Date: Mar 8, 2013 7:23 PM
Author: quasi
Subject: Re: Cardinality of turning wheel
netzweltler wrote:

>quasi wrote:

>> netzweltler wrote:

>> >quasi wrote:

>> >>

>> >> Any infinite set of pairwise disjoint intervals on the real

>> >> line is countably infinite since each interval contains a

>> >> distinct rational number.

>> >

>> >t = 0 s: I am marking

>> >[0, 0.5] #0, [0.5, 0.75] #-1, [0.75, 0.875] #-2, ...

>> >

>> >t = 0.5 s: I am marking

>> >[0.5, 0.75] #0, [0.75, 0.875] #-1, [0.875,0.9375] #-2, ...

>> >

>> >t = 0.75 s: I am marking [0.75, 0.875] #0, [0.875, 0.9375] #-1,

>> >[0.9375, 0.96875] #-2, ...

>> >...

>>

>> >Which segments have been marked #0 after 1 s?

>>

>> I don't understand the mechanics or the intent of the above

>> marking scheme.

>>

>> But I think you've fallen victim to a common fallacy.

>>

>> When trying to determine whether or not an infinite set is

>> countable, a failed counting _doesn't_ disprove countability.

>

>I am not trying to prove or disprove countability of these

>segments in [0, 1]. Remember, we have been discussing if the

>choice of #1 (or #0 as you suggested) is arbitrary. Let me give

>the answer to my question instead: All segments in [0, 1] have

>been marked #0 at t = 1 s. So,there are no segments left to be

>marked #-1, #-2, ..., are there?

I don't understand the intent of your marking scheme.

I thought the original question was, assuming finite, positive

speeds, what is the cardinality of the set of revolutions of a

wheel?

Answer: aleph-0

In other words, the set of revolutions is countably infinite.

The idea is that each revolution corresponds to an interval

of time on the real line (with arbitrary choice of the location

of time t = 0), and that infinite set of non-overlapping

intervals on the real line is countably infinite.

Your attempt to match up intervals with markers in such a

was as to leave some intervals unmarked has no impact on

the original question.

For the purposes of determining countability, all we need

is _some_ way of marking the intervals, not necessarily

_your_ way, and not necessarily a way which matches the order

of the markers with the order of the left-to-right placement

of the intervals.

quasi