Elements of Torelli topology: I. The rank of abelian subgroups

30 Pages·2016·0.32 MB·English

Elements of Torelli topology:
I. The rank of abelian subgroups
Nikolai V. Ivanov
Contents
Preface
1. Surfaces, circles, and diffeomorphisms
2. Dehn twists
3. Action of Dehn twists on homology
4. Dehn multi-twists in Torelli groups
5. The rank of multi-twist subgroups of Torelli groups
6. Pure diffeomorphisms and reduction systems
7. The rank of abelian subgroups of Torelli groups
8. Dehn and Dehn–Johnson twists in Torelli groups: I
9. Dehn and Dehn–Johnson twists in Torelli groups: II
Appendix. Necklaces
References
© Nikolai V. Ivanov, 2016. Neithertheworkreportedinthispaper, noritspreparationweresupported
byanygovernmentalornon-governmentalagency, foundation, orinstitution.
1
Preface
By Torelli topology the author understands aspects of the topology of surfaces (poten-
tially) relevant to the study of Torelli groups.
v1
The present paper is devoted to a new approach to the results of W. Vautaw [ ] about
Dehn multi-twists in Torelli groups and abelian subgroups of Torelli groups. The main
resultsareacompletedescriptionofDehnmulti-twistsinTorelligroupsandthetheorem
to the effect that the rank of any abelian subgroup of the Torelli group of a closed
surface of genus g is (cid:54) 2g − 3. In contrast with W. Vautaw’s paper [v1], which
heavilyreliesonthegraph-theoreticlanguage, thepresentpaperisbasedontopological
methods. The resulting proofs are more transparent and lead to stronger estimates of
the rank of an abelian subgroup when some additional information is available. A key
role is played by the notion of a necklace of a system of circles on a surface.
8 9
As an unexpected application of our methods, in Sections and we give a new
proof of the algebraic characterization of the Dehn twist about separating circles and
fi
the Dehn-Johnson twists about bounding pairs of circles from the paper [ ] of B. Farb
and the author. This proof is much shorter than the original one and bypasses one
of the main difficulties, which may be called the extension problem, specific to Torelli
groups as opposed to the Teichmüller modular groups. This algebraic characterization
v2 v2
is also contained in the Ph.D. Thesis of W. Vautaw [ ]. But the proof in [ ] is not
quite complete because the extension problem is ignored. The extension problem is
i3
discussed in details in the second paper [ ] of this series.
In many respects Torelli groups look similar to Teichmüller modular groups from a
sufficient distance. For example, the algebraic characterization of the Dehn twist about
separating circles and the Dehn-Johnson twists about bounding pairs of circles in Torelli
i1
groups is morally the same as author’s [ ] algebraic characterization of Dehn twists in
Teichmüllermodulargroups. Buttheanalogybreaksdowntoabigextentifonefocuses
on the details. The extension problem is one of such details.
The new proof this algebraic characterization is simpler thanks not to any technical
advances, but to a few shifts in the point of view. It may well be the case that key
51
new insight is in Theorem . below, which is a trivial corollary of our form of the
44
description of Dehn multi-twists in the Torelli groups (see Theorem . ). Another
novel aspect of this proof is the shift of emphasis from the canonical reduction systems
6
to the pure reduction systems. See Section .
For the experts, one perhaps should point out that the centers of the centralizers, a fa-
vorite tool of the author, are replaced by more elegant bicommutants. On the technical
2
level these notions are equivalent in the situation of the present paper, but bicommu-
tants lead to a slightly different mindset.
1 3
Sections – are devoted mostly to a review of terminology and prerequisites, except
3 6
of an instructive example in Section . To a big extent this is also true for Section ,
devoted to the reduction systems, except of rephrasing some well known results in the
terms of pure reduction systems. The Appendix is devoted to some motivation for the
term necklace, and is not used in the main part of the paper. The table of contents
should serve as a sufficient guide for the rest of the paper.
1
. Surfaces, circles, and diffeomorphisms
Surfaces. By a surface we understand a compact orientable 2-manifold with (possibly
empty) boundary. The boundary of a surface S is denoted by ∂S, and its genus by
g(S). If ∂S = ∅, then S is called a closed surface. By a subsurface of a surface S
we understand a codimension 0 submanifold Q of S such that each component of
∂Q is either equal to a component of ∂S, or disjoint from ∂S.
