Characteristics of Eulerian Graph/Sufficient Condition/Proof 1

Theorem
Let $G$ be a finite (undirected) graph which is connected

Let each vertex of $G$ be even.

Then $G$ is an Eulerian graph.

Note that the definition of graph here includes:
 * Simple graph
 * Loop-graph
 * Multigraph
 * Loop-multigraph

but does not include directed graph.

Proof
Suppose that an (undirected) graph $G$ is connected and its vertices all have even degree.

If there is more than one vertex in $G$, then each vertex must have degree greater than $0$.

Begin at a vertex $v$.

From Graph with Even Vertices Partitions into Cycles, we know that $v$ will be on at least one cycle.

Since $G$ is connected, there must be an edge $\left\{{v, v_1}\right\}$ for some vertex $v_1 \ne v$.

Since $v_1$ has even degree greater than $0$, there is an edge $\left\{{v_1, v_2}\right\}$ where $v_2 \ne v_1$.

These two edges make a trail from $v$ to $v_2$.

Continue this trail, leaving each vertex on an edge that was not previously used, until returning to $v$.

This is always possible, because $v$ is on a cycle.

Call the circuit formed by this process $C_1$.

If $C_1$ covers all the edges of $G$, then the proof is complete.

Otherwise, remove all the edges that contribute to $C_1$ from $G$, leaving the graph $G_0$.

The remaining vertices are still even, and since $G$ is connected there is some vertex $u$ in both $G_0$ and $C_1$.

Repeat the same process as before, beginning at $u$.

The new circuit, $C_2$, can be added to $C_1$ by starting at $v$, moving along $C_1$ to $u$, travelling around $C_2$ back to $u$ and then along the remainder of $C_1$ back to $v$.

Repeat this process, adding each new circuit found to create a larger circuit.

Since $G$ is finite, this process must end at some point, and the resulting circuit will be an Eulerian circuit.