# Products of subgroups that generate a finite group

Consider the following general problem. There is a finite group $G$ and $H_1,H_2 < G$. Suppose we know that $\langle H_1, H_2 \rangle = G$, i.e. $G$ is generated by $H_1$ and $H_2$. Denote by $n_0$ the minimal $n \in \mathbb{N}$ such that the following holds: $$G = \underbrace{H_1 H_2 H_1 H_2 \ldots H_1}_{\text{2n-1 subgroups}}$$ Problem: How to estimate $n_0$?

This is, definitely, too general.

Question: Do you know about any results of such kind regarding any specific group $G$ and corresponding specific $H_1$ and $H_2$? Maybe matrix groups?

• We have $|H_1H_2| = |H_1||H_2|/|H_1 \cap H_2|$, so you can determine whether $G = H_1H_2$ just by looking at orders of subgroups. Also, knowing how many steps it takes to cover the commutator group $[H_1,H_2]$ will give a good estimate, since $G = [H_1,H_2]H_1H_2$. Jul 12 '14 at 15:04

Finite groups for which $n_0=1$ have been studied in various guises by various people:

• Geometric ABA-groups by Higman and McLauglin. This paper connects the study of groups $G$ that factorize as $G=ABA$, with the study of automorphism groups of designs. This is a famous paper that has been cited many times. Note, though, that to make the connection to designs, one needs to place restrictions on the intersection $A\cap B$. The nature of this restriction varies.

• Bruhat decompositions. This is due first to Bruhat, and later to Chevalley, and is a result for reductive linear algebraic groups $G$ (and finite groups of Lie type). The precise statement varies depending on what assumptions you are making about your group but, roughly speaking, it says that one can write $G=BNB$ where $B$ is a Borel subgroup, and $N$ is the normalizer of a maximal split torus.

• Finally, the following paper...

Alavi, S. Hassan; Praeger, Cheryl E. On triple factorizations of finite groups. J. Group Theory 14 (2011), no. 3, 341–360.

... studies the general question of which groups $G$ can be written as $G=ABA$. The results connect the factorization to the associated permutation representations of $G$ (on the set of cosets of $A$ and $B$).

• There are also Zappa-Szep products, that is, groups of the form $AB$ for subgroups $A$ and $B$ such that $A \cap B$ is trivial. See for instance math.stackexchange.com/questions/107781/… Jul 15 '14 at 2:56