Proper families for Anosov flows - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-20T00:50:03Z http://mathoverflow.net/feeds/question/7315 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/7315/proper-families-for-anosov-flows Proper families for Anosov flows Steve Huntsman 2009-11-30T22:43:30Z 2010-02-01T22:22:49Z <p>So I've been skimming Bowen's 1972 paper "Symbolic Dynamics for Hyperbolic Flows" hoping it would give me some insight into how to build a Markov family for the cat flow (i.e., the Anosov flow obtained by suspension of the cat map with unit height). For the sake of completeness, the cat flow $\phi$ is obtained as follows:</p> <p>i. Consider the cat map $A$ on the 2-torus and identify points $(Ax,z)$ and $(x,z+1)$ to obtain a 3-manifold $M$</p> <p>ii. Equip $M$ with a suitable metric (e.g., $ds^2 = \lambda_+^{2z}dx_+^2 + \lambda_-^{2z}dx_-^2 + dz^2$, where $x_\pm$ are the expanding and contracting directions of $A$ and $\lambda_\pm$ are the corresponding eigenvalues.)</p> <p>iii. Consider the flow generated by the vector field $(0,1)$ on $M$--that's the cat flow.</p> <p>Unfortunately I'm getting stuck at the first part of Bowen's quasi-constructive proof, which requires finding a suitable set of disks and subsets transverse to the flow. Rather than rehash the particular criteria for a set of disks and subsets used in Bowen's construction, I will relay a simpler but very similar set of criteria, for a <em>proper family</em> (which if it meets some auxiliary criteria is also a Markov family):</p> <p>$\mathcal{T} =$ {$T_1,\dots,T_n$} is called a proper family (of size $\alpha$) iff there are differentiable closed disks $D_j$ transverse to the flow s.t.</p> <ol> <li>the $T_j$ are closed</li> <li>$M = \phi_{[-\alpha, 0]}\Gamma(\mathcal{T})$, where $\Gamma(\mathcal{T}) = \cup_j T_j$</li> <li>$\dim D_j = \dim M - 1$</li> <li>diam $D_j &lt; \alpha$</li> <li>$T_j \subset$ int $D_j$ and $T_j = \bar{T_j^*}$ where $T_j^*$ is the relative interior of $T_j$ in $D_j$</li> <li>for $j \ne k$, at least one of the sets $D_j \cap \phi_{[0,\alpha]}D_k$, $D_k \cap \phi_{[0,\alpha]}D_j$ is empty. </li> </ol> <p>I've been stuck on even constructing such disks and subsets (let alone where the subsets are rectangles in the sense of hyperbolic dynamics). Bowen said this sort of thing is easy and proceeded under the assumption that the disks and subsets were already in hand. I haven't found it to be so. The thing that's killing me is 6, otherwise neighborhoods of the Adler-Weiss Markov partition for the cat map would fit the bill and the auxiliary requirements for the proper family to be a Markov family. </p> <p>I've really been stuck in the mud on this one, could use a push.</p> http://mathoverflow.net/questions/7315/proper-families-for-anosov-flows/7325#7325 Answer by Andrey Gogolev for Proper families for Anosov flows Andrey Gogolev 2009-12-01T00:47:25Z 2009-12-01T02:15:13Z <p>Take Adler-Weiss on $0\times\mathbb T^2$, $1/3\times\mathbb T^2$ and $2/3\times\mathbb T^2$. Take neighborhoods of this tripled Adler-Weiss. Then this collection would satisfy all the properties with $\alpha=1/3$.</p> <p>I am not sure why are you particularly interested in suspension flow, everything is determined by the base Anosov diffeo. </p> <p>Edit: Indeed, this has to be tinkered a bit. Say Adler Weiss has two rectangles with neighborhoods $D_1$ and $D_2$. Then take collection $0\times D_1$, $1/3\times D_1$, $2/3\times D_1$, $\varepsilon\times D_2$, $1/3+\varepsilon\times D_2$, $2/3+\varepsilon\times D_2$.</p> <p>To ensure the second property take $\alpha=1/3+\varepsilon$.</p> http://mathoverflow.net/questions/7315/proper-families-for-anosov-flows/11933#11933 Answer by Steve Huntsman for Proper families for Anosov flows Steve Huntsman 2010-01-15T22:11:34Z 2010-01-15T22:11:34Z <p>Don't upvote this. I just figured since this is getting some attention from <a href="http://mathoverflow.net/questions/11846/has-mathoverflow-yet-led-to-mathematical-breakthroughs/11869#11869" rel="nofollow">another question</a> on MO I'd communicate the blurb that is going to go into what I'm writing now. (See also <a href="http://meta.mathoverflow.net/discussion/64/where-to-keep-track-of-math-overflow-success-stories/" rel="nofollow">a related bit on meta</a>.) A comment won't allow TeX at this point, hence the use of an answer.</p> <blockquote> <p>Given a Markov partition $\mathcal{R} =$ {$R_1,\dots, R_n$} for the cat map and $m \ge 3$, consider the sets $R_{jk} := R_j \times$ {$\frac{k}{m} - j\epsilon$}, where $1 \le k \le m$ and $\epsilon &lt; \frac{1}{mn}$. The family $\mathcal{R}' :=$ {$R_{jk}$} is readily seen to be a proper family for the cat flow. [A. Gogolev, private communication] The Poincaré map for $\mathcal{R}'$ sends $R_{jk}$ to $R_{j,k+1}$ for $1 \le k \le m-1$, and because $\mathcal{R}$ is a Markov partition for the cat map it follows that $\mathcal{R}'$ is a Markov family for the cat flow.</p> </blockquote>