---
title: Cubic Knödel Graphs
url: https://www.emergentmind.com/topics/cubic-knodel-graphs
type: topic
---

# Cubic Knödel Graphs

A cubic Knödel graph $W_{3,n}$ is a 3-regular, bipartite, vertex-transitive graph of even order $n \geq 8$, arising as a key case within the broader family of Knödel graphs $W_{\Delta, n}$. These structures are notable for combining algebraic regularity, combinatorial symmetry, and extremal properties that make them prototypical objects in the study of domination, diameter, and related invariants in graph theory. Their precise construction and associated structural results furnish a rich testing ground for the analysis of domination-type parameters and distance-related metrics [1804.02532], [2004.05435].

## 1. Definition and Construction

For every even integer $n \geq 8$, the cubic Knödel graph $W_{3,n}$ is defined on vertex set $V = U \cup V$, where
- $U = \{u_j \mid 0 \leq j \leq n/2-1\}$,
- $V = \{v_j \mid 0 \leq j \leq n/2-1\}$.

Edges are defined by adjacency rules that reflect 3-regularity and bipartition:
- Each $u_j \in U$ is adjacent to $v_j$, $v_{j+1}$, and $v_{j+3}$ modulo $n/2$.
- Each $v_j \in V$ is adjacent to $u_j$, $u_{j-1}$, and $u_{j-3}$ modulo $n/2$.

Alternatively, labeling vertices as $(1,j)$ and $(2,j)$ for $0 \leq j \leq n/2-1$, edges join $(1,j)$ to $(2, (j+2^k-1) \bmod (n/2))$ for $k=0,1,2$. The resulting graph is bipartite, 3-regular, and vertex-transitive [1804.02532], [2004.05435].

## 2. Structural Properties

Cubic Knödel graphs exhibit strong algebraic and combinatorial features:
- They are bipartite, with both parts $U$ and $V$ of size $n/2$.
- Each vertex has degree 3.
- The graph is vertex-transitive.
- For any two vertices in the same part, the intersection of their neighborhoods is at most one, and exactly one if and only if their index-distance is in $\{1,2,4\}$.
- $W_{3, n}$ is $K_{2,3}$-free (does not contain a complete bipartite subgraph $K_{2,3}$ as an induced subgraph).

Table: Basic Parameters of $W_{3, n}$
| Parameter                       | Value                | Reference         |
|----------------------------------|----------------------|-------------------|
| Order ($n$)                     | even $\geq 8$        | [1804.02532]      |
| Degree                          | 3                    | [1804.02532]      |
| Vertex-transitive                | Yes                  | [1804.02532]      |
| Bipartite                        | Yes                  | [1804.02532]      |

## 3. Distance and Diameter

For $n \geq 10$, the diameter of $W_{3,n}$ is given by
\[
\operatorname{diam}(W_{3,n}) = 1 + \left\lceil \frac{n-2}{6} \right\rceil.
\]
This exact bound follows from a general distance analysis of Knödel graphs. The formulas and procedures for computing distances within and between partite sets are as follows [2004.05435]:
- For $u_i, u_j \in U$:
  \[
  d(u_i, u_j) = 2 \lceil \Delta_U / 3 \rceil
  \]
  where $\Delta_U = \min\{(j-i) \bmod (n/2), (i-j) \bmod (n/2)\}$.
- For $u_i \in U$, $v_j \in V$:
  \[
  d(u_i, v_j) = 1 + \min \{ 2 \lceil ((j-i) \bmod (n/2))/3 \rceil, 2 \lceil ((i-(j-1)) \bmod (n/2))/3 \rceil, 2 \lceil ((i-(j-3)) \bmod (n/2))/3 \rceil \}
  \]
- For $v_i, v_j \in V$, the formula is analogous.

The graph strictly increases in diameter as $n$ increases by increments of 6. For example, $n = 10, 12, 14$ all have diameter 3, while $n = 16, 18, 20$ all have diameter 4 [2004.05435].

