# Probability axioms

The **Kolmogorov axioms** are a fundamental part of Andrey Kolmogorov's probability theory.
In it, the probability *P* of some event *E*, denoted , is usually defined as to satisfy these axioms.
The axioms are described below.

These assumptions can be summarised as follows: Let (Ω, *F*, *P*) be a measure space with *P*(Ω) = 1. Then (Ω, *F*, *P*) is a probability space, with sample space Ω, event space *F* and probability measure *P*.

An alternative approach to formalising probability, favoured by some Bayesians, is given by Cox's theorem.

## Contents

## AxiomsEdit

### First axiomEdit

The probability of an event is a non-negative real number:

where is the event space. It follows that is always finite, in contrast with more general measure theory. Theories which assign negative probability relax the first axiom.

### Second axiomEdit

This is the assumption of unit measure: that the probability that at least one of the elementary events in the entire sample space will occur is 1.

### Third axiomEdit

This is the assumption of σ-additivity:

- Any countable sequence of disjoint sets (synonymous with
*mutually exclusive*events) satisfies

Some authors consider merely finitely additive probability spaces, in which case one just needs an algebra of sets, rather than a σ-algebra. Quasiprobability distributions in general relax the third axiom.

## ConsequencesEdit

From the Kolmogorov axioms, one can deduce other useful rules for calculating probabilities.

### The probability of the empty setEdit

In some cases, is not the only event with probability 0.

### MonotonicityEdit

If A is a subset of, or equal to B, then the probability of A is less than, or equal to the probability of B.

### The numeric boundEdit

It immediately follows from the monotonicity property that

## ProofsEdit

The proofs of these properties are both interesting and insightful. They illustrate the power of the third axiom, and its interaction with the remaining two axioms. When studying axiomatic probability theory, many deep consequences follow from merely these three axioms. In order to verify the monotonicity property, we set and , where and for . It is easy to see that the sets are pairwise disjoint and . Hence, we obtain from the third axiom that

Since the left-hand side of this equation is a series of non-negative numbers, and since it converges to which is finite, we obtain both and . The second part of the statement is seen by contradiction: if then the left hand side is not less than infinity

If then we obtain a contradiction, because the sum does not exceed which is finite. Thus, . We have shown as a byproduct of the proof of monotonicity that .

## Further consequencesEdit

Another important property is:

This is called the addition law of probability, or the sum rule.
That is, the probability that *A* *or* *B* will happen is the sum of the probabilities that *A* will happen and that *B* will happen, minus the probability that both *A* *and* *B* will happen. The proof of this is as follows:

Firstly,

- (by Axiom 3)

So,

- (by ).

Also,

and eliminating from both equations gives us the desired result.

An extension of the addition law to any number of sets is the inclusion–exclusion principle.

Setting *B* to the complement *A ^{c}* of

*A*in the addition law gives

That is, the probability that any event will *not* happen (or the event's complement) is 1 minus the probability that it will.

## Simple example: coin tossEdit

Consider a single coin-toss, and assume that the coin will either land heads (H) or tails (T) (but not both). No assumption is made as to whether the coin is fair.

We may define:

Kolmogorov's axioms imply that:

The probability of *neither* heads *nor* tails, is 0.

The probability of *either* heads *or* tails, is 1.

The sum of the probability of heads and the probability of tails, is 1.

## See alsoEdit

- Borel algebra
- σ-algebra
- Set theory
- Conditional probability
- Quasiprobability
- Fully probabilistic design

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (November 2010) (Learn how and when to remove this template message) |

## Further readingEdit

- DeGroot, Morris H. (1975).
*Probability and Statistics*. Reading: Addison-Wesley. pp. 12–16. ISBN 0-201-01503-X. - McCord, James R.; Moroney, Richard M. (1964). "Axiomatic Probability".
*Introduction to Probability Theory*. New York: Macmillan. pp. 13–28.

## External linksEdit

- Kolmogorov`s probability calculus, Stanford Encyclopedia of Philosophy.
- Formal definition of probability in the Mizar system, and the list of theorems formally proved about it.