Condition for Bipartite Graph to be Hamiltonian

Theorem
Let $G = \struct {A \mid B, E}$ be a bipartite graph.

Let $G$ be Hamiltonian.

Then $\card A = \card B$.

That is, there is the same number of vertices in $A$ as there are in $B$.

Proof
Let $G = \struct {A \mid B, E}$ be a bipartite graph.

To be Hamiltonian, a graph $G$ needs to have a Hamilton cycle: that is, one which goes through all the vertices of $G$.

As each edge in $G$ connects a vertex in $A$ with a vertex in $B$, any cycle alternately passes through a vertex in $A$ then a vertex in $B$.

, suppose that $\card A > \card B$, that is, that there are more vertices in $A$ than in $B$.

Let $\card A = m, \card B = n$.

Suppose $G$ has a Hamilton cycle $C$.

Let that cycle start at $u \in B$.

After $2 n$ edges have been traversed, we will have arrived back at $u$ again, and all the vertices of $B$ will have been visited.

But there will still be $m - n$ vertices in $A$ which have not been visited.

Hence $C$ can not be a Hamilton cycle.

Note
The implication does not go both ways.

This graph:


 * NonHamiltonianBipartite.png

clearly fulfils the conditions (that is, it is a bipartite graph such that $\card A = \card B$) and is equally clearly not Hamiltonian.