Graph::Maker::Keller - create Keller graphs
use Graph::Maker::Keller; $graph = Graph::Maker->new ('Keller', N => 2);
Graph::Maker::Keller creates a
Graph.pm graph of a Keller graph. This is a graph form of Keller's conjecture on N-dimensional tilings by unit hypercubes.
The Keller graph N has 4^N vertices numbered 0 to 4^N-1. These vertices are treated as base-4 integers. Edges are between vertices differing in 2 or more digit positions, at least one of which is difference = 2 mod 4.
N 0 1 2 3 4 5 vertices 1, 4, 16, 64, 256, 1024, ... 4^N degree 0, 0, 5, 34, 171, 776, ... 4^N - 3^N - N edges 0, 0, 40, 1088, 21888, 397312, ... (4^N-3^N-N)*4^N / 2
Each vertex has the same degree (is regular) since digit differences can all be taken mod 4. Each vertex degree is 4^N - 3^N - N. This is edges to 4^N vertices (including self) except no edge to those differing by only 0,1,3 in any or all digit positions, which is 3^N combinations (including no differences at all for vertex itself), and also no edge to vertices differing by 2 mod 4 at a single digit, which is N possible digit positions.
N=0 and N=1 have no edges since there are only 0 digits and 1 digit respectively, so nothing differs in 2 digit positions.
N=2 (16 vertices and 40 edges) is the Clebsch graph, which is the 16-cyclotomic graph.
subgraph => 1 gives the subgraph of the Keller graph induced by neighbours of vertex 0. This means the neighbours of vertex 0 (and not 0 itself) and the edges among those neighbours.
subgraph => 1 N 0 1 2 3 4 5 vertices 0, 0, 5, 34, 171, 776, ... 4^N - 3^N - N edges 0, 0, 0, 261, 9435, 225990, ...
The number of vertices is the degree above. This subgraph is of interest since the Keller graph is vertex transitive and so the problem of finding a clique of size k in the Keller graph is reduced to finding a clique of size k-1 in this subgraph. The size of the maximum clique is related to Keller's conjecture.
N=2 subgraph has no edges. Its vertices are 12, 21, 22, 23, 32 in base-4 and any pair of them either differ in only 1 digit or differ in 2 digits but without a difference 2 mod 4.
$graph = Graph::Maker->new('Keller', key => value, ...)
The key/value parameters are
N => integer subgraph => 0 or 1, default 0 graph_maker => subr(key=>value) constructor, default Graph->new
Other parameters are passed to the constructor, either
If the graph is directed (the default) then edges are added both forward and backward between vertices. Option
undirected => 1creates an undirected graph and for it there is a single edge between vertices.
House of Graphs entries for graphs here include
- N=2, https://hog.grinvin.org/ViewGraphInfo.action?id=975 (Clebsch)
- N=3 subgraph, https://hog.grinvin.org/ViewGraphInfo.action?id=22730
- N=4 subgraph, https://hog.grinvin.org/ViewGraphInfo.action?id=22732
Entries in Sloane's Online Encyclopedia of Integer Sequences related to these graphs include
A202604 clique numbers (size of maximum clique)
O. H. Keller, "Uber die luckenlose Einfullung des Raumes mit Wurfeln", Journal für die Reine und Angewandte Mathematik (Crelle's journal), volume 163, 1930, pages 231-248.
The second DIMACS challenge 1993 included the N=4 subgraph among its benchmarks for comparing clique-finding algorithms, http://dimacs.rutgers.edu/Challenges/ =over
Copyright 2015, 2016, 2017, 2018, 2019 Kevin Ryde
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