Brinkmann graph: Difference between revisions
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{{infobox graph | |||
| name = Errera graph | |||
| image = [[Image:Errera graph alt.svg|220px]] | |||
| image_caption = The Errera graph | |||
| namesake = Alfred Errera | |||
| vertices = 17 | |||
| edges = 45 | |||
| automorphisms = 20 ([[dihedral group|D]]<sub>10</sub>) | |||
| girth = 3 | |||
| radius = 3 | |||
| diameter = 4 | |||
| chromatic_number = 4 | |||
| chromatic_index = 6 | |||
| properties = [[Planar graph|Planar]]<br>[[Hamiltonian graph|Hamiltonian]]<ref>{{MathWorld|urlname=HamiltonianGraph|title=Hamiltonian Graph}}</ref> | |||
}} | |||
In the [[mathematics|mathematical]] field of [[graph theory]], the '''Errera graph''' is a graph with 17 [[vertex (graph theory)|vertices]] and 45 [[edge (graph theory)|edge]]s discovered by Alfred Errera.<ref>{{MathWorld|urlname=ErreraGraph|title=Errera graph}}</ref> Published in 1921, it provides an example of how [[Alfred Kempe|Kempe]]'s proof of the [[four color theorem]] cannot work.<ref>Errera, A. "Du coloriage des cartes et de quelques questions d'analysis situs." Ph.D. thesis. 1921.</ref><ref>Peter Heinig. [http://www-m9.ma.tum.de/foswiki/pub/Allgemeines/PeterHeinig/erreraGraphIsNarrowProof.pdf Proof that the Errera Graph is a narrow Kempe-Impasse]. 2007.</ref> | |||
Later, the Fritsch graph and Soifer graph provide two smaller counterexamples.<ref>Gethner, E. and Springer, W. M. II. "How False Is Kempe's Proof of the Four-Color Theorem?" Congr. Numer. 164, 159-175, 2003.</ref> | |||
The Errera graph is [[Planar graph| planar]] and has [[chromatic number]] 4, [[chromatic index]] 6, [[radius (graph theory)|radius]] 3, [[diameter (graph theory)|diameter]] 4 and [[girth (graph theory)|girth]] 3. All its vertices are of degree 5 or 6 and it is a 5-[[k-vertex-connected graph|vertex-connected graph]] and a 5-[[k-edge-connected graph|edge-connected graph]]. | |||
==Algebraic properties== | |||
The Errera graph is not a [[vertex-transitive graph]] and its full automorphism group is isomorphic to the [[dihedral group]] of order 20, the group of symmetries of a [[decagon]], including both rotations and reflections. | |||
The [[characteristic polynomial]] of the Errera graph is <math>-(x^2-2 x-5) (x^2+x-1)^2 (x^3-4 x^2-9 x+10) (x^4+2 x^3-7 x^2-18 x-9)^2</math>. | |||
==Gallery== | |||
<gallery> | |||
Image:Errera graph 4COL.svg|The [[chromatic number]] of the Errera graph is 4. | |||
Image:Errera graph 6color edge.svg|The [[chromatic index]] of the Errera graph is 6. | |||
Image:Errera graph.svg|The Errera graph is [[Planar graph| planar]]. | |||
</gallery> | |||
== References == | |||
{{reflist}} | |||
[[Category:Individual graphs]] | |||
[[Category:4-chromatic graphs]] | |||
Latest revision as of 06:38, 13 April 2013
In the mathematical field of graph theory, the Errera graph is a graph with 17 vertices and 45 edges discovered by Alfred Errera.[1] Published in 1921, it provides an example of how Kempe's proof of the four color theorem cannot work.[2][3]
Later, the Fritsch graph and Soifer graph provide two smaller counterexamples.[4]
The Errera graph is planar and has chromatic number 4, chromatic index 6, radius 3, diameter 4 and girth 3. All its vertices are of degree 5 or 6 and it is a 5-vertex-connected graph and a 5-edge-connected graph.
Algebraic properties
The Errera graph is not a vertex-transitive graph and its full automorphism group is isomorphic to the dihedral group of order 20, the group of symmetries of a decagon, including both rotations and reflections.
The characteristic polynomial of the Errera graph is .
Gallery
-
The chromatic number of the Errera graph is 4.
-
The chromatic index of the Errera graph is 6.
-
The Errera graph is planar.
References
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Hostgator tutorials on how to install Wordpress need not be complicated, especially when you will be dealing with a web hosting service that is friendly for novice webmasters and a blogging platform that is as intuitive as riding a bike. After that you can get Hostgator to host your domain and use the wordpress to do the blogging. Once you start site flipping, trust me you will not be able to stop. I cut my webmaster teeth on Control Panel many years ago, but since had left for other hosting companies with more commercial (cough, cough) interfaces. If you don't like it, you can chalk it up to experience and go on. First, find a good starter template design. When I signed up, I did a search for current "HostGator codes" on the web, which enabled me to receive a one-word entry for a discount. Your posts, comments, and pictures will all be imported into your new WordPress blog. - ↑ Errera, A. "Du coloriage des cartes et de quelques questions d'analysis situs." Ph.D. thesis. 1921.
- ↑ Peter Heinig. Proof that the Errera Graph is a narrow Kempe-Impasse. 2007.
- ↑ Gethner, E. and Springer, W. M. II. "How False Is Kempe's Proof of the Four-Color Theorem?" Congr. Numer. 164, 159-175, 2003.