Let $G$ be a Lie group (not necessarily connected) and let $H$ be a closed subgroup of $G$. I am after an algebraic (group theoretic) characterization of when the homogeneous space $G/H$ is connected.


I found the following necessary condition in Onishchik/Vinberg, Lie groups and algebraic groups: Let $G\_0$ denote the connected component of the identity in $G$ then $G/G\_0$ is a discrete group. If $G/H$ as above is connected then $G/G\_0$ is isomorphic (as a group) to $H/(H\cap G\_0)$.


Does anybody know if this is also sufficient? Onishchik/Vinberg do not give a proof. Any idea of how to go about that? I was quite surprised that I could not find anything relevant on this question in other books (Kobayashi/Nomizu, Helgason, Hilgert/Neeb, ...) and also a google search throws up nothing.


The above necessary condition seems to formalize the intuitive idea that $H$ needs to contain an element from every connected component of $G$ for $G/H$ to be connected. 


Ultimately, I want to be able to decide this algorithmically (at least for matrix Lie groups), but this is not part of this question.


Edit: clarified wording, I did not know how to formally link the idea in paragraph four to the quotient group objects when I asked the question



When is a homogeneous space connected?
As Ryan says, the idea is in the first sentence of the fourth paragraph, so it's just a matter of formalizing it into a proof. 


There is a natural isomorphism $\pi\_0(G) \cong G/G\_0$ for any Lie group $G$. The inclusion $i: H \to G$ induces by functoriality a function $\pi\_0(i): \pi\_0(H) \to \pi\_0(G)$; by the natural isomorphism it is given by a composite 


$$H/H\_0 \to H/H \cap G\_0 \hookrightarrow G/G\_0$$ 


where the first map is well-defined as $H\_0 \subseteq H \cap G\_0$ and is clearly an epi. Thus the composite gives the epi-mono factorization of $\pi\_0(i)$, and the necessary condition cited in the question (as corrected by Anton Petrunin), which is that the inclusion is onto, is thus sufficient to guarantee that $\pi\_0(i)$ is surjective, or that $H$ intersects each path component of $G$ in a nonempty set. 


So given $g, g' \in G$, we may pick $h \in H$ so that $g$ and $g'h$ belong to the same path component. Pick a path $\alpha: I \to G$ connecting them; then the composite $I \stackrel{\alpha}{\to} G \stackrel{\pi}{\to} G/H$ is a path connecting $gH$ to $g'H$. 


