Probabilistic method: Difference between revisions

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===First example===
Suppose we have a [[complete graph]] on {{mvar|n}} vertices. We wish to show (for small enough values of {{mvar|n}}) that it is possible to [[GraphEdge coloring|color the edges]] of the graph in two colors (say red and blue) so that there is no complete subgraph on {{mvar|r}} vertices which is monochromatic (every edge colored the same color).
 
To do so, we color the graph randomly. Color each edge independently with probability {{math|1/2}} of being red and {{math|1/2}} of being blue. We calculate the expected number of monochromatic subgraphs on {{mvar|r}} vertices as follows: