Hello,  

One of the subgrouops of $SO(n)$ which acts transitively on the sphere $S^{n-1}$ is the (compact) symplectic group $Sp(n/4)$. The center of $Sp(m)$ is isomorphic to $\mathbb{Z}\_2$. Can we embed $Sp(n/4)/\mathbb{Z}\_2$ in $SO(n)$ (as a Lie subgroup)? If the answer is yes, what happens to its orbits? Precisely, is $S^{n-1}$ one of its orbits?  

Thanks



Transitive action on the sphere
(I add details to my comments.) The answer depends on $n=4r$. Write $G=Sp(r)/\mu\_2$. If $r=1$, then $G\simeq SO\_3$, so $G$ admits a faithful 4-dimensional representation into $SO\_4$. Similarly, if $r=2$, then $G\simeq SO\_5$, hence $G$ admits a faithful 8-dimensional representation into $SO\_8$. (Of course, in these cases $S^{4r-1}$ is not an orbit.) For $r\ge 3$ the group $G$ has no nontrivial representations of dimension $4r$, see below, hence it cannot be embedded into $SO\_{4r}$.


An irreducible real $n$-dimensional representation of the real algebraic group $G$ induces an irreducible complex $n$-dimensional representation
of $G\_{\mathbb C}=Sp\_{r,{\mathbb C}}/\mu\_2$.
The irreducible complex representations of the simple group 
$\widetilde G\_{\mathbb C} =Sp\_{r,{\mathbb C}}$ 
of type $C\_r$ 
for $r>1$ of dimension $n<{\rm dim}\ \widetilde{G}\_{\mathbb C}$
are listed in the [paper of Andreev, Vinberg, and Elashvili](http://link.springer.com/content/pdf/10.1007%2FBF01076005), 
Table 1. 
They are the fundamental irreducible representations $R(\pi\_1)$ of dimension $2r$, $R(\pi\_2)$ of dimension $2r^2-r-1$, 
and, for $r=3$, $R(\pi\_3)$ of dimension 14. Since the representations $R(\pi\_1)$ and $R(\pi\_3)$ 
are nontrivial on the center $Z(\widetilde G\_{\mathbb C})\simeq\mu\_2$,
we see that the only nontrivial irreducible representation of $G\_{\mathbb C}$ of dimension $n<{\rm dim}\ G\_{\mathbb C}$
is the representation $R(\pi\_2)$ of dimension $2r^2-r-1$,
hence $R(\pi\_2)$ is the irreducible representation of $G\_{\mathbb C}$ of the smallest dimension.
For $r\ge 3$ we have $2r^2-r-1>4r$, hence $G\_{\mathbb C}$ has no nontrivial representations of dimension $4r$.