For a subsurface Q of S we will denote by cQ the closure of its set-theoretic comple-
ment S(cid:114)Q. Clearly, cQ is also a subsurface of S, and ∂Q(cid:114)∂S = ∂cQ(cid:114)∂S. We
will say that cQ is the subsurface complementary to Q.
Teichmüller modular groups and Torelli groups. The Teichmüller modular group Mod(S)
of an orientable surface S is the group of isotopy classes of orientation-preserving dif-
feomorphisms S −→ S. Both diffeomorphisms and isotopies are required to preserve
the boundary ∂S only set-wise (this is automatic for diffeomorphisms and standard
forisotopies). Forclosedsurfaces S thesubgroupofelements Mod(S) actingtrivially
on the homology group H1(S, Z) is called the Torelli group of S and is denoted by
I(S). Usually we will denote the homology group H1(S, Z) simply by H1(S).
There are several candidates for the definition of the Torelli groups of surfaces with
boundary, but none of them is completely satisfactory. It seems that this is so not for
the lack of trying to find a “right definition”, but because there is no such definition.
By technical reasons we will need also the subgroups I (S) of Mod(S) consisting of
m
the isotopy classes of diffeomorphisms acting trivially on H1(S, Z/mZ), where m is
an integer. Clearly, I(S) ⊂ I (S) for every m ∈ Z.
m
Circles. A circle on a surface S is defined as a submanifold of S diffeomorphic to the
1
standard circle S and disjoint from ∂S. A circle in S is called non-peripheral if does
3
not bound an annulus together with a component of the boundary ∂S. If S is a closed
surface, then, obviously, all circles in S are non-peripheral. A circle on S is said to
be non-trivial in S if it is if does not bound an annulus together with a component of
the boundary ∂S and does not bound a disc in S.
Separating circles. A circle D in a connected surface S is called separating if the
set-theoretic difference S(cid:114)D is not connected. In this case S(cid:114)D consist of two
components. The closures of these components are subsurfaces of S having D as a
boundary component. The other boundary components of these subsurfaces are at the
sametimeboundarycomponentsof S. Wewillcallthesesubsurfaces thepartsintowhich
D divides S. If Q is one of these parts, then, obviously, cQ is the other part.
Bounding pairs of circles. A bounding pair of circles on a connected surface S is defined
as an unordered pair C, C(cid:48) of disjoint non-isotopic circles in S such that both circles
C, C(cid:48) are non-separating, but S(cid:114)(C∪C(cid:48)) is not connected. In this case the difference
S(cid:114)(C∪C(cid:48)) consist of two components. The closures of these components are subsur-
faces of S having both C and C(cid:48) as their boundary components. The other boundary
components of these subsurfaces are at the same time boundary components of S. We
will call these subsurfaces the parts into which the bounding pair of circles C, C(cid:48) divides
S. If Q is one of these parts, then, obviously, cQ is the other part.
Cutting surfaces and diffeomorphisms. Every one-dimensional closed submanifold c
of a surface S leads to a new surface S//c obtained by cutting S along c. The
components of S//c are called the parts into which c divides S. The canonical map
p//c: S//c −→ S.
induces a diffeomorphism (p//c)−1(S(cid:114)c) −→ S(cid:114)c and a double covering map
(p//c)−1(c) −→ c.
We will treat the diffeomorphism (p//c)−1(S(cid:114)c) −→ S(cid:114)c as an identification. If
p//c is injective on a component Q of S//c, then we will treat as an identification also
the induced map Q −→ p//c(Q), and will treat Q as a subsurface of S.
Since S is orientable, the covering (p//c)−1(c) −→ c is actually trivial. Namely, for
every component C of c its preimage (p//c)−1(C) consists of two boundary circles of
S//c, and each of these circles is mapped by p//c diffeomorphically onto C.
Any diffeomorphism F: S −→ S such that F(c) = c induces a diffeomorphism
F//c: S//c −→ S//c.
4
If F//c leaves a component Q of S//c invariant, then F//c induces a diffeomorphism
F : Q −→ Q,
Q
called the restriction of F to Q.
Systems of circles. A one-dimensional closed submanifold c of a surface S is called
a system of circles on S if the components of c are all non-trivial circles on S and are
pair-wise non-isotopic. As usual, we will denote by π0(c) the set of components of
c. If c is a system of circles, then the elements of π0(c) are pair-wise disjoint and
pair-wise non-isotopic circles, and if U is a set of pair-wise disjoint and non-isotopic
circles, then the union of these circles is a system of circles.
