"WM" wrote in message
news:1b25cb2c-a0d0-4412...@googlegroups.com...
>
> Of course. But there is no fixed quantity, namely a set that is the
> union of all FISONs and is larger than all FISONs.
N is the set that is the union of all of its initial segments and N is
larger than any of its initial segments. To see this, just note that
CARD(N)=ALEPH_0 and card(n)=n for any natural number n and n<ALEPH_0 for any
natural number n.
> >
> > {0} U {0,1} U {0,1,2} U {0,1,2,3} U {0,1,2,3,4} U {0,1,2,3,4,5} U ...
> > = N
>
> No that is blatantly wrong. A union of inclusion-monotonic sets cannot
> be larger than all sets. |N may be understood as the limit but not as
> the union. It is like 0.999... that is an infinite sequence converging
> towards the limit 1.
{0} U {0,1} U {0,1,2} U {0,1,2,3} U {0,1,2,3,4} U {0,1,2,3,4,5} U ... = N
No, what I wrote is blatantly correct. It is a theorem of ZF that if L is a
limit ordinal then UL=L (the union of the sets in L equals L itself). A
proof of this can be found on page 210 of the textbook "Classic Set Theory"
by Derek Goldrei. In this example, think of it this way: pick any natural
number you want out of the set N (the right-hand side of the above equation)
and you can find the first set which includes it on the left-hand side of
the same above equation. In fact, once you have found the first set that
includes it then all subsequent sets in this ordered union also includes it.
It is true that no set on the left-hand side of the equation includes all
the members in N but there are an infinite number of such sets which is why
the equation is true. Note that each member of N can be found on the
left-hand side by simply looking at the last member in each set. Also note
that the below equation is true:
{0} U {1} U {2} U {3} U {4} U {5} U {6} U {7} U {8} U {9} U ... = N
because
{0} U {1} U {2} U {3} U {4} U {5} U {6} U {7} U {8} U {9} U ... =
{0,1,2,3,4,5,6,7,8,9,...}
and {0,1,2,3,4,5,6,7,8,9,...} = N.
> In {0} U {0,1} U {0,1,2} U {0,1,2,3} U {0,1,2,3,4} U {0,1,2,3,4,5} U ... =
> N
> all FISONs can be omitted without changing the result, yielding U{ } = |N.
No. If each set in the union is omitted then no union is taking place at
all. U{}={} since there are no sets in the empty set and taking the union
of nothing yields nothing.
k