The Banach–Ulam problem asks: are there any nontrivial measures on a set equipped with the σ-algebra of all subsets of ?
All measures are by definition countably additive.
Here a measure on is trivial if it can be obtained by the following construction. Start with a set , a map , and a σ-ideal on . Set if and otherwise. Then is a measure on .
Once trivial measure on are excluded, we can further reduce to the case of probability measures on that vanish on all singleton subsets of .
Recall that the additivity of a poset is the smallest cardinal such that has a subset of cardinality without an upper bound.
Recall that the additivity of a measure is the smallest cardinal such that there is a disjoint family of cardinality of measurable subsets such that the additivity property of fails for this family.
Recall that any measure on induces a σ-ideal such that if and there is such that and .
A cardinal is real-valued-measurable if there is a -additive probability measure that vanishes on all singleton subsets of .
If the probability measure only takes values 0 and 1, then is known as a measurable cardinal.
A cardinal is atomlessly-measurable if there is an atomless -additive probability measure .
Here an atom of a measure on is such that and if satisfies , then either or .
(Ulam?.) Suppse is a nontrivial probability space.
The additivity of equals the additivity of the σ-ideal and is a real-valued measurable cardinal.
Any real-valued-measurable cardinal is either atomlessly measurable or measurable.
Any atomlessly measurable cardinal is weakly inaccessible? and not greater than .
Any measurable cardinal is strongly inaccessible?.
The Lebesgue measure on extends to if and only if there is an atomlessly measurable cardinal.
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Last revised on May 15, 2020 at 21:44:59. See the history of this page for a list of all contributions to it.