Reduction systems. A system of circles c on S is called a reduction system for a diffeo-
morphism F: S −→ S if F(c) = c. A system of circles c on S is called a reduction
system for an element f ∈ Mod(S) if c is a reduction system for some diffeomorphism
in the isotopy class f, i.e. if the isotopy class f can be represented by a diffeomor-
phism F: S −→ S such that F(c) = c.
Reducible and pseudo-Anosov elements. A non-trivial element f ∈ Mod(S) is said to
be reducible if there exists a non-empty reduction system for f, and irreducible other-
wise. An irreducible element of infinite order is called a pseudo-Anosov element. This is
an easy, but hardly enlightening way to define pseudo-Anosov elements. The original
t
definition of Thurston [ ] looks more like a theory than a short definition.
2
. Dehn twists
Twist diffeomorphisms of an annulus. Let A be an annulus, i.e. a surface diffeomor-
phic to S1×[0, 1]. As is well known, the group of diffeomorphisms of A, fixed in a
neighborhood of ∂A and considered up to isotopies fixed in a neighborhood ∂A, is
an infinite cyclic group. A diffeomorphism of A is called a twist diffeomorphism of A
if it is fixed in a neighborhood of ∂A and its isotopy class is a generator of this group.
Fixing an orientation of A allows to choose a preferred generator of this group. A
twist diffeomorphism of A −→ A is called a left twist diffeomorphism if its isotopy
class is the preferred generator, and a right twist diffeomorphism otherwise. Up to an
isotopy, the right twist diffeomorphism is the inverse of the left one.
Twist diffeomorphisms of a surface. If A is annulus contained in S as a subsurface,
then any twist diffeomorphism of A can be extended by the identity to a diffeomor-
5
phism of S. Such extensions are called twist diffeomorphisms of S.
If S isoriented, thentheextensionofaleft (respectively, right) twistdiffeomorphism
of an annulus in S with the induced orientation is called a left (respectively, right)
twist diffeomorphism of S.
Dehn twists. Suppose that S is oriented. Let A be an annulus contained in S, and
let C be a circle in S contained in A as a deformation retract. Such a circle C is
unique up to isotopy.
It turns out that isotopy class of a left (respectively, right) twist diffeomorphism of S
obtainedbyextensionofaleft (respectively, right) twistdiffeomorphismof A depends
only on the isotopy class of C in S. In particular, it is uniquely determined by C.
Thisisotopyclassiscalledthe left (respectively, right) Dehntwist of S aboutthecircle
C. It is an element of the group Mod(S).
The left Dehn twist about a circle C is denoted by t . The right Dehn twist about C
C
is the inverse of the left one and hence is equal to t−1.
C
The Dehn twist t is equal to 1 ∈ Mod(S) if and only if the circle C is trivial. Let
C
G be a diffeomorphism of S and let g ∈ Mod(S) be its isotopy class. Then
gt g−1 = t .
C G(C)
Inparticular, if G(C) = C, or if G(C) isisotopicto C, then t and g commute.
C
3
. Action of Dehn twists on homology
The explicit formula. From now on we will assume that the surface S is closed and oriented.
The orientation of S allows to define a skew-symmetric pairing on H1(S), known as
the intersection pairing. It is denoted by
(a, b) (cid:55)−→ (cid:104)a, b(cid:105).
Let C beacircleon S. Letusorient C anddenoteby [C] theimageofthefundamental
class of C in H1(S). Then tC acts on H1(S) by the formula
(3.1) (cid:0)t (cid:1) (a) = a + (cid:104)a, [C](cid:105)[C].
C ∗
Changing the orientation of C replaces [C] by −[C] and hence does not change the
6
right hand side of (3.1). The powers of tC act on H1(S) by the formula
(3.2) (cid:0)tm(cid:1) (a) = a + m(cid:104)a, [C](cid:105)[C].
C ∗
If [C] (cid:54)= 0, then (cid:104)a, [C](cid:105) =(cid:54) 0 for some a ∈ H1(S), and hence
a + (cid:104)a, [C](cid:105)[C] (cid:54)= a.
On the other hand, if [C] = 0, then a + (cid:104)a, [C](cid:105)[C] = a. Therefore, t ∈ I(S)
C
if and only if [C] = 0. i.e. if and only if C is a separating circle.
