Graph is Bipartite iff No Odd Cycles

Theorem
Let $$G$$ be a graph.

Then $$G$$ is bipartite iff it has no cycles whose length is odd.

Sufficient Condition
Let $$G = \left({V, E}\right)$$ be bipartite.

So, let $$V = A \cup B$$ such that $$A \cap B = \varnothing$$ and that all edges $$e \in E$$ are such that $$e$$ is of the form $$\left\{{a, b}\right\}$$ where $$a \in A$$ and $$b \in B$$.

(This is what being bipartite means.)

Suppose $$G$$ has (at least) one cycle $$C$$ whose length $$n$$ is odd.

Let $$C = \left({v_1, v_2, \ldots, v_n, v_1}\right)$$.

WLOG, let $$v_1 \in A$$. It follows that $$v_2 \in B$$ and hence $$v_3 \in A$$, and so on.

Hence we see that $$\forall k \in \left\{{1, 2, \ldots, n}\right\}$$, we have:

$$v_k \in \begin{cases} A : & k \text{ odd} \\ B : & k \text{ even} \end{cases} $$

But as $$n$$ is odd, $$v_n \in A$$.

But $$v_1 \in A$$, and $$v_n v_1 \in C_n$$.

So $$v_n v_1 \in E$$ which contradicts the assumption that $$G$$ is bipartite.

Hence if $$G$$ is bipartite, it has no odd cycles.

Necessary Condition
It is enough to consider $$G$$ as being connected, as otherwise we could consider each component separately.

Suppose $$G$$ has no cycles whose length is odd.

Choose any vertex $$v \in G$$.

Divide $$G$$ into two sets of vertices like this:


 * Let $$A$$ be the set of vertices such that the shortest path from each element of $$A$$ to $$v$$ is of odd length;
 * Let $$B$$ be the set of vertices such that the shortest path from each element of $$B$$ to $$v$$ is of even length.

Then $$v \in B$$ and $$A \cap B = \varnothing$$.

Suppose $$a_1, a_2 \in A$$ are adjacent.

But then there would be a cycle $$\left({v, \ldots, a_1, a_2, v}\right)$$ which would be of odd length.

So no two vertices in $$A$$ can be adjacent.

Similarly, suppose $$b_1, b_2 \in B$$ are adjacent.

Then there would be a cycle $$\left({v, \ldots, b_1, b_2, v}\right)$$ which would be of odd length.

So no two vertices in $$B$$ can be adjacent.

Thus $$A$$ and $$B$$ satisfy the conditions for $$G = \left({A \cup B, E}\right)$$ to be bipartite.