# The Legendrian surgery conjecture

Update: Andy Wand has posted a preprint of his proof on the arXiv.  Obviously, definitions and content there supercede what was originally written in this series of blog posts.

Update: (12 May 2014) Some minor technical details have been corrected (Thanks to Patrick Massot for pointing these out)

The Legendrian surgery conjecture is one of the biggest problems in contact topology. The question is, does (-1) Legendrian surgery on Legendrian knots in a contact 3-manifold preserve tightness? Andy Wand has an approach to proving this by characterizing the tightness of some $(M, \xi)$ in terms of the monodromy of an open book supporting the contact structure $\xi$. He gave a seminar talk last month at Georgia Tech that I attended and there is also a video of a similar talk at the Simons Center. He hasn’t posted a written proof yet, an hour-long lecture is not sufficient to cover all the details and I don’t completely understand every part yet, so I can’t verify the proof for myself. However, I do understand some suggestive examples that motivate his approach and I want to blog that. I’ll start by reviewing some background in contact topology, motivating the conjecture and describing Honda-Kazez-Matic’s work on non-right-veering monodromies before moving on to Andy’s work.

Let’s start from the beginning. An overtwisted disk in a contact 3-manifold in an embedded disk $D^2$ such that the contact structure $\xi$ is tangent to the disk along its boundary. For example, consider the unit disk in the contact structure $\xi = \text{ker}(\cos \pi r dz + \sin \pi r d \theta)$. A contact structure is overtwisted if it contains an overtwisted disk and is tight otherwise.

There is nice classification of overtwisted contact structures, due to Eliashberg.

Theorem. (Eliashberg) Let M be a closed, oriented 3-manifold. Then there is 1-1 correspondence between overtwisted contact structures on M, up to isotopy, and homotopy classes of 2-plane fields on M.

Therefore, the classification of overtwisted contact structures is governed by algebraic topology and is not really interesting from a geometric perspective. Understanding and classifying tight contact structures on 3-manifolds is a big remaining topic in contact topology.

Motivation for Legendrian surgery conjecture

One way to construct contact 3-manifolds is by performing Dehn surgery but in a manner that respects the geometry. Recall that $p/q$-Dehn surgery on a knot $K \subset M$ consists of removing a tubular neighborhood $S^1 \times D^2$ of the knot, and then regluing via the diffeomorphism $\phi: T^2 \rightarrow T^2$ that sends the meridian $\mu = pt \times \partial D^2$ to $p \mu + q \lambda$, where $\lambda = S^1 \times pt$ is a longitude (equivalently a framed pushoff of the knot). This operation is well-defined up to isotopy but depends upon the (framing), the explicit identification of $\nu(K) \simeq S^1 \times D^2$. Denote the new manifold as $(M_K(p/q))$

In contact topology, a Legendrian knot has a framing, the Thurston-Bennequin  framing, that is well-defined up to Legendrian isotopy and neighborhoods of all Legendrian knots are contactomorphic. This framing is given by any vector field in M along K that is positively transverse to the contact planes, such as the Reeb vector field for any contact form. When there is a well-defined null-homologous framing, the Thurston-Bennequin framing is often written the integer giving the difference between the null-homologous framing and the TB framing. To properly keep track of surgery framings, coefficients in parentheses will indicate framing relative to the TB framing and coefficients without parentheses will indicate framing relative to the null-homologous framing.

When the surgery coefficient is $(1/k)$ for some integer k, the contact structure on the complement $M \setminus \eta(L)$ extends uniquely across the surgery torus. When this coefficient is (-1), this operation is called Legendrian surgery.

In 3-manifold topology, the Lickorish-Wallace theorem states that every closed, oriented 3-manifold can be obtained by integral surgery on a link in $S^3$. There is an analogous result in contact topology:

Theorem (Ding-Geiges) Every closed contact 3-manifold $(M, \xi)$ is given by $(\pm 1)$-surgery on a Legendrian link in $(S^3, \xi_{std})$.

Here $(\pm 1)$ means that we perform (-1) surgery on some link components and (+1) surgery on the remaining components.

For example, the standard contact structure on $S^1 \times S^2$ is given by (+1) surgery on the $tb=-1$ unknot. The standard overtwisted contact structure on $S^3$, which is contactomorphic to the one described above in the definition of overtwisted disks, is given by (+1) surgery on the Hopf link of $tb=-1$ unknots. The former is tight, the latter is overtwisted and the standard contact structure on $S^3$ is tight (a result due to Bennequin), so (+1) surgery neither preserves tightness nor necessarily creates overtwistedness.

Integral knot surgery can be interpreted in low-dimensional topology as a cobordism from $M$ to $M_K(n)$ given by attaching a 4-dimensional 2-handle to M along K with framing n. This also has a geometric counterpart.  A Stein cobordism between contact manifold $(M, \xi), (M', \xi')$ is a cobordism X that is a complex manifold, equipped with a J-convex function f such that M, M’ are regular level sets of f.

Lemma. There is a Stein cobordism from $(M,\xi)$ to $(M_L(-1), \xi_L(-1))$ given by attaching a handle along each component in L with framing (-1).

Theorem. A cobordism from $(M, \xi)$ to $(M', \xi')$ is Stein if and only if it can be obtained by Legendrian surgery on a link in $(M, \xi)$ [edit: plus 1-handles].

For details, see Cieliebak and Eliashberg’s book.

A contact structure $(M, \xi)$ is Stein fillable if there exists a Stein bordism with boundary $(M,\xi)$. Stein fillability is the strongest notion of fillability (see Laura’s post) and all fillable contact 3-manifolds are tight.

Proposition. Legendrian surgery preserves fillability.

So, Legendrian surgery preserves a large subclass of tight contact structures. Yet there are tight contact 3-manifolds that are not fillable. For example, (+1) surgery on the max TB right-handed trefoil in $S^3$. What about them?

In addition, Legendrian surgery seems to make things more tight. Consider the following lemma:

Lemma. There is a Stein cobordism from every contact 3-manifold to a Stein fillable contact structure.
Proof. (-1) and (+1) surgeries cancel each other out, topologically and geometrically. By Ding-Geiges, there exists a link $L = L_- \cup L_+ \subset (S^3, \xi_{std})$ such that $(\pm 1)$ surgery on $L_{\pm}$ gives $(M,\xi)$. Now performing (-1) surgery on $L_+$ removes all of the (+1) surgeries, leaving a contact manifold obtained by (-1) surgery on a Legendrian link in the Stein fillable $(S^3, \xi_{std})$, hence it is Stein fillable.

This leads us to the following conjecture:

Conjecture. The contact structure given by Legendrian surgery on a Legendrian knot in a tight contact structure is tight.

Filed under Uncategorized

### 3 responses to “The Legendrian surgery conjecture”

1. lstarkston

Thanks for posting on this. I wanted to write about this when Andy Wand gave a talk at UT awhile back, but I couldn’t follow quickly enough to take notes.

One small note on this part. I’m not sure that it is entirely true that every Stein cobordism can be obtained from Legendrian surgery. The 4-manifold may need 1-handles in its decomposition though the rest of the cobordism will be made up of 2-handle attachments that correspond to Legendrian surgery.

• That’s true, I’m always forgetting the 1-handles. Fixed that bit.

2. Gary

Is your overtwisted structure defined on R^3 ?