Table of simple cubic graphs

Connectivity

The number of simple cubic graphs on 4, 6, 8, 10, ... vertices is 1, 2, 5, 19, ... (sequence A002851 in the OEIS). A classification according to edge connectivity is made as follows: the 1-connected and 2-connected graphs are defined as usual. This leaves the other graphs in the 3-connected class because each 3-regular graph can be split by cutting all edges adjacent to any of the vertices. To refine this definition in the light of the algebra of coupling of angular momenta (see below), a subdivision of the 3-connected graphs is helpful. We shall call

This declares the numbers 3 and 4 in the fourth column of the tables below.

Pictures

Ball-and-stick models of the graphs in another column of the table show the vertices and edges in the style of images of molecular bonds. Comments on the individual pictures contain girth, diameter, Wiener index, Estrada index and Kirchhoff index. A Hamiltonian circuit (where present) is indicated by enumerating vertices along that path from 1 upwards. (The positions of the vertices have been defined by minimizing a pair potential defined by the squared difference of the Euclidean and graph theoretic distance, placed in a Molfile, then rendered by Jmol.)

LCF notation

The LCF notation is a notation by Joshua Lederberg, Coxeter and Frucht, for the representation of cubic graphs that are Hamiltonian.

The two edges along the cycle adjacent to any of the vertices are not written down.

Let v be the vertices of the graph and describe the Hamiltonian circle along the p vertices by the edge sequence v₀v₁,v₁v₂, ...,v_p-2v_p-1,v_p-1v₀. Halting at a vertex v_i, there is one unique vertex v_j at a distance d_i joined by a chord with v_i,

The vector [d₀,d₁, ..., d_p-1] of the p integers is a suitable, although not unique, representation of the cubic Hamiltonian graph. This is augmented by two additional rules:

1. If a d_i >p/2, replace it by d_i-p;
2. avoid repetition of a sequence of d_i if these are periodic and replace them by an exponential notation.

Since the starting vertex of the path is of no importance, the numbers in the representation may be cyclically permuted. If a graph contains different Hamiltonian circuits, one may select one of these to accommodate the notation. The same graph may have different LCF notations, depending on precisely how the vertices are arranged.

Often the anti-palindromic representations with

are preferred (if they exist), and the redundant part is then replaced by ";-". The LCF notation [5,-9,7,-7,9,-5]⁴, for example, and would at that stage be condensed to [5,-9,7;-]⁴.

12 vertices

The LCF entries are absent above if the graph has no Hamiltonian cycle, which is rare (see Tait's conjecture). In this case a list of edges between pairs of vertices labeled 0 to n-1 in the third column serves as an identifier.

Vector coupling coefficients

Each 4-connected (in the above sense) simple cubic graph on 2 n vertices defines a class of quantum mechanical 3 n − j symbols. Roughly speaking, each vertex represents a 3-jm symbol, the graph is converted to a digraph by assigning signs to the angular momentum quantum numbers j, the vertices are labelled with a handedness representing the order of the three j (of the three edges) in the 3-jm symbol, and the graph represents a sum over the product of all these numbers assigned to the vertices.

There are 1 (6j), 1 (9j), 2 (12j), 5 (15j), 18 (18j), 84 (21j), 607 (24j), 6100 (27j), 78824 (30j), 1195280 (33j), 20297600 (36j), 376940415 (39j) etc. of these (sequence A175847 in the OEIS).

If they are equivalent to certain vertex-induced binary trees (cutting one edge and finding a cut that splits the remaining graph into two trees), they are representations of recoupling coefficients, and are then also known as Yutsis graphs (sequence A111916 in the OEIS).