Definition:Probability Measure/Definition 2

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Let $\EE$ be an experiment.

Let $\Omega$ be the sample space on $\EE$.

Let $\Sigma$ be the event space of $\EE$.

A probability measure on $\EE$ is a mapping $\Pr: \Sigma \to \R$ which fulfils the Kolmogorov axioms:

\((1)\)   $:$     \(\ds \forall A \in \Sigma:\)    \(\ds 0 \)   \(\ds \le \)   \(\ds \map \Pr A \le 1 \)      The probability of an event occurring is a real number between $0$ and $1$
\((2)\)   $:$      \(\ds \map \Pr \Omega \)   \(\ds = \)   \(\ds 1 \)      The probability of some elementary event occurring in the sample space is $1$
\((3)\)   $:$      \(\ds \map \Pr {\bigcup_{i \mathop \ge 1} A_i} \)   \(\ds = \)   \(\ds \sum_{i \mathop \ge 1} \map \Pr {A_i} \)      where $\set {A_1, A_2, \ldots}$ is a countable (possibly countably infinite) set of pairwise disjoint events
  That is, the probability of any one of countably many pairwise disjoint events occurring
  is the sum of the probabilities of the occurrence of each of the individual events

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