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In-Class Notes to Accompany Applied Combinatorics

Handout 5.3 Graph Coloring

Peer instruction questions 1–3.
Let \(\bfG=(V,E)\) be a graph. Then \(\phi\colon V\to C\) is a proper coloring of \(\bfG\) if
  1. \(\phi\) is one-to-one
  2. \(xy\in E\) implies \(\phi(x)\neq \phi(y)\)
  3. \(\phi\) uses as few colors from \(C\) as possible
  4. None of the above
Let \(\bfG=(V,E)\) be a graph and \(\phi\colon V\to C\) be a proper coloring of \(\bfG\text{.}\) Let \(S\) be all vertices colored \(1\in C\text{.}\) How many edges does the subgraph of \(\bfG\) induced by \(S\) contain?
  1. \(\displaystyle |S|\)
  2. Any number is possible.
What is the chromatic number of the complete graph on \(n\) vertices \(\bfK_{n}\text{?}\)
  1. \(\displaystyle n-1\)
  2. \(\displaystyle n\)
  3. \(\displaystyle n+1\)
  4. There is no fixed formula depending on \(n\text{.}\)

Definition 5.9.

A graph is bipartite provided that its chromatic number is \(2\text{.}\)
Peer instruction question 4.
First Fit