Cube complexes, Reidemeister 3, zonohedra and the missing 8th region

There is an old puzzle which starts by asking: what is the next number in the sequence 1,2,4,? We are supposed to recognize the start of the sequence and answer that the next number is surely 8, because the first three numbers are consecutive powers of 2, and so the next number should be the cube of 2 which is 8. The puzzler then explains (contrary to expectations) that the successive terms in the sequence are actually the number of regions into which the plane is divided by a collection of lines in general position (so that any two lines intersect, and no three lines intersect in a single point). Thus:

So the “correct” answer to the puzzle is 7 (and the sequence continues 11, 26, ). This is somehow meant to illustrate some profound point; I don’t quite see it myself. Anyway, I would like to suggest that there is a natural sense in which the “real” answer should actually be 8 after all, and it’s the point of this short blog post to describe some connections between this puzzle, the theory of cube complexes (which is at the heart of Agol’s recent proof of the Virtual Haken Conjecture), and the location of the missing 8th region.

Agol’s Virtual Haken Theorem (part 3): return of the hierarchies

Ian gave his second and third talks this afternoon, completing his (quite detailed) sketch of the proof of the Virtual Haken Theorem. Recall that after work of Kahn-Markovic, Wise, Haglund-Wise and Bergeron-Wise, the proof reduces to showing the following:

Theorem (Agol): Let G be a hyperbolic group acting properly discontinuously and cocompactly on a CAT(0) cube complex X. Then there is a finite index subgroup G’ so that X/G’ is special; in other words, G is virtually special.

Agol’s Virtual Haken Theorem (part 1)

I am in Paris attending a workshop at the IHP where Ian Agol has just given the first of three talks outlining his proof of the Virtual Haken Conjecture and Virtual Fibration Conjecture in 3-manifold topology (hat tip to Henry Wilton at the Low Dimensional Topology blog from whom I first learned about Ian’s announcement last week). I think it is no under overstatement to say that this marks the end of an era in 3-manifold topology, since the proof ties up just about every loose end left over on the list of problems in 3-manifold topology from Thurston’s famous Bulletin article (with the exception of problem 23 — to show that volumes of closed hyperbolic 3-manifolds are not rationally related — which is very close to some famous open problems in number theory). The purpose of this blog post is to say what the Virtual Haken Conjecture is, and some of the background that goes into Ian’s argument. I hope to follow this up with more details in another post (after Agol gives talks 2 and 3 this coming Wednesday). Needless to say this post has been written in a bit of a hurry, and I have probably messed up some crucial details; but if that caveat is not enough to dissuade you, then read on.

Filling geodesics and hyperbolic complements

Patrick Foulon and Boris Hasselblatt recently posted a preprint entitled “Nonalgebraic contact Anosov flows on 3-manifolds”. These are flows which are at the same time Anosov (i.e. the tangent bundle splits in a flow-invariant way into stable, unstable and flow directions) and contact (i.e. they preserve a contact form — that is, a 1-form for which is a volume form). Their preprint gives some very interesting new constructions of such flows, obtained by surgery along a Legendrian knot (one tangent to the kernel of the contact form) which is transverse to the stable/unstable foliations of the Anosov flow.

Quasigeodesic flows on hyperbolic 3-manifolds

My student Steven Frankel has just posted his paper Quasigeodesic flows and Mobius-like groups on the arXiv. This heartbreaking work of staggering genius interesting paper makes a deep connection between dynamics, hyperbolic geometry, and group theory, and represents the first significant progress that I know of on a conjectural program I formulated a few years ago.

