## When does a hypersurface have contact-type?

In a symplectic manifold $(X^{2n},\omega)$, a hypersurface $Y\subset X$ has contact-type if there is a contact form $\lambda$ such that $d\lambda=\omega|_Y$. Recall that a contact form is a 1-form with $\lambda\wedge(d\lambda)^{n-1}>0$, i.e. the opposite of a foliation. For example, a starshaped hypersurface has contact-type in $\mathbb{R}^{2n}$, or more generally an $Y\subset\mathbb{R}^{2n}$ transverse to a Liouville vector field defined in a neighborhood of $Y$. In particular, any contact manifold $Y$ is a contact-type hypersurface in a symplectic manifold (the symplectization $\mathbb{R}\times Y$).

Now it's nice and useful to consider symplectic manifolds where its boundary has contact-type. And this can usually be done given a compact symplectic manifold (cobordism between contact manifolds).

This leads to the question of whether or not you can always build such a space. In other words:
Given a random hypersurface in $\mathbb{R}^{2n}$, is it of contact-type? How do you tell when it's not of contact-type?

Edit: In light of the posted responses, I think it would be appropriate to tweak one of the questions above. In particular, it was essentially pointed out twice that in the "set of hypersurfaces" there are open neighborhoods which contain no contact-type ones. But, are "most" hypersurfaces in $\mathbb{R}^{2n}$ of contact-type? i.e. Should I expect my hypersurface to be of contact-type?

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Not all hypersurfaces in $\mathbb{R}^{2n}$ are of contact type.

Weinstein, in the paper: "On the hypothesis of Rabinowitz' periodic orbit theorems", where he defined the concept of contact type, gives a criterion. If $H^1(\Sigma)=0$, and $\Sigma$ is of contact type, then the characteristic line bundle comes with a distinguished orientation, determined by those vectors $\xi$ such that $\alpha(\xi)>0$ for all contact forms $\alpha$. This is independent of the contact form. For periodic orbits this induces a positivity criterion. In the same paper he also constructs an hypersurface in $\mathbb{R}^4$ which is not of contact type, by showing that the criterion is violated.

Many hypersurfaces are of contact type, as you remarked. Another nice example are mechanical hypersurfaces. These are hypersurfaces arising as level sets from hamiltonians $H=T+V$, where $T$ is the kinetic energy term $T=\frac{1}{2}\vert p\vert^2$, and $V$ is a potential depending only on $q$. This works in general cotangent bundles.

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I haven't read Weinstein's paper, so this example may be essentially the same, but Hofer and Zehnder give an example of a wine bottle $S^3$ in $\mathbb{R}^4$ in their book (at the end of section 4.3). – Sam Lisi Aug 24 at 9:09
Also, to emphasize something implicit in the examples of Weinstein and Hofer-Zehnder is that a small perturbation won't allow you to make them contact. (This is discussed somewhat in Hofer-Zehnder, if I am not mistaken.) This tells you that being of contact-type is not generic for a hypersurface in $\mathbb{R}^4$. – Sam Lisi Aug 24 at 9:26

The condition is clearly quite restricting however it is obvious that no effective proceedure could exist to determine if a hypersurface is or is not of contact type.

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 @Luttinger: welcome to MO! I had some questions, I was wondering if we could chat over email (if you have time and are willing to). I cannot locate your email but mine is on my website. – Chris Gerig Jan 1 at 19:01

Here is an example of a surface which cannot be made of contact type even after isotopy. The elliptic surface $E(1)$ is obtained by blowing up $\mathbb{C}\mathbb{P}^1$ nine times. It is a symplectic four-manifold with fibre of genus 1 and 12 singular fibres. If we make a fibred connected sum between two copies of E(1) we obtain the elliptic surface $E(2)$, which is a symplectic four-manifold with fibre of genus 1 and 24 singular fibres.

Then $E(2)$ is separated by a torus $T^3$ into two pieces which are diffeomorphic to the complement of a regular fibre in $E(1)$. This shows that the separating torus cannot be made of contact type because, by a result of Chris Wendl, all strong fillings of $T^3$ are diffeomorphic to a blow up of $T^*T^2$.

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There are also examples that are characterized dynamically. By Viterbo, every contact-type hypersurface of $\mathbb{R}^{2n}$ carries a closed Reeb orbit, i.e. a closed leaf of the characteristic line bundle. (Actually, Hofer and Zehnder show this is true for any stable hypersurface.) There are examples of autonomous Hamiltonians on $\mathbb{R}^{2n}$ that fail to have any closed orbits on a specific level set, thus providing examples of hypersurfaces that fail to be stable. These are due to Ginzburg and Gurel.

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