108 research outputs found

### Every knot has characterising slopes

Let K be a knot in the 3-sphere. A slope p/q is said to be characterising for
K if whenever p/q surgery on K is homeomorphic, via an orientation-preserving
homeomorphism, to p/q surgery on another knot K' in the 3-sphere, then K and K'
are isotopic. It was an old conjecture of Gordon, proved by Kronheimer, Mrowka,
Ozsvath and Szabo, that every slope is characterising for the unknot. In this
paper, we show that every knot K has infinitely many characterising slopes,
confirming a conjecture of Baker and Motegi. In fact, p/q is characterising for
K provided |p| is at most |q| and |q| is sufficiently large.Comment: 15 pages, no figures; final versio

### Expanders, rank and graphs of groups

Let G be a finitely presented group, and let {G_i} be a collection of finite
index normal subgroups that is closed under intersections. Then, we prove that
at least one of the following must hold: 1. G_i is an amalgamated free product
or HNN extension, for infinitely many i; 2. the Cayley graphs of G/G_i (with
respect to a fixed finite set of generators for G) form an expanding family; 3.
inf_i (d(G_i)-1)/[G:G_i] = 0, where d(G_i) is the rank of G_i.
The proof involves an analysis of the geometry and topology of finite Cayley
graphs. Several applications of this result are given.Comment: 13 pages; to appear in Israel J. Mat

### New lower bounds on subgroup growth and homology growth

We establish new strong lower bounds on the (subnormal) subgroup growth of a
large class of groups. This includes the fundamental groups of all
finite-volume hyperbolic 3-manifolds and all (free non-abelian)-by-cyclic
groups. The lower bound is nearly exponential, which should be compared with
the fastest possible subgroup growth of any finitely generated group. This is
achieved by free non-abelian groups and is slightly faster than exponential. As
a consequence, we obtain good estimates on the number of covering spaces of a
hyperbolic 3-manifold with given covering degree. We also obtain slightly
weaker information on the number of covering spaces of closed 4-manifolds with
non-positive Euler characteristic. The results on subgroup growth follow from a
new theorem which places lower bounds on the rank of the first homology (with
mod p coefficients) of certain subgroups of a group. This is proved using a
topological argument.Comment: 39 pages, 2 figures; v3 has minor changes from v2, incorporating
referee's comments; v2 has minor changes from v1; to appear in the
Proceedings of the London Mathematical Societ

### The Heegaard genus of amalgamated 3-manifolds

Let M and M' be simple 3-manifolds, each with connected boundary of genus at
least two. Suppose that M and M' are glued via a homeomorphism between their
boundaries. Then we show that, provided the gluing homeomorphism is
`sufficiently complicated', the Heegaard genus of the amalgamated manifold is
completely determined by the Heegaard genus of M and M' and the genus of their
common boundary. Here, a homeomorphism is `sufficiently complicated' if it is
the composition of a homeomorphism from the boundary of M to some surface S,
followed by a sufficiently high power of a pseudo-Anosov on S, followed by a
homeomorphism to the boundary of M'. The proof uses the hyperbolic geometry of
the amalgamated manifold, generalised Heegaard splittings and minimal surfaces.Comment: 7 pages, 2 figure

### Covering spaces of 3-orbifolds

Let O be a compact orientable 3-orbifold with non-empty singular locus and a
finite volume hyperbolic structure. (Equivalently, O is the quotient of
hyperbolic 3-space by a lattice in PSL(2,C) with torsion.) Then we prove that O
has a tower of finite-sheeted covers {O_i} with linear growth of p-homology,
for some prime p. This means that the dimension of the first homology, with mod
p coefficients, of the fundamental group of O_i grows linearly in the covering
degree. The proof combines techniques from 3-manifold theory with
group-theoretic methods, including the Golod-Shafarevich inequality and results
about p-adic analytic pro-p groups.
This has several consequences. Firstly, the fundamental group of O has at
least exponential subgroup growth. Secondly, the covers {O_i} have positive
Heegaard gradient. Thirdly, we use it to show that a group-theoretic conjecture
of Lubotzky and Zelmanov would imply that O has large fundamental group. This
implication uses a new theorem of the author, which will appear in a
forthcoming paper. These results all provide strong evidence for the conjecture
that any closed orientable hyperbolic 3-orbifold with non-empty singular locus
has large fundamental group.
Many of the above results apply also to 3-manifolds commensurable with an
orientable finite-volume hyperbolic 3-orbifold with non-empty singular locus.
This includes all closed orientable hyperbolic 3-manifolds with rank two
fundamental group, and all arithmetic 3-manifolds.Comment: 26 pages. Version 3 has only minor changes from versions 1 and 2. To
appear in Duke Mathematical Journa

### Some 3-manifolds and 3-orbifolds with large fundamental group

We provide two new proofs of a theorem of Cooper, Long and Reid which asserts
that, apart from an explicit finite list of exceptional manifolds, any compact
orientable irreducible 3-manifold with non-empty boundary has large fundamental
group. The first proof is direct and topological; the second is
group-theoretic. These techniques are then applied to prove a string of results
about (possibly closed) 3-orbifolds, which culminate in the following theorem.
If K is a knot in a compact orientable 3-manifold M, such that the complement
of K admits a finite-volume hyperbolic structure, then the orbifold obtained by
assigning a singularity of order n along K has large fundamental group, for
infinitely many positive integers n. We also obtain information about this set
of values of n. When M is the 3-sphere, this has implications for the cyclic
branched covers over the knot. In this case, we may also weaken the hypothesis
that the complement of K is hyperbolic to the assumption that K is non-trivial.Comment: 14 pages, 1 figur

### Detecting large groups

Let G be a finitely presented group, and let p be a prime. Then G is 'large'
(respectively, 'p-large') if some normal subgroup with finite index
(respectively, index a power of p) admits a non-abelian free quotient. This
paper provides a variety of new methods for detecting whether G is large or
p-large. These relate to the group's profinite and pro-p completions, to its
first L2-Betti number and to the existence of certain finite index subgroups
with 'rapid descent'. The paper draws on new topological and geometric
techniques, together with a result on error-correcting codes.Comment: 31 pages, 2 figure

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