One of the main results of the paper is to show that every quasigeodesic flow on a closed hyperbolic 3-manifold either has a closed orbit, or the fundamental group of the manifold admits an action on a circle with some very peculiar properties, namely that it is Mobius-like but not Mobius. The problem of giving necessary and sufficient conditions on a vector field on a 3-manifold to guarantee the existence of a closed orbit is a long and interesting one, and the introduction to the paper gives a brief sketch of this history as follows:

Rotation numbers and the Jankins-Neumann ziggurat

I’m in Melbourne right now, where I recently attended the Hyamfest and the preceding workshop. There were many excellent talks at both the workshop and the conference (more on that in another post), but one thing that I found very interesting is that both Michel Boileau and Cameron Gordon gave talks on the relationships between taut foliations, left-orderable groups, and L-spaces. I haven’t thought seriously about taut foliations in almost ten years, but the subject has been revitalized by its relationship to the theory of Heegaard Floer homology. The relationship tends to be one-way: the existence of a taut foliation on a manifold implies that the Heegard Floer homology of is nontrivial. It would be very interesting if Heegaard Floer homology could be used to decide whether a given manifold admits a taut foliation or not, but for the moment this seems to be out of reach.

Anyway, both Michel and Cameron made use of the (by now 20 year old) classification of taut foliations on Seifert fibered 3-manifolds. The last step of this classification concerns the case when the base orbifold is a sphere; the precise answer was formulated in terms of a conjecture by Jankins and Neumann, proved by Naimi, about rotation numbers. I am ashamed to say that I never actually read Naimi’s argument, although it is not long. The point of this post is to give a new, short, combinatorial proof of the conjecture which I think is “conceptual” enough to digest easily.

Hyperbolic Geometry Notes #5 – Mostow Rigidity

1. Mostow Rigidity

For hyperbolic surfaces, Moduli space is quite large and complicated. However, in three dimensions Moduli space is trivial:

Theorem 1 If is a homotopy equivalence of closed hyperbolic manifolds with , then is homotopic to an isometry.

In other words, Moduli space is a single point.

This post will go through the proof of Mostow rigidity. Unfortunately, the proof just doesn’t work as well on paper as it does in person, especially in the later sections.

1.1. Part 1

First we need a definition familiar to geometric group theorists: a map between metric spaces (not necessarily Riemannian manifolds) is a quasi-isometry if for all , we have

Without the term, would be called bilipschitz.

First, we observe that if is a homotopy equivalence, then lifts to a map in the sense that is equivariant with respect to (thought of as the desk groups of and , so for all , we have .

Now suppose that and are hyperbolic. Then we can lift the Riemannian metric to the covers, so and are specific discrete subgroups in , and maps equivariantly with respect to and .

Lemma 2 is a quasi-isometry.

Proof: Since is a homotopy equivalence, there is a such that . Perturbing slightly, we may assume that and are smooth, and as and are compact, there exists a constant such that and . In other words, paths in and are stretched by a factor of at most : for any path , . The same is true for going in the other direction, and because we can lift the metric, the same is true for the universal covers: for any path , , and similarly for .

Thus, for any in the universal cover ,

and

We see, then, that is Lipschitz in one direction. We only need the for the other side.

Since , we lift it to get an equivariant lift For any point , the homotopy between gives a path between and . Since this is a lift of the homotopy downstairs, this path must have bounded length, which we will call . Thus,

Putting these facts together, for any in ,

And

By the triangle inequality,

This is the left half of the quasi-isometry definition, so we have shown that is a quasi-isometry.

Notice that the above proof didn’t use anything hyperbolic—all we needed was that and are Lipschitz.

Our next step is to prove that a quasi-isometry of hyperbolic space extends to a continuous map on the boundary. The boundary of hyperbolic space is best thought of as the boundary of the disk in the Poincare model.

Lemma 3 A quasi-isometry extends to a continuous map on the boundary .

The basic idea is that given a geodesic, it maps under to a path that is uniformly close to a geodesic, so we map the endpoints of the first geodesic to the endpoints of the second. We first need a sublemma:

Lemma 4 Take a geodesic and two points and a distance apart on it. Draw two perpendicular geodesic segments of length from and . Draw a line between the endpoints of these segments such that has constant distance from the geodesic. Then the length of is linear in and exponential in .

