Euler's formula for a connected plane graph $G$ states that

$n - m + f = 2$

where:

• $n$ is the number of vertices,
• $m$ is the number of edges, and
• $f$ is the number of faces.

If $G$ is not connected and has $k(G)$ connected components, we need to generalize Euler's formula. Let each connected component $G_i$ of $G$ have $n_i$ vertices, $m_i$ edges, and $f_i$ faces.

For each connected component $G_i$, Euler's formula holds:

$n_i - m_i + f_i = 2$

Now, summing this equation over all $k(G)$ components, we get:

$\sum_{i=1}^{k(G)} (n_i - m_i + f_i) = \sum_{i=1}^{k(G)} 2$

This simplifies to:

$\left(\sum_{i=1}^{k(G)} n_i\right) - \left(\sum_{i=1}^{k(G)} m_i\right) + \left(\sum_{i=1}^{k(G)} f_i\right) = 2k(G)$

Since the total number of vertices $n$, edges $m$, and faces $f$ in $G$ are just the sums over all components, we can write:

$n - m + f = 2k(G)$

Thus, Euler's formula for a graph with $k(G)$ connected components is:

$n - m + f = 2k(G)$

This generalized Euler's formula shows the relationship between the number of vertices, edges, faces, and connected components in any plane graph.

Would you like more details or have any questions?

Here are 5 questions you might want to ask next:

1. How can Euler's formula be applied to non-planar graphs?
2. Can you give an example of applying Euler's formula to a specific plane graph?
3. What are some applications of Euler's formula in real-world problems?
4. How does Euler's formula change when applied to graphs on surfaces other than planes?
5. What are the implications of Euler's formula for graph coloring problems?

Tip: Euler's formula is a powerful tool in graph theory and topology. Understanding its applications and generalizations can provide deep insights into the properties of different types of graphs and surfaces.