# Block (permutation group theory)

In mathematics and group theory, a block system for the action of a group G on a set X is a partition of X that is G-invariant. In terms of the associated equivalence relation on X, G-invariance means that

x ~ y implies gx ~ gy

for all gG and all x, yX. The action of G on X induces a natural action of G on any block system for X.

The set of orbits of the G-set X is an example of a block system. The corresponding equivalence relation is the smallest G-invariant equivalence on X such that the induced action on the block system is trivial.

The partition into singleton sets is a block system and if X is non-empty then the partition into one set X itself is a block system as well (if X is a singleton set then these two partitions are identical). A transitive (and thus non-empty) G-set X is said to be primitive if it has no other block systems. For a non-empty G-set X the transitivity requirement in the previous definition is only necessary in the case when |X|=2 and the group action is trivial.

## Characterization of blocks

Each element of some block system is called a block. A block can be characterized as a non-empty subset B of X such that for all gG, either

• gB = B (g fixes B) or
• gBB = ∅ (g moves B entirely).

Proof: Assume that B is a block, and for some gG it's gBB ≠ ∅. Then for some xX it's gx ~ x. Let yB, then x ~ y and from the G-invariance it follows that gx ~ gy. Thus y ~ gy and so gB = B. In the other direction, if the set B satisfies the given condition then it's easy to see that the system {gB | gG} together with the complement of the union of these sets is a block system containing B.

In particular, if B is a block then gB is a block for any gG, and if G acts transitively on X then the set {gB | gG} is a block system on X.

## Stabilizers of blocks

If B is a block, the stabilizer of B is the subgroup

GB = { gG | gB = B }.

The stabilizer of a block contains the stabilizer Gx of each of its elements. Conversely, if xX and H is a subgroup of G containing Gx, then the orbit H.x of x under H is a block contained in the orbit G.x and containing x.

For any xX, block B containing x and subgroup HG containing Gx it's GB.x = BG.x and GH.x = H.

It follows that the blocks containing x and contained in G.x are in one-to-one correspondence with the subgroups of G containing Gx. In particular, if the G-set X is transitive then the blocks containing x are in one-to-one correspondence with the subgroups of G containing Gx. In this case the G-set X is primitive if and only if either the group action is trivial (then X = {x}) or the stabilazer Gx is a maximal subgroup of G (then the stabilizers of all elements of X are the maximal subgroups of G conjugate to Gx because Ggx = gGxg1).