Proof: Here is a representative picture:

So we see that . By Gauss-Bonnet,

Where the on the left is the sum of the turning angles, and is the geodesic curvature of the segment . What is this geodesic curvature ? If we imagine increasing , then the derivative of the length with respect to is the geodesic curvature times the length , i.e.

So . Therefore, by the Gauss-Bonnet equality,

so . Therefore, , which proves the lemma

With this lemma in hand, we move on the next sublemma:

Lemma 5 If is a quasi-isometry, there is a constant depending only on and such that for all on the geodesic from to in , is distance less than from any geodesic from to .

Proof: Fix some , and suppose the image of the geodesic from to goes outside a neighborhood of the geodesic from to . That is, there is some segment on between the points and such that maps completely outside the neighborhood.

Let’s look at the nearest point projection from to . By the above lemma, . Thus means that

On the other hand, because is a quasi-isometry,

and

So we have

Which implies that

That is, the length of the offending path is uniformly bounded. Thus, increase by times this length plus , and every offending path will now be inside the new neighborhood of .

The last lemma says that the image under of a geodesic segment is uniformly close to an actual geodesic. Now suppose that we have an infinite geodesic in . Take geodesic segments with endpoints going off to infinity. There is a subsequence of the endpoints converging to a pair on the boundary. This is because the visual distance between successive pairs of endspoints goes to zero. That is, we have extended to a map , where is the diagonal . This map is actually continuous, since by the same argument geodesics with endpoints visually close map (uniformly close) to geodesics with visually close endpoints.

1.2. Part 2

Now we know that a quasi-isometry extends continuously to the boundary of hyperbolic space. We will end up showing that is conformal, which will give us the theorem.

We now introduce the Gromov norm. if is a topological space, then singular chain complex is a real vector space with basis the continuous maps . We define a norm on as the norm:

This defines a pseudonorm (the Gromov norm) on by:

This (pseudo) norm has some nice properties:

Lemma 6 If is continuous, and , then .

Proof: If represents , then represents .

Thus, we see that if is a homotopy equivalence, then .

If is a closed orientable manifold, then we define the Gromov norm of to be the Gromov norm .

Here is an example: if admits a self map of degree , then . This is because we can let represent , so , so represents . Thus . Notice that we can repeat the composition with to get that is as small as we’d like, so it must be zero.

Theorem 7 (Gromov) Let be a closed oriented hyperbolic -manifold. Then . Where is a constant depending only on .

We now go through the proof of this theorem. First, we need to know how to straighten chains:

Lemma 8 There is a map (the second complex is totally geodesic simplices) which is -equivariant and – equivariantly homotopic to .

Proof: In the hyperboloid model, we imagine a simplex mapping in to . In , we can connect its vertices with straight lines, faces, etc. These project to being totally geodesics in the hyperboloid. We can move the original simplex to this straightened one via linear homotopy in ; now project this homotopy to .

Now, if represents , then we can straighten the simplices, so represents , and , so when finding the Gromov norm it suffices to consider geodesic simplices. Notice that every point has finitely many preimages, and total degree is 1, so for any point , .

Next, we observe:

Lemma 9 If given a chain , there is a collection such that and is a cycle homologous to .

Proof: We are looking at a real vector space of coefficients, and the equations defining what it means to be a cycle are rational. Rational points are therefore dense in it.

By the lemma, there is an integral cycle , where is some constant. We create a simplicial complex by gluing these simplices together, and this complex comes together with a map to . Make it smooth. Now by the fact above, , so . Then

on the one hand, and on the other hand,

The volume on the right is at most , the volume of an ideal simplex, so we have that

i.e.

This gives the lower bound in the theorem. To get an upper bound, we need to exhibit a chain representing with all the simplices mapping with degree 1, such that the volume of each image simplex is at least .

We now go through the construction of this chain. Set , and fix a fundamental domain for , so is tiled by translates of . Let be the set of all simplices with side lengths with vertices in a particular -tuple of fundamental domains . Pick to be a geodesic simplex with vertices , and let be the image of under the projection. This only depends on up to the deck group of .

