Examples
An example of a σ-additive function is the function μ defined over the power set of the real numbers, such that
If is a sequence of disjoint sets of real numbers, then either none of the sets contains 0, or precisely one of them does. In either case the equality
holds.
See measure and signed measure for more examples of σ-additive functions.
An example of an additive function which is not σ-additive is obtained by considering μ, defined over the power set of the real numbers by the slightly modified formula
where the bar denotes the closure of a set.
One can check that this function is additive by using the property that the closure of a finite union of sets is the union of the closures of the sets, and looking at the cases when 0 is in the closure of any of those sets or not. That this function is not σ-additive follows by considering the sequence of disjoint sets
for n=1, 2, 3, ... The union of these sets is the interval (0, 1) whose closure is and μ applied to the union is then infinity, while μ applied to any of the individual sets is zero, so the sum of μ(An) is also zero, which proves the counterexample.
Read more about this topic: Sigma Additivity
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