1
$\begingroup$

Edit: According to the comment of Michael Albanese we revise the question.

Assume that $n$ is an odd integer and $M\subset \mathbb{R}^{2n}$ is a compact orientable $n$ dimensional submanifold.

Does there exist a non vanishing smooth vector field $X$ on $M$ with the following property?

$$\forall p\in M\quad \omega(X(p), V_p)=0,\quad \forall V_p\in T_pM$$

where $\omega$ is the standard symplectic structure of $\mathbb{R}^{2n}$.

$\endgroup$
2
  • 1
    $\begingroup$ Do you want $M$ to be a hypersurface? As written, $M$ need not be odd-dimensional (which your title suggests you intend it to be). $\endgroup$ May 7, 2019 at 12:10
  • $\begingroup$ @MichaelAlbanese Thanks for your comment. I forgot to mention that $M$ is $n$ dimensional. Now oddness assumption is meaning full by two reasons: first we are sure that $M$ admit a non vanishing section second fiberise there is a vector which lies in $\omega$- angilator of the fiber. Now the question seeks for a global anhilator section. Please see my last comment to the answer to this questio mathoverflow.net/questions/330686/… $\endgroup$ May 7, 2019 at 18:34

1 Answer 1

4
$\begingroup$

This is not necessarily true, even locally. Consider $M \subset \mathbb{R}^6$ given by the graph of the function $f : \mathbb{R}_x^3 \rightarrow \mathbb{R}_y^3$ with

$$f(x_1,x_2,x_3) = (x_2x_3, 0, -x_1x_2)$$

Then at a point $p = (\vec{x},f(\vec{x})) \in M \subset \mathbb{R}_x^3 \times \mathbb{R}_y^3$, we compute $T_pM$ is given by the span of the vectors

$$e_1 = \partial_{x_1} -x_2\partial_{y_3}$$ $$e_2 = \partial_{x_2} + x_3\partial_{y_1} - x_1\partial_{y_3}$$ $$e_3 = \partial_{x_3} + x_2\partial_{y_1}$$

Explicitly, we have that in this basis,

$$\omega = \begin{pmatrix} 0 & -x_3 & -x_2 \\ x_3 & 0 & -x_1 \\ x_2 & x_1 & 0 \end{pmatrix}$$

from which it is easy to compute that so long as $p \neq 0$, any $X$ such that $\omega(X,v) = 0$ for all $v \in T_pM$ (i.e. $X$ is in the nullspace of the above matrix) must satisfy that $$X(p) \in \mathrm{span}\left\langle\begin{pmatrix}x_1 \\ - x_2 \\ x_3\end{pmatrix}\right\rangle$$ By continuity, $X(0)$ would need to be a multiple of $e_1$ (by looking at $X(p)$ along the axis $x_2 = x_3 = 0$) and also a multiple of $e_3$ (similarly). So there can be no nonzero $X(p)$ in a neighborhood of $0$.

For $M$ in higher dimensions, one can repeat the argument by successively taking $M \times \mathbb{R}^2 \subset \mathbb{R}^{2n} \times \mathbb{R}^4$, where the $\mathbb{R}^2 \subset \mathbb{R}^4$ we choose is symplectic. For a compact $M$, simply choose one which matches this model near $0$.

$\endgroup$
3
  • 1
    $\begingroup$ I have answered this in the last sentence: yes. Simply take a compact $M$ which matches this model near $0$. $\endgroup$
    – KSackel
    May 10, 2019 at 19:58
  • $\begingroup$ Thanks again for your interesting answer. i erroneously deleted my previous comment which was a result of not reading your last sentence. $\endgroup$ May 10, 2019 at 20:27
  • $\begingroup$ what about if we modify the question as follows: can a compact odd($n$) dimensional manifold be embedded in $\mathbb{R}^{2n}$ which admit a non zero section which $\omega$-annihilates the tangent space? $\endgroup$ May 10, 2019 at 20:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.