Now define the chain:

With the to make it orientation-preserving, and where is an -invariant measure on the space of regular simplices of side length . If the diameter of is every simplex with has edge length in , so:

1. The volume of each simplex is if is large enough.
2. is finite — fix a fundamental domain; then there are only finitely many other fundamental domains in .

Therefore, we just need to know that is a cycle representing : to see this, observe that every for every face of every simplex, there is an equal weight assigned to a collection of simplices on the front and back of the face, so the boundary is zero.

By the equality above, then,

Taking to zero, we get the theorem.

1.3. Part 3 (Finishing the proof of Mostow Rigidity

We know that for all , there is a cycle representing such that every simplex is geodesic with side lengths in , and the simplices are almost equi-distributed. Now, if , and represents , then represents , as is a homotopy equivalence.

We know that extends to a map . Suppose that there is an tuple in which is the vertices of an ideal regular simplex. The map takes (almost) regular simplices arbitrarily close to this regular ideal simplex to other almost regular simplices close to an ideal regular simplex. That is, takes regular ideal simplices to regular ideal simplices. Visualizing in the upper half space model for dimension 3, pick a regular ideal simplex with one vertex at infinity. Its vertices form an equilateral triangle in the plane, and takes this triangle to another equilateral triangle. We can translate this simplex around by the set of reflections in its faces, and this gives us a dense set of equilateral triangles being sent to equilateral triangles. This implies that is conformal on the boundary. This argument works as long as the boundary sphere is at least 2 dimensional, so this works as long as is 3-dimensional.

Now, as is conformal on the boundary, it is a conformal map on the disk, and thus it is an isometry. Translating, this means that the map conjugating the deck group to is an isometry of , so is actually an isometry, as desired. The proof is now complete.

Knots with small rational genus

I recently uploaded a paper to the arXiv entitled Knots with small rational genus, joint with Cameron Gordon. The genesis of this paper was a couple of nice (and related) talks at Caltech by Matthew Hedden and Jake Rasmussen in 2007. They both talked about potential applications of the theory of knot Floer homology to the Berge conjecture. A Berge knot is a (tame) knot in the 3-sphere which lies on a genus two Heegaard surface, and with the property that on each side of the Heegaard surface there is a meridian disk that the knot intersects exactly once. Equivalently, the inclusion of the knot into each (closed) handlebody sends the generator of to a generator of . Note that since the 3-sphere admits a unique (up to isotopy) Heegaard splitting of any genus, one may think of such a knot as lying on a specific genus 2 surface in . Such knots were classified by Berge; they admit (Dehn) surgeries which result in (nontrivial) Lens spaces. The Berge conjecture is the converse; i.e.:

Berge Conjecture: Let be a knot in which admits a nontrivial Lens space surgery; i.e. there is a Lens space and a knot in for which is homeomorphic to . Then is a Berge knot.

An equivalent formulation (of course) is to try to classify knots in Lens spaces which admit an surgery, i.e. to identify the knots as in the formulation of the conjecture above. The equivalent formulation says that these knots should be 1-bridge. The strategy of Hedden-Rasmussen (building on work of Ken Baker and Eli Grigsby) to approach the Berge conjecture depends on characterizing such knots by properties which can be detected by topological invariants that behave well under surgery. An example of such a topological invariant is the Casson invariant , a -valued invariant of integer homology spheres which satisfies the surgery formula where denotes the result of surgery on some integral homology sphere along a fixed knot , and is the Arf invariant. For more sophisticated invariants like knot Floer homology, the surgery formula is replaced by an exact triangle. One important piece of topological information that is detected by knot Floer homology is the genus of a knot. The approach to the Berge conjecture thus rests on Ken Baker’s impressive paper showing that small genus knots (in a sense to be made precise) in Lens spaces have small bridge number.

