Cycle does not Contain Subcycles
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Although this article appears correct, it's inelegant. There has to be a better way of doing it. In particular: Write this in the context of cycles, not cycle graphs You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by redesigning it. To discuss this page in more detail, feel free to use the talk page. When this work has been completed, you may remove this instance of {{Improve}} from the code.If you would welcome a second opinion as to whether your work is correct, add a call to {{Proofread}} the page. |
Theorem
Let $G$ be a cycle graph.
Then the only cycle graph that is a subgraph of $G$ is $G$ itself.
Proof
Aiming for a contradiction, suppose $G$ contains a subgraph $C$ such that:
- $C$ is a cycle graph
- $C \ne G$ is non-empty.
Then there exists some vertex $v$ that is not in $C$.
Let $u$ be any vertex of $C$.
Since $G$ is a cycle graph, it is connected.
Therefore there is a walk from $u$ to $v$ in $G$.
There must be some vertex $x$ that is the last vertex in $C$ along that walk.
Therefore, $x$ is adjacent to a vertex not in $C$.
Thus it has a degree of at least $3$.
But $G$ is a cycle graph and every vertex in a cycle graph has degree $2$.
The result follows by Proof by Contradiction.
$\blacksquare$