Formal Book

45 Probability makes counting (sometimes) easy

Theorem 45.1
#

Every family of at most \(2^{d-1}\) \(d\)-sets is \(2\)-colorable, that is, \(m(d) {\gt} 2^{d-1}\).

Proof

TODO

Theorem 45.2
#

Every family of at most \(2^{d-1}\) \(d\)-sets is \(2\)-colorable, that is, \(m(d) {\gt} 2^{d-1}\).

Proof

TODO

Theorem 45.3

For every \(k \geq 2\), there exists a graph \(G\) with chromatic number \(\chi (G) {\gt} k\) and girth \(\gamma (G) {\gt} k\).

Proof

TODO

Theorem 45.4

Let \(G\) be a simple graph with \(n\) vertices and \(m\) edges, where \(m \ge 4n\). Then

\[ \operatorname {cr}(G) \ge \frac{1}{64}\frac{m^3}{n^2}. \]
Proof

TODO