Index: The Book of Statistical ProofsGeneral TheoremsProbability theoryProbability axioms ▷ Probability of the empty set

Theorem: The probability of the empty set is zero:

Proof: Let E_i = \emptyset for i = 1,2,\ldots Then, E_i \cap E_j = \emptyset \cap \emptyset = \emptyset for i,j \geq 1 and \bigcup_{i=1}^\infty E_i = \bigcup_{i=1}^\infty \emptyset = \emptyset. Thus, E_1, E_2, \ldots is a countable sequence of disjoint events so that, with the third axiom of probability, it holds that:

\label{eq:prob-emp-qed} \begin{split} P\left(\bigcup_{i=1}^\infty E_i \right) &= \sum_{i=1}^\infty P(E_i) \\ P(\emptyset) &= \sum_{i=1}^\infty P(\emptyset) \; . \end{split}

Since, by the first axiom of probability, probabilities are non-negative, i.e. P(\emptyset) \geq 0, we are searching for a non-negative number which, when added to itself infinitely, is equal to itself. The only such number is zero, i.e. P(\emptyset) = 0.

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Metadata: ID: P464 | shortcut: prob-emp2 | author: JoramSoch | date: 2024-08-08, 11:41.