Probability Applications
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# Probability Applications ## 10.1 Probabilistic Method Show existence of an object by proving the probability of its existence is positive. **Example:** Lower bound for Ramsey numbers: If $\binom{n}{k} 2^{1-\binom{k}{2}} n$.

Probability Applications
10.1 Probabilistic Method
Show existence of an object by proving the probability of its existence is positive.
Example: Lower bound for Ramsey numbers: If (kn)21−(2k)<1, then R(k,k)>n.
10.2 Expectation and Variance
Linearity of expectation: E[X+Y]=E[X]+E[Y] (always, even for dependent variables).
Example: Expected number of fixed points in a random permutation of n elements: Let Xi=1 if i is fixed, 0 otherwise. E[Xi]=1/n. By linearity, E[∑Xi]=n⋅1/n=1.
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