On November 12 2005 23:30 HeadBangaa wrote:Show nested quote +On November 12 2005 22:02 BigBalls wrote:
speaking of set theory, this reminds me of extraordinary sets.
a set is extraordinary if it contains itself. For example, A = {1,2,3,A} is extraordinary.
Define S to be the set that contains all ordinary sets and nothing else. Suppose S is ordinary. Then S contains itself because it contains all ordinary sets, which makes it extraordinary, which is a contradiction. Now suppose S is extraordinary. That means it contains itself. But S contains nothing but extraordinary sets, so it cannot contain itself. Another contradiction.
It simply means that S cannot be ordinary in the first place. It's an extraordinary set that contains all ordinary sets.
This example does illustrate some of the seemingly strange characteristics of set theory. Like that one about successive power sets of the empty set:
A power set of S, P(S), is the set of all subsets of S.
if S = (a, b, c)
P(S) = {a, b, c, {a,b}, {a,c}, {b,c}, {a,b,c}}
(I think I'm missing one element, but you get the point)
The empty set, Ø, contains nothing. It's "null space".
Ø = {}. But...
P(Ø) = {Ø}
P({Ø}) = {Ø, {Ø}}
P({Ø, {Ø}}) = {Ø, {Ø}, {Ø, {Ø}}, {{Ø}}}
etc...
I think that's freakin weird and it has no actual application that I can think of.