Dehn–Johnson twists and multi-twists. Let C, D be a pair of disjoint circles on S
such that the union C ∪ D is equal to the boundary ∂Q of some subsurface Q of S.
Such pairs of circles are called bounding pairs. The circles C and D can be oriented
in such a way that [C] = [D]. Therefore (3.1) implies that the maps
(cid:0) (cid:1) (cid:0) (cid:1)
tC ∗, tD ∗ : H1(S) −→ H1(S)
are equal, and hence t t−1, t t−1 ∈ I(S). Both these elements of I(S) are called
C D D C
the Dehn–Johnson twists about the bounding pair C, D.
Let c be a one-dimensional closed submanifold of S. A Dehn multi-twist about about
c is defined as a product t of the form
(cid:89)
(3.3) t = tmO,
O O
where O runs over all components of c, and m are integers.
O
A siren song. Suppose that Q is a subsurface of S and c = ∂Q. The orientation of
S defines an orientation of Q, which, in turn, defines an orientation of c = ∂Q.
Let t be defined by (3.3). If c has (cid:54) 2 components, then t ∈ I(S) if and only if
(cid:88)
(3.4) m [O] = 0,
O
O
where O runs over components of c, and these components are considered with ori-
entations induced from c = ∂Q. It is tempting to believe that this is true in general.
Infact, thisisveryfarfrombeingtrue. Thefollowingexample, explainedtotheauthor
1992 44
in few words by R. Hain in , illustrates the reason behind this, and Theorem .
below provides a simple necessary and sufficient condition for t ∈ I(S).
It is much more difficult to find reasons for such a belief.
7
Example. Suppose that both Q and the complementary subsurface cQ are con-
nected and that c = ∂Q consists of three circles C, D, E. Let us orient these circles
as boundary components of Q. Then the only relation between [C], [D], [E] is
[C] + [D] + [E] = 0.
Inparticular, theclasses [C], [D], [E] arenon-zeroandanytwoofthemarelinearly
independent. Let A be a circle in S disjoint from E and intersecting each of the
circles C, D transversely at one point. Let us orient A and let a = [A]. By the
choice of A the intersection number (cid:104)a, [E](cid:105) = 0, one of the intersection numbers
(cid:104)a, [C](cid:105), (cid:104)a, [D](cid:105) is equal to 1, and the other is equal to −1. We may assume that
(cid:104)a, [C](cid:105) = 1 and (cid:104)a, [D](cid:105) = −1. Let m , m , m ∈ Z and
C D E
m m m
t = t C t D t D.
C D C
Then t (a) = a + m (cid:104)a, [C](cid:105)[C] + m (cid:104)a, [D](cid:105)[D] + m (cid:104)a, [E](cid:105)[E]
∗ C D E
= a + m [C] − m [D].
C D
It follows that if t ∈ I(S), then m = m = 0. A similar argument using D, E
C D
instead of C, D shows that if t ∈ I(S), then also m = m = 0. It follows that
D E
t ∈ I(S) if and only if m = m = m = 0, i.e. if and only if t = 1.
C D E
4
. Dehn multi-twists in Torelli groups
Homology equivalence. Two circles C, C(cid:48) in S are said to be homology equivalent if
they can be oriented in such way that their homology classes in H1(S) are equal,
[C] = [C(cid:48)].
Changing the orientation of a circle C replaces [C] by −[C]. It follows that the
homology equivalence is indeed an equivalence relation. If C is a separating circle,
then [C] = 0, and hence all separating circles are homology equivalent.
Necklaces. Let s be a one-dimensional closed submanifold of S. The homology
equivalence induces an equivalence relation on the set π0(s) of components of s. A
necklace of s is an equivalence class of this equivalence relation containing a non-
1
separating circle. If a necklace consists of more than circle, then the union of all
2
circles in it is a BP-necklace in the sense of Appendix. See Corollary A. . The descrip-
tion of BP-necklaces in Appendix is the motivation behind the term necklace.
