D Recurrences

KennyTM~ kennytm at gmail.com
Thu Jun 9 13:43:49 PDT 2011


On Jun 10, 11 03:20, Timon Gehr wrote:
> Timon Gehr wrote:
>> Steve Schveighoffer wrote:
>>> On Thu, 09 Jun 2011 14:45:49 -0400, Andrej Mitrovic
>>> <andrej.mitrovich at gmail.com>  wrote:
>>>
>>>> I never understood why it's called "open" interval. What does it 'open'?
>>>
>>> Look at the terminology section of
>>> http://en.wikipedia.org/wiki/Interval_(mathematics)
>>>
>>> No clue as to the "why" :)
>>>
>>> -Steve
>>
>> Actually it does not make too much sense to use the terms open or closed on
>> integer intervals in a strict mathematical sense. If your universe is the
>> integers, all subsets of the integers are neither open nor closed, with the
>> exception of the empty set and the set of all integers, which are both open>  and
>> closed. :o)
>
> I'm sorry, this was misinformation. Actually *all* subsets are both open and closed.
>
> Timon

That depends on how you define the topology on Z anyway.


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