# Graph on the cover of Bollobás's “Combinatorics”

I was browsing the library and I found Bela Bollobás book "Combinatorics: Set Systems, Hypergraphs, Families of Vectors and Probabilistic Combinatorics" and, on its cover, it has a graph that I don't quite recognize:

Is this graph any "interesting", as used for counterexamples (e.g., Petersen's Graph)? I could determine some of its characteristics (9 vertices, regular of valency 4, non-planar, diameter 2 etc.), but is it distinguished in any way for being featured in an important book?

-
It possesses a certain kind of regularity: If $x,y$ are neighbors, there is always $1$ path of length $2$ between them, while if they are not neighbors, there is always $2$ paths of length two between them. I forget what this sort of regularity is called, however. –  Thomas Andrews Dec 20 '12 at 19:44
Ah, the term I was thinking of is "strongly regular." en.wikipedia.org/wiki/Strongly_regular_graph –  Thomas Andrews Dec 20 '12 at 20:19
@ThomasAndrews, Great. Translating from your description to wikipedia's notation: for any two adjacent vertices, there is exactly 1 path of length 1 between then, which means that any 2 adjacent vertices have exactly 1 neighbor in common (in wikipedia's notation, $\lambda = 1$). For any 2 non-adjacent vertices, there is exactly 2 (vertex disjoint) paths of length 2 between them, which means that 2 non-adjacent vertices have 2 neighbors in common (in wikipedia's notation, $\mu = 2$). The graph is regular of degree $k = 4$ with $v = 9$ vertices and is, therefore, $sgr(9, 4, 1, 2)$. –  rbrito Dec 21 '12 at 9:00
BTW, all of the answers are superb and I don't know what to choose as "the" answer to this question... –  rbrito Dec 21 '12 at 9:06

This graph is known as the generalized quadrangle (2,1), it is also the line graph of the $K_{3,3}$, as well as the Paley-9 graph. See here for more information. I am not aware that it is used as a prominent counterexample.

-

It's isomorphic to the rook graph on a 3 by 3 grid:

The Wikipedia page lists some nice properties of square rook graphs. For example, the graph is perfect, completely symmetric (i.e. looks the same from every vertex), it's the line graph of $K_{3,3}$, ...

-

It's also the Cartesian product of a 3-cycle with another 3-cycle.

-

It's the unit distance embedding of $K_3 \times K_3$, one of the few strongly regular graphs that has a unit distance embedding in $\mathbb{R}^2$. It is also known as $3 \times 3$ rook graph (though sometimes mistakenly as $3 \times 3$ grid graph). It also is a generalized quadrangle as pointed out above by A. Schulz.

-