8
Graphassociatedwithasubmanifold. Let c beaone-dimensionalclosedsubmanifold
of S. One can associate with the pair (S, c) a graph G(S, c) as follows. Its set of
vertices is the set π0(S//c) of components of the cut surface S//c, and its set of edges
is the set π0(c) of components of c. Every component C of c is the image under
the canonical map p = p//c: S//c −→ S of two components of ∂S//c. Let C1, C2
be these components, and let Q1, Q2 be the components of S//c containing C1, C2
respectively (itmayhappenthat Q1 = Q2). Thecomponent C consideredasanedge
of G(S, c) connects the components Q1, Q2 considered as vertices. If Q1 = Q2,
then the edge C is a loop, and if the intersection of the images p(Q1), p(Q2) consists
of several components of c, then the vertices Q1, Q2 are connected by several edges.
Separating edges. Let a, b be two vertices of a connected graph G. A set T of
edges of G is separating edge set for a and b if every path in G contains at least
one edge from T. The vertices a, b are said to be τ-edge separated if τ is the
minimal number of elements in a separating edge set for a and b.
4.1. Theorem. Iftwovertices a, b ofaconnectedgraph G are τ-edgeseparated, thenthere
exists τ simple paths connecting a with b and having pair-wise disjoint sets of edges.
Proof. See, for example, Theorem 12.3.1 in the classical book of O. Ore [o]. (cid:4)
4.2. Corollary. If two vertices a, b of a connected graph G remain connected by a path
after removing any edge of G, then there exists at least two 2 simple paths connecting a
with b and having disjoint sets of edges.
Remark. We will use only Corollary 4.2. But Theorem 4.1 is so beautiful that the
authorcouldnotresistincludingit. Itisanedge-separationversionofvertex-separation
m w
results of K. Menger [ e] and H. Whitney [ ]. It is worth to note that K. Menger’s
m
paper [ e] is a paper in topology, and H. Whitney is better known not for his seminal
contributions to graph theory, but as one of the creators of differential topology.
42
The idea to apply Corollary . to Dehn multi-twists in Torelli groups is due to W.
v1
Vautaw [ ]. Unfortunately, the application of this result is hidden deep inside of a
v1
technical argument in [ ], and no references related to this result are provided.
42
For a textbook exposition the vertex-separation version of Corollary . , known as
bm 32 br 337
Whitney’s theorem, see [ ], Theorem . , or [ ], Theorem . . . The Corollary
42 39 bm
. itself is the Exercise . in [ ].
4.3. Lemma. Suppose that c is a one-dimensional submanifold of S such that all compo-
nents of c are non-separating and no two components are homology equivalent. Let D be a
9
component of c. Then there exists two circles A, B on S intersecting each component of c
transversely in no more than one point and such that D intersects both circles A, B and no
other component of c does.
Proof. Let us consider the graph G(S, c). Let Q1, Q2 be the components of S//c
connected by the edge D of G(S, c). If Q1 = Q2, then there is a circle in S
intersecting D transversely in one point and disjoint from all other components of c.
In this case we can take this circle as both A and B.
Suppose now that Q1 (cid:54)= Q2. Let G be the result of removing the edge D from the
graph G(S, c). Since the circle D is non-separating, the graph G is connected. If
after removing an edge C of G the vertices Q1, Q2 are not connected, then C and
D together separate S, and hence C and D together bound a subsurface of S. In
this case the circles C and D are homology equivalent, contrary to the assumption.
Thereforethevertices Q1, Q2 of G remainconnectedafterremovinganyedgeof G.
Let us choose some points x1, x2 in the interior of surfaces Q1, Q2 respectively. Let
J be an arc in S connecting x1 with x2, intersecting D transversely at one point,
and disjoint from all other components of c.
A path in G connecting Q1 with Q2 can be “realized” by an arc in S connecting
x1 with x2, contained in the union
(cid:91)
p(Q),
Q
where Q runs over the set of vertices of this path, intersecting only those components
of c which are the edges of this path, and intersecting each of these components
transversely at one point.
By Corollary 4.2, there exist two paths in G connecting Q1 with Q2 and having
disjoint sets of edges. Therefore, there exist two arcs in S connecting x1 with x2,
disjoint from D, and such that no other component of c intersects both of these arcs.
By taking the unions of these two arcs with the arc J we get circles A, B with the
(cid:4)
required properties.
Difference maps. The difference map of a diffeomorphism G: S −→ S is the map
∆G : H1(S) −→ H1(S)
defined by ∆ = G (a) − a. Clearly, ∆ depends only on the isotopy class g of
G ∗ G
G and hence may be denoted by ∆ . The isotopy class g belongs to I(S) if and only
g
if the difference map ∆ is equal to 0.
g
10