Mappings of mapping class groups - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T00:48:55Z http://mathoverflow.net/feeds/question/7622 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/7622/mappings-of-mapping-class-groups Mappings of mapping class groups algori 2009-12-03T00:12:46Z 2012-03-12T20:45:59Z <p>Let $X$ be a compact non-orientable surface, maybe with boundary, and let $\tilde X$ be the orienting cover of $X$. If I understand correctly, any smooth automorphism of $X$ lifts naturally to an automorphism of $\tilde X$ (since $\tilde X$ can be viewed as the space of couples (a point $x$ of $X$, an orientation of $T_xX$)). Moreover, composition lifts to composition and isotopic automorphisms lift to isotopic automorphisms. So we get a map $MCG(X)\to MCG(\tilde X)$ where $MCG$ stands for the mapping class group. (In the definition of the mapping class groups we do not require that maps or isotopies should be the identity on the boundary.)</p> <ol> <li><p>Is there a simple way to describe the kernel of the above map?</p></li> <li><p>Is it true that the mapping class group of a of non-orientable surface injects into the mapping class group of some orientable surface (not necessarily the orienting cover)?</p></li> <li><p>(a slightly unrelated question which nonetheless involves mapping class groups) Is it true that the mapping class group of an orientable surface without boundary injects into the mapping class group of some orientable surface with boundary?</p></li> </ol> http://mathoverflow.net/questions/7622/mappings-of-mapping-class-groups/7630#7630 Answer by Ryan Budney for Mappings of mapping class groups Ryan Budney 2009-12-03T01:53:13Z 2009-12-03T01:53:13Z <p>Your question (1) is pretty classical -- the Birman Hilden Annals paper from 1973 essentially answers it, no? I mean, they don't deal with the non-orientable case in their paper as far as I recall but the techniques still work. </p> <p>Re (3), your notion of mapping class group for a surface with boundary would typically be called the mapping class group of a closed surface with marked points, in the literature I'm familiar with, denoted something like $MCG(X,n)$ for a closed surface $X$ and $n$ marked points -- in your case they would be circle boundary components. </p> <p>In that regard there are extensions ($X$ a boundaryless surface)</p> <p>$\pi_1 Diff(X) \to \pi_1 C_n X \to MCG(X,n) \to MCG(X) \to 0$</p> <p>$C_n(X)$ is the configuration space of $n$ points in the surface $X$. </p> <p>$\pi_1 Diff(X)$ is typically trivial but there are some non-trivial cases like the torus, sphere $\mathbb RP^2$, the Klein bottle. Some of these special cases give you injectivity $MCG(X,n) \to MCG(X)$ for $n$ small, but not many.</p> <p>Anyhow, it's a natural map and is frequently useful. Are you really more interested in knowing exactly when there is such an embedding or not, or are you more interested in general relations? ie: do you have a reason for wanting to know the answer to these questions?</p> http://mathoverflow.net/questions/7622/mappings-of-mapping-class-groups/7637#7637 Answer by Richard Kent for Mappings of mapping class groups Richard Kent 2009-12-03T03:40:33Z 2009-12-03T03:40:33Z <p>3) is an open problem. With notation as in comments to Ryan's post, if $n=1$ and $g$ is at least two, then there's no embedding, as every finite subgroup of the target is cyclic. If $n=0$, then there is a $g'$ and an injection, see Aramayona, Leininger, Souto, Injections of mapping class groups, Geometry &amp; Topology 13 (2009) 2523--2541. They also have references to what's known.</p> http://mathoverflow.net/questions/7622/mappings-of-mapping-class-groups/7646#7646 Answer by Oscar Randal-Williams for Mappings of mapping class groups Oscar Randal-Williams 2009-12-03T08:30:34Z 2009-12-03T08:30:34Z <p>The second of your questions is answered in</p> <p>Graham Hope and Ulrike Tillmann "On the Farrell cohomology of the mapping class group of non-orientable surfaces" Proc. Amer. Math. Soc. 137 (2009), no. 1, 393--400. </p> <p>as Lemma 2.1, using results of Birman and Chillingworth. It can be realised as a subgroup of the mapping class group of its orientation cover. I <strong>think</strong> the injection is induced by the "lift diffeomorphisms to the orientation cover" map, which answers the first question also.</p> http://mathoverflow.net/questions/7622/mappings-of-mapping-class-groups/91018#91018 Answer by Frédéric Mangolte for Mappings of mapping class groups Frédéric Mangolte 2012-03-12T20:45:59Z 2012-03-12T20:45:59Z <p>You should have a look at the paper <a href="http://www.springerlink.com/content/xkf2bqn5p7bndcrd/" rel="nofollow">http://www.springerlink.com/content/xkf2bqn5p7bndcrd/</a> which deals with nonorientable surfaces with boundary.</p>