## 4. Total Domination Number

A total dominating set $D$ in $W_{3,n}$ is a subset such that every vertex has a neighbor in $D$. The minimum cardinality of such a set, denoted $\gamma_t(W_{3,n})$, is determined exactly for all even $n \geq 8$. Writing $t = \lceil n/10 \rceil$,
\[
\gamma_t(W_{3,n}) = \begin{cases}
4t & \text{if } n \equiv 0,6,8 \pmod{10}, \\
4t - 2 & \text{if } n \equiv 2,4 \pmod{10}.
\end{cases}
\]
Equivalently,
\[
\gamma_t(W_{3,n}) = 4 \lceil n/10 \rceil - \delta(n),
\]
where $\delta(n) = 0$ when $n \equiv 0,6,8 \pmod{10}$ and $\delta(n) = 2$ when $n \equiv 2,4 \pmod{10}$ [1804.02532].

The construction of extremal total dominating sets is explicit: for $k = 0, …, t-1$, select $\{u_{5k+1}, u_{5k+2}, v_{5k+1}, v_{5k+2}\}$, and for certain congruence classes of $n$ additional vertices are chosen to cover remaining uncovered vertices.

This result, completing the determination of total domination in the cubic case (the ordinary domination number $\gamma(W_{3,n})$ had been previously obtained for all $n$), serves as a paradigm for domination-type invariant analyses in regular bipartite graphs.

## 5. Proof Techniques and Combinatorial Tools

The bounds on $\gamma_t(W_{3,n})$ utilize both explicit constructions and lower bounds established via combinatorial arguments:
- The pigeonhole principle partitions any candidate set into $D_U$ and $D_V$, observing that each $u \in U$ has three neighbors in $V$.
- Sharpness in counting arguments is achieved by introducing the *cyclic-sequence* of subset $A \subseteq U$ and the index-distance function $\mathrm{id}(u_i, u_j) = \min\{|i-j|, n/2-|i-j|\}$.
- Lemma 2.3 specifies necessary and sufficient conditions for vertices in $U$ to have common neighbors in $V$, in terms of membership of the index-distance in $M_3 = \{1,2,4\}$.
- Lemmata 2.7 and 2.8 detail neighborhood intersections and control the gaps of cyclic-sequences, making extremal arguments on dominating sets tight.

These methods are not only instrumental for the cubic ($\Delta=3$) case but are suggested to generalize toward higher-regularity Knödel graphs $W_{k,n}$ for $k \geq 4$ [1804.02532].

## 6. Examples and Special Cases

Specific computations for small values of $n$ illustrate the structural results:
- For $n=8$, $\gamma_t(W_{3,8})=4$ (direct verification).
- For $n=10$, $t=1$, so $\gamma_t(W_{3,10})=4$ (example: $\{u_1,u_2,v_1,v_2\}$).
- For $n=12$, $t=2$, $n\equiv2\pmod{10}$, so $\gamma_t(W_{3,12})=6$ (example: $\{u_1,u_2,u_6,v_1,v_2,v_6\}$).

There is a unique threshold value for the diameter: for $n=8$, $W_{3,8}$ is exceptional, with diameter 3, divergent from the formula valid for $n \geq 10$ [2004.05435].

## 7. Broader Implications and Related Directions

Cubic Knödel graphs constitute canonical instances of highly regular, symmetric, bipartite graphs amenable to explicit combinatorial analysis. The mutual constraints between local and global structure (e.g., $K_{2,3}$-freeness, sharp dominating sets, explicit diameter) make them central objects for exploring extremal questions in graph domination and communication. The machinery developed extends naturally to domination-type invariants and suggests that combinatorial techniques—such as cyclic-sequence analysis and index-distance tools—are broadly applicable to deeper studies of $\Delta$-regular bipartite graphs and their algorithmic properties [1804.02532], [2004.05435].

Source: https://www.emergentmind.com/topics/cubic-knodel-graphs