Automorphisms of Graphs

First Example: The graph of a square

Let's look at the graph of a square. Since a square is a 2-cube, we can create the polytope and look at its graph:

polytope > $c=cube(2);
polytope > $c->GRAPH->VISUAL;

To study the automorphisms of this graph, we create a props::Graph object refering to the C++ class named Graph (see the tutorial on graphs for more details):

polytope > $g=new props::Graph($c->GRAPH->ADJACENCY);

The picture of the graph shows that the node with label 0 is adjacent to the nodes 1 and 2, Node 1 is adjacent to 0 and 3, and so on. For the complete adjacency information you can print $c→GRAPH→ADJACENCY or just the props::Graph object $g:

polytope > print rows_labeled($g);
0:1 2
1:0 3
2:0 3
3:1 2

Now, we compute the generators of the automorphism group of this graph:

polytope > $aut=automorphisms($g);

In this case, the automorphism group has two generators:

polytope > print $aut;
0 2 1 3
1 0 3 2

Each generator is a permutation on the nodes. The first generator fixes the nodes 0 and 3, and exchanges the nodes 1 and 2, i.e., it describes the reflection along the diagonal through 0 and 3. The second generator is the reflection along the horizontal line.

In order to be able to work with the group, we create a new Group object, which lives in the application group:

polytope > $autgroup=new group::Group(GENERATORS=>$aut);

Now we can ask for basic properties of the group, e.g., the number of elements:

polytope > print $autgroup->ORDER;
8

Sometimes, it is useful to know which elements of the group fix a specific set of indices, that is, we are interested in the subgroup which is the stabilizer of the given set. In the first case, we just fix the index 0:

polytope > $s0=new Set<Int>(0);
polytope > $stab0=group::stabilizer_of_set($autgroup,$s0);

We learn that the node 0 is only fixed by the permutation 0 2 1 3:

polytope > print $stab0->ORDER;
2
polytope > print $stab0->GENERATORS;
0 2 1 3

In the second case, we look at the subgroup which leaves the set {1,2} invariant:

polytope > $s12=new Set<Int>(1,2);
polytope > $stab12=group::stabilizer_of_set($autgroup,$s12);

Now, we obtain a group of order 4:

polytope > print $stab12->ORDER;
4
polytope > print $stab12->GENERATORS;
3 1 2 0
0 2 1 3

Finally, we compute the orbits of the indices under the three different groups:

polytope > print group::compute_domain_orbits($stab0);
{0}
{1 2}
{3}

polytope > print group::compute_domain_orbits($stab12);
{0 3}
{1 2}

polytope > print group::compute_domain_orbits($autgroup);
{0 1 2 3}
tutorial/aut_of_graphs.txt · Last modified: 2017/06/13 11:22 by oroehrig
Except where otherwise noted, content on this wiki is licensed under the following license: CC Attribution-Noncommercial-Share Alike 4.0 International
Recent changes RSS feed Donate Powered by PHP Valid XHTML 1.0 Valid CSS Driven by DokuWiki