# Density and irreducibility

Let G be an linear algebraic group, and let H be a zariski dense subgroup of G. Then does H have to be a irreducible subgroup of G? Here H being irreducible means that H has no nontrivial invariant subspaces. Assume that G comes with a faithful irreducible representation. For example we can take G=SL(n,R) or SO(n,R). In these cases subgroup H being irrreducible means that when H acts on R^n there is no nontrivial invariant subspace.

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Subspaces of what? Does G come with a faithful irrep, that remains irreducible for H? In which case you want somehow to exclude G = the circle. – Allen Knutson Oct 30 '10 at 2:42
Yes G comes with a faithful irreducible representation. For example we can take G=SL(n,R) or SO(n,R). In these cases subgroup H being irrreducible means when H acts on R^n there is no nontrivial invariant subspaces. – user9552 Oct 30 '10 at 4:14
@unknown Mixing Lie group and algebraic group language is risky, as when you take matrix groups over the reals as examples: these are better viewed as the real points of certain algebraic groups (or group schemes). The Zariski topology comes into play when you look at polynomial or rational function properties, so "faithful irreducible representation" in the question should refer to a "rational" (algebraic group) representation. – Jim Humphreys Oct 30 '10 at 19:12

If $G$ is a linear algebraic group, then you can recognize the coordinate algebra of $G$ as the algebra generated by all matrix entries of all algebraic representations of $G$. (In other words, every matrix entry is a function on $G$, they are all regular functions, and they generate the coordinate ring $O(G)$.) This implies that if $H \subseteq G$ is Zariski dense, then the $H$-decomposition of every algebraic representation of $G$ is exactly the same as the $G$-decomposition. Because, if $W \subseteq V$ is an $H$-subrepresentation, then this says that in a basis of $V$ that extends a basis of $W$, certain matrix entries vanish on elements of $H$; then since $H$ is Zariski dense, the same matrix entries vanish on all of $G$. In particular, every $G$-irreducible representation is also $H$-irreducible.
More simply, perhaps (without referring to coordinate algebras): with $W\subset V$ as above, the stabilizer of $W$ is an algebraic subgroup of $G$, hence equal to $G$ if it contains $H$. – Laurent Moret-Bailly Nov 1 '10 at 12:13
Let me add another perspective. $H = SL_2(\mathbb{Z})$ is Zariski dense in $G=SL_2(\mathbb{R})$, but the restriction of a holomorphic discrete series representation of $G$ to $H$ is never irreducible. This is OK because these are not algebraic reps of $G$. [1] is a good reference about this.