# Countably infinite sigma-algebra

1. Feb 24, 2005

### Zaare

Problem: Is there a countably infinite sigma-algebra?
I can assume there are countably number of disjoint subsets included in the sigma-algebra.

2. Feb 24, 2005

### mathman

Given there are a countable number of disjoint subsets, the number of sets in the sigma algebra has to be non-countable. It is a very simple one to one mapping of all the combinations from the countable number of subsets to all binary numbers between 0 and 1. Specifically, order the countable subsets. For any combination of subsets, the equivalent binary number has zeros for the sets omitted and 1 for sets included.

Example: .001110101.... is the image of the set where the 3rd, 4th, 5th, 7th, 9th, etc. sets are include while the 1st, 2nd, 6,th, 8th, etc. sets are omitted (from the union).

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