Hedden remarked in his talk that his work, and that of his collaborators “gave the first examples of an infinite family of knots that were characterized by their knot Floer homology”. Though technically true, I think this overstates the role of knot Floer homology in this case, since the knots (1-bridge knots in Lens spaces) are entirely characterized (up to isotopy) by their genus (and therefore by any topological invariant which detects genus). My immediate instinct was to think that knots with small genus in any 3-manifold should always be quite special, and that a complete classification might even be feasible. My paper with Cameron confirms this suspicion, and gives such a classification. Let me admit at this point that I am not especially interested in the Berge conjecture per se, although I find it interesting that new ideas in 3-manifold topology are starting to have something meaningful to say about it. In any case, I shall not have anything else to say about it (meaningful or otherwise) in this post.

First I should say that I have been using the word “genus” in a somewhat sloppy manner. For an oriented knot in , a Seifert surface is a compact oriented embedded surface whose boundary is . The genus of such a surface is a non-negative integer, and the least such genus over all Seifert surfaces is (said to be) the genus of , denoted . Such a surface represents the generator in the relative homology group which equals since has vanishing homology in dimensions 1 and 2. This relative homology group is dual to , which is parameterized by homotopy classes of maps from to a circle (which is a ). The preimage of a regular value under a smooth map dual to the homology class is a smooth proper surface in whose closure is a Seifert surface. It is immediate that if and only if is an unknot; in other words, the unknot is “characterized” by its genus. There are infinitely many knots of any positive genus in ; on the other hand, there are only two fibered genus 1 knots — the trefoil and the figure 8 knot (three if you distinguish the left-handed from the right-handed trefoil), and it is worth remarking (from the point of view of the motivation of characterizing knots by topological invariants) that a theorem of Yi Ni says that fiberedness of knots can be detected by knot Floer homology.

For knots in integral homology -spheres, the situation is very similar: every knot admits a Seifert surface, and the least genus of such a surface is the genus of a knot. The unknot is (always) characterized by the fact that it has genus , but there are infinitely many knots of every positive genus. For a knot in a general -manifold it is not so easy to define genus. A necessary and sufficient condition for to bound an embedded surface in its complement is that in . However, if has finite order, one can find an open properly embedded surface in the complement of whose “boundary” wraps some number of times around . Technically, let be a compact oriented surface, and a map which restricts to an embedding from the interior of into , and which restricts to an oriented covering map from to (note that we allow to have multiple boundary components). If is the degree of the covering map , we call a -Seifert surface, and define the rational genus of to be , where denotes Euler characteristic, and (for a connected surface ). The reason to use Euler characteristic instead of genus is that Euler characteristic is multiplicative under coverings (unlike genus), and behaves well with respect to “local” operations on surfaces like cut-and-paste. Moreover, (negative) Euler characteristic, unlike genus, is a good measure of complexity for surfaces with possibly many boundary components. The coefficient of in the denominator reflects the fact that genus is “almost” times Euler characteristic. With this definition, we say that the rational genus of , for any knot with of finite order in , is the infimum of over all -Seifert surfaces for and all . The purpose of our paper is to give a complete classification of knots with sufficiently small rational genus, and to show that such knots are always “geometric” — i.e. they can be isotoped into a normal form which is sensitive to the geometric decomposition of the ambient -manifold . Thus the concept of rational genus makes contact between the homological world of the Thurston norm, knot Floer homology and such invariants, and the geometric world of hyperbolic structures, JSJ decompositions and so on.

It is worth pointing out at this point that knots with small rational genus are not special by virtue of being rare: if is any knot in (for instance) of genus , and in is obtained by Dehn surgery on , then the knot has order in , and . Since for “most” coprime the integer is arbitrarily large, it follows that “most” knots obtained in this way have arbitrarily small rational genus.

