# Mathematics 676-3 Homework (due Mar 3) 18) 19)

```Mathematics 676-3
Homework (due Mar 3)
A. Hulpke
18) Show that an iterated subdirect product of simple groups must be a direct product.
19) Let G = TransitiveGroup(24, 40)
= h(1, 11, 9, 19, 17, 3)(2, 12, 10, 20, 18, 4)(5, 15, 14, 24, 22, 7)(6, 16, 13, 23, 21, 8),
(1, 14)(2, 13)(3, 4)(5, 17)(6, 18)(9, 22)(10, 21)(11, 12)(19, 20)i .
Find all subgroups StabG (1) ≤ S ≤ G. (Hint: Use AllBlocks in GAP.)
20) a) Let n = a + b with a &lt; b. Show that Sn acts primitively on the a-element subsets of n.
b) Show that Sa &times; Sb is a maximal subgroup of Sn .
c) Why is a) and b) false if a = b?
21) Let G be transitive on Ω with two unrefinable block systems (i.e. any finer block system is
the trivial system B1 ). Show that G can be written as a subdirect product of groups of smaller
degree.
22) Let F be a finite field and G = GLn (F), acting on the nonzero vectors of F n .
a) Show that this action is imprimitive, by describing a nontrivial block system.
b) Describe (e.g. by describing the matrices) the stabilizer of a vector, and the stabilizer of one of
a block.
c) Show that the action of G on the blocks is primitive. (Thus the block stabilizer is a maximal
subgroup of G.)
23) Let Ω = {1, . . . , n}, G ≤ SΩ be a transitive permutation group and S = StabG (1). Let
∆ = {ω ∈ Ω | ωg = ω∀g ∈ S} .
a) Show that ∆ is a block for G on Ω.
b) Show that StabG (∆) = NG (S).
```