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# Permutation groups

Permutation groups appear in many areas of design theory, in particular as automorphism groups of designs.

The specification of an permutation group is:

```permutation_group = element permutation_group {
attribute degree	{ xsd:positiveInteger } ,
attribute order	{ xsd:positiveInteger } ,
attribute domain	{ "points" },
generators ,
permutation_group_properties?
}
```

There are four compulsory properties:

degree

An attribute giving the number of points on which the permutations are defined (the permutation group will then act on the indices ).

order

An attribute giving the number of permutations in the group.

domain
An attribute specifying the domain indexed by the points .

generators

A list of permutations which generate the group. A permutation is represented by the ordered list of its values (the images of the points under the permutation).

For example, the permutation group which is the automorphism group of our Fano plane can be given as:

```<permutation_group degree="7" domain="points" order="168">
<generators>
<permutation>
<z>1</z>
<z>0</z>
<z>2</z>
<z>3</z>
<z>5</z>
<z>4</z>
<z>6</z>
</permutation>
<permutation>
<z>0</z>
<z>2</z>
<z>1</z>
<z>3</z>
<z>4</z>
<z>6</z>
<z>5</z>
</permutation>
<permutation>
<z>0</z>
<z>3</z>
<z>4</z>
<z>1</z>
<z>2</z>
<z>5</z>
<z>6</z>
</permutation>
<permutation>
<z>0</z>
<z>1</z>
<z>2</z>
<z>5</z>
<z>6</z>
<z>3</z>
<z>4</z>
</permutation>
<permutation>
<z>0</z>
<z>1</z>
<z>2</z>
<z>4</z>
<z>3</z>
<z>6</z>
<z>5</z>
</permutation>
</generators>
</permutation_group>
```

There are also various properties which can optionally be specified:

primitive

True if the group acts primitively on points. A permutation group is primitive if it preserves no non-trivial equivalence relation. By convention, we assume that a primitive group is transitive (that is, any point can be carried to any other by some group element). (So the trivial group acting on two points is not primitive.)

generously_transitive, multiplicity_free, stratifiable

Each orbit of the group acting on the set of ordered pairs of points can be represented by a matrix of zeros and ones of order (which can be thought of as the characteristic function of the orbit). These basis matrices span the centraliser algebra of the group (the algebra of all matrices commuting with the group elements). Now the group is generously transitive if all the basis matrices are symmetric; it is multiplicity-free if the basis matrices commute; and it is stratifiable if the symmetrised basis matrices commute. Each concept implies its successor in the order given.

A transitive permutation group is generously transitive iff any two points can be interchanged by some element of the group; it is multiplicity-free iff no irreducible constituent of the permutation character occurs with multiplicity greater than 1; and it is stratifiable iff the orbits of the group on unordered pairs form an association scheme. All these properties are false if the group is not transitive.

no_orbits

The number of orbits on points. The group is transitive exactly when there is just one orbit on points.

degree_transitivity

The maximum number such that the group is -transitive on points (that is, any -tuple of distinct points can be carried to any other by some group element).

rank

The number of orbits of the group on the set of ordered pairs of points. Note that this is defined for any permutation group; if the group is transitive, it is equal to the number of orbits of the stabiliser of a point.

cycle_type_representatives

see below

The cycle type of a permutation is the multiset of its cycle lengths (when it is written as a product of disjoint cycles). The element cycle_type_representative consists of a cycle type and an element of the group having that cycle type, and optionally the number of elements of the group having that cycle type. cycle_type_representatives is a list of these cycle_type_representative elements, one for each cycle type represented by an element of the group.

For the example above, there are five cycle types, , and (the last being the identity). The cycle type representative for the second type is:

```<cycle_type_representative>
<permutation>
<z>0</z>
<z>2</z>
<z>1</z>
<z>5</z>
<z>6</z>
<z>4</z>
<z>3</z>
</permutation>
<cycle_type ordered="true"><z>1</z><z>2</z><z>4</z></cycle_type>
<no_having_cycle_type>
<z>42</z>
</no_having_cycle_type>
</cycle_type_representative>
```

Next: Numerical Data Types Up: The External Representation of Previous: Functions and Index Flags   Contents
Peter Dobcsanyi 2003-12-15