-
M
and k*"M
M,"
such thatpu,
k’=k*ph,. We
mayassume thatk*ph,(Dh
{i}),k*ph,(Oq))
(pg,(Dg
{i}),pg,(cq)),
i=-1,1, whereN(D_)
corresponds
toD_
[-1,
1].
Now
(phlN(Dh))(Dh
{i})
and(paIN(Da))(Da
{i})
arefaces ofthe 3-cellsph(N(Dh))
andp(N(Dg)),
respectively. The liftable isomorphism k*M’,
M,*
induces an isomorphism]c
between these pairs of faces.We
notethat]c
extendstoahomeomorphism]c’N(Dh)*
N(D)’,
suchfixedpoint imagesaretakento fixedpoint images and
Now
letf*
be:(ph,(N(Dh)
0(A
{0})))
ps(N(Da)
91(A
x{0})).
(k*
tO])"
M*,
UN(Dh)*
M7
UN(D)*
taking
ph(Fix(h))
top(Fix(g)).
Thenf"
lifts tof"
M
M,
sothat h g.REFERENCES
1.
KIM,
P.
K.
andTOLLEFSON,
J. L.
PL
involutionsoffibered3-manifolds, Trans.
Amer.
Math.Soc.
232(1977),
221-237.2.
KWUN,
K. W.
andTOLLEFSON,
J.
L. Extending
aPL
involutionoftheinteriorofacompactmanifold, Am.
J.
Math. 99(1977),
No. 5,
995-1001.3.
NELSON,
R. B. Some
fiber preservinginvolutions of orientable 3-dimensional handlebodies,Houston J.
Math9(1983),
255-269.4.
PRZTYCKI,
JOZEF H. Free
ActionsofZn
onhandlebodiesandsurfaces,
Bulleti______nd__e
L’Academie PolonaisedesSciences,Serie dessciejcesmaths, ast____r_r, e_tp_hy_. 26(1978),
No.7,
617-624. 5.ROURKE,
C. P.
andSANDERSON,
B.J.
Introductionto PiecewisLinea_______rTopology,