Is there a simple proof that there is no Anosov flow on $S^2$? Where can I find it?
1 Answer
The usual definition of Anosov flow requires three invariant subbundles, so I guess you are actually asking about the 3sphere?
Plante and Thurston have proved in
Plante, J. F.; Thurston, W. P., Anosov flows and the fundamental group, Topology 11, 147150 (1972). ZBL0246.58014.
that if a manifold admits a codimension 1 Anosov flow, then its fundamental group has exponential growth.

2$\begingroup$ You're right! What about Anosov diffeomorphisms? $\endgroup$– UagiNov 11, 2022 at 11:52

3$\begingroup$ The only (orientable) surface admitting an Anosov diffeomorphism is the torus. One way to see this is using the Euler characteristic: the unstable foliation has no closed leaf (otherwise the diffeomorphism would be expanding on the closed leaf), and the only surface admitting such a foliation is the torus. $\endgroup$ Nov 11, 2022 at 12:20

$\begingroup$ @Uagi  If you admit "poles" (oncepronged singularities of the foliations) then you can obtain pseudoAnosov homoemorphisms. This is why "pseudoAnosov braids" can exist. $\endgroup$– Sam NeadNov 23, 2022 at 11:33