There is a precise connection between rational genus and the Thurston norm. There is an exact sequence in homology, which contains the fragment . Since , the kernel of is generated by some class , and one can define the affine subspace . By excision, we identify with where is a tubular neighborhood of . Under this identification, the rational genus of is equal to where denotes the (relative) Thurston norm, and the infimum is taken over classes in in the affine subspace corresponding to . Since the Thurston norm is a convex piecewise rational function, this infimum is realized at some rational point. In other words, rational genus of any knot is rational, and is realized by some -Seifert surface, where as above divides (note: if is a rational homology sphere, then necessarily , but if the rank of is positive, this is not necessarily true, and might be arbitrarily large). This relationship to the Thurston norm also gives a straightforward algorithm to compute rational genus, since one can compute Thurston norm e.g. by linear programming in normal surface space relative to any triangulation.

The precise statement of results depends on the geometric decomposition of the ambient manifold . By the geometrization theorem (of Perelman), a closed, orientable -manifold is either reducible (i.e. contains an embedded sphere that does not bound a ball), or is a Lens space, or is hyperbolic, or is a small Seifert fiber space, or is toroidal (i.e. contains an essential (-injective) embedded torus). For the record, the complete “classification” is as follows:

Reducible Theorem: Let be a knot in a reducible manifold . Then either

1. ; or
2. there is a decomposition , and either
   1. is irreducible, or
   2. 

Lens Theorem: Let be a knot in a lens space . Then either

1. ; or
2. lies on a Heegaard torus in ; or
3. is of the form and lies on a Klein bottle in as a non-separating orientation-preserving curve.

Hyperbolic Theorem: Let be a knot in a closed hyperbolic -manifold . Then either

1. ; or
2. is trivial; or
3. is isotopic to a cable of the core of a Margulis tube.

Small SFS Theorem: Let be an atoroidal Seifert fiber space over with three exceptional fibers and let be a knot in . Then either

1. ; or
2. is trivial; or
3. is a cable of an exceptional Seifert fiber of ; or
4. is a prism manifold and is a fiber in the Seifert fiber structure of over with at most one exceptional fiber.

Toroidal Theorem: Let be a closed, irreducible, toroidal 3-manifold, and let be a knot in . Then either

1. ; or
2. is trivial; or
3. is contained in a hyperbolic piece of the JSJ decomposition of and is isotopic either to a cable of a core of a Margulis tube or into a component of ; or
4. is contained in a Seifert fiber piece of the JSJ decomposition of and either
   1. is isotopic to an ordinary fiber or a cable of an exceptional fiber or into , or
   2. contains a copy of the twisted bundle over the Möbius band and is contained in as a fiber in this bundle structure;

or

5. is a -bundle over with Anosov monodromy and is contained in a fiber.

The constant is presumably not optimal, but reflects the coarseness of certain geometric estimates at a particular step in the argument. Broadly speaking, there are two cases to consider: when the knot complement is hyperbolic, and when it is not. The complement is hyperbolic unless it contains an essential subsurface of non-negative Euler characteristic.

The case that is hyperbolic is conceptually easiest to analyze. Let be a surface, embedded in and with boundary wrapping some number of times around , realizing the rational genus of . The complete hyperbolic structure on may be deformed, adding back as a cone geodesic. Just as a cone can be obtained from a wedge of paper by gluing the two edges together, the geometry of a cone geodesic is locally modeled on the quotient space obtained from a (3-dimensional hyperbolic) wedge by gluing the two flat faces together. The thinner the wedge, the smaller the cone angle along the geodesic. For all sufficiently small angles , Thurston proved that there exists a unique hyperbolic metric on which is singular along a cone geodesic, isotopic to , with cone angle . Call this metric space . The cone angle can be increased, deforming the geometry in a family of spaces, until one of the following three things happens:

1. The cone angle is increased all the way to , resulting in the complete hyperbolic structure on , in which is isotopic to an embedded geodesic; or
2. The volume of the family of manifolds goes to zero (and either converges after rescaling to a Euclidean cone manifold, or converges after rescaling to have fixed diameter and injectivity radius going to zero everywhere); or
3. The cone locus bumps into itself (this can only happen for ).

