Maarten Maartensz:    Philosophical Dictionary | Filosofisch Woordenboek

 P - Powerset


Powerset: The powerset of a set is the set of all subsets of the set. In something close to the standard notation of set theory:
Powerset(X) = {Xi : Xi
a X} = the set of sets Xi that are included in X.

It is a convenient way of referring to all possibilities inherent in the set. In standard set theory this requires an axiom, namely the axiom of powersets, which asserts that every set has a powerset - equivalently: If one has a set, one also has the set of its subsets.

This is easily illustrated for small sets. Thus, the set of {a, b, c} has the powerset {, {a}, {b}, {c}, {a,b}, {a,c}, {b,c}, {a,b,c}}, where represents the empty set, that may also be written as {}.

It should be noted that the elements of a powerset of a set of elements (of any kind) are sets, and that it is not difficult to prove that for finite sets, the number of sets in the powerset of a set of n elements is 2n i.e. 2 to the power of n. In the above example, n=3 and 23=8. This explains the name of the powerset and implies that the number of sets in the powerset of a set is always greater than the number of elements in the set.

Cantor has an elegant argument by which one can prove that if one admits infinite sets, then for these sets also 2n > n. And this in turn implies that then there are infinities upon infinities of ever larger infinite sets, simply by taking powersets of powersets without limit.


See also:


Cantor, Halmos, Quine, Suppes

 Original: Aug 20, 2004                                                Last edited: 12 September 2005.   Top