As the cone angle along increases, so does the length of the cone geodesic. Simultaneously, the diameter of an embedded tube about this diameter decreases. While the diameter of the tube is big, the deformation can continue. Hodgson-Kerckhoff analyzed the kinds of degenerations that can occur, and obtained universal geometric control on how fast the tube diameter can shrink, or the length of the cone geodesic grow. They showed that the cone angle can be increased (giving rise to a family of singular hyperbolic structures ) either until , or until the product , where is the length of the cone geodesic, is at least , at which point the diameter of an embedded tube about this cone geodesic is at least . Since in the latter case, one obtains a lower bound on both the length of the cone geodesic and the diameter of an embedded tube, independent of or .

Now, one would like to use this big tube to conclude that is large. This is accomplished as follows. Geometrically, one constructs a -form which agrees with the length form on the cone geodesic, which is supported in the tube, and which satisfies pointwise for some (universal) constant . Then one uses this -form to control the topology of . By Stokes theorem, for any surface homotopic to in one has an estimate

In particular, the area of divided by can’t be too small. However, it turns out that one can find a surface as above with ; such an estimate is enough to obtain a universal lower bound on . Such a surface can be constructed either by the shrinkwrapping method of Calegari-Gabai, or the (related) PL-wrapping method of Soma. Roughly speaking, one uses the cone geodesic as an “obstacle”, and finds a surface of least area homotopic to (rel. boundary) subject to the constraint that it cannot cross the geodesic. Away from the cone geodesic, looks like an ordinary minimal surface. In particular, its intrinsic curvature is no more than the extrinsic curvature of hyperbolic space, which is everywhere. Along the geodesic, looks like a bedsheet hanging on a clothesline; in particular, it does not accumulate any corners or atoms of positive curvature along this singularity, so the Gauss-Bonnet theorem gives the desired bound on .

This leaves the case that is not hyperbolic to analyze. As remarked above, this only occurs when contains an essential surface (which might be closed or proper) of non-negative Euler characteristic, i.e. a sphere, a disk, an annulus or a torus. In this case, one tries to make the intersection of with this essential surface as simple as possible; if one arranges this just right, every intersection contributes a definite amount to the topology of , and one can conclude either that is complicated (in which case is large), or that the intersection is simple, and therefore draw some topological conclusion.

To actually do this in practice is quite complicated, but fortunately it relies on (largely combinatorial) methods developed at length by Gabai, Scharlemann, Gordon and others over the last 30 years to analyze (so-called) “exceptional surgeries”. Of course, the argument is still complicated, and this analysis takes up most of the length of the paper. It is also worth pointing out that every case provided for by the classification above actually occurs, with examples of arbitrarily small rational genus.

This paper raises several natural questions, the most obvious of which is whether the explicit (but quite small) constants can be improved in any way. The constant in the statement of the Toroidal Theorem is really only there to take care of a knot sitting inside a hyperbolic piece in the decomposition; a knot that interacts in a meaningful way with an essential torus necessarily has rational genus at least (for a precise statement, see the paper). As remarked above, knots of (ordinary) genus are very plentiful, even in , and do not “see” any of the ambient geometry, so the wildest and most optimistic guess might be that there is a chance of classifying knots of rational genus at most . There are some (very weak) reasons to think that this fraction is critical, at least in some cases, not least of which is the papers of Hedden and Ni mentioned above. But in the hyperbolic case, it is probably not easy to get a better estimate using purely geometric arguments.

Another approach might be to try to substitute another conclusion (again in the hyperbolic case) than that be isotopic to the cable of a core of a Margulis tube. For instance, one might ask for to admit an insulator family (of the kind Gabai used here), or one might merely ask that be unknotted in the universal cover, or satisfy some other condition. This goes to the heart of a very, very difficult and important question, namely how to identify geometric features of codimension 2 objects in (especially hyperbolic) geometric 3-manifolds from purely topological properties. If I am optimistic, then I can imagine that this paper makes a contribution, however small, to this ongoing project.