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Math Help - Why are these not elements of W

  1. #1
    Senior Member bugatti79's Avatar
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    Why are these not elements of W

    Folks, Simple questions.

    Why are these not elements of W?

    (1,2,3,....1000,0,0,0). The finite numbers stop at 1000 and the rest are 0, hence is it not a element of W?

    (1,1,1....1, 0,0,0) Not sure of this one?

    Thanks
    Last edited by bugatti79; October 21st 2011 at 09:05 AM. Reason: changed word subset to element
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  2. #2
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    Re: Why are these not elements of W

    I think that before anyone can tell you why those "are not elements of W", you will have tell us what W is! How is "W" defined?
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  3. #3
    Senior Member bugatti79's Avatar
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    Re: Why are these not elements of W

    Quote Originally Posted by HallsofIvy View Post
    I think that before anyone can tell you why those "are not elements of W", you will have tell us what W is! How is "W" defined?
    Sorry,

    The subset W consisting of infinite real sequences with only finitely many non 0 entries...
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  4. #4
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    Re: Why are these not elements of W

    Quote Originally Posted by bugatti79 View Post
    Folks, Simple questions.

    Why are these not elements of W?

    (1,2,3,....1000,0,0,0). The finite numbers stop at 1000 and the rest are 0, hence is it not a element of W?
    Well, last time I looked, "1000" was pretty darn finite!

    (1,1,1....1, 0,0,0) Not sure of this one?

    Thanks
    With (1, 1, 1, ..., 1, 0, 0, 0) you are apparently not told exactly how many "1"s there are but you should be able to see that, whatever that number is, it is a number and so finite.
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  5. #5
    Senior Member bugatti79's Avatar
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    Re: Why are these not elements of W

    Quote Originally Posted by HallsofIvy View Post
    Well, last time I looked, "1000" was pretty darn finite!


    With (1, 1, 1, ..., 1, 0, 0, 0) you are apparently not told exactly how many "1"s there are but you should be able to see that, whatever that number is, it is a number and so finite.
    I am just not getting it!

    (1,2,7,0,0...) is an element of W but(1,2,3....1000,0,0,0...) is not! Why? The '3' just goes up as far as '1000' whats the difference?
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    Re: Why are these not elements of W

    where are you getting your information, and exactly which words are being used? it feels like we are not gettting the whole story, here.
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    Re: Why are these not elements of W

    Quote Originally Posted by bugatti79 View Post
    I am just not getting it!

    (1,2,7,0,0...) is an element of W but(1,2,3....1000,0,0,0...) is not! Why? The '3' just goes up as far as '1000' whats the difference?
    Who told you that (1, 2, 3, ...., 1000, 0, 0, 0,...) is not? It obviously is an infinite sequence and it obviously has only a finite number of non-zero entries.
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  8. #8
    Senior Member bugatti79's Avatar
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    Re: Why are these not elements of W

    Quote Originally Posted by HallsofIvy View Post
    Who told you that (1, 2, 3, ...., 1000, 0, 0, 0,...) is not? It obviously is an infinite sequence and it obviously has only a finite number of non-zero entries.
    because thats whats in my lecture notes. okay, how about this one

    (1,1,1,....1,0,0,0). THis is not an element of W, right? because the '1's followed by dots imply it goes on indefinitely and hence the sequence does not have finite many non 0 terms....?
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    Re: Why are these not elements of W

    are these printed lecture notes? is there more text surrounding these statements? context is important. a symbol like:

    (1,1,1,....,1,0,0,0) could mean many things.

    the presence of a right parenthesis suggests this is not an infinite sequence, as infinite sequences do not terminate.

    even if "what was meant" was: an infinite number of 1's and then 3 zeros, that is still not an infinite sequence.

    an infinite sequence is just a function \mathbb{N} \to X where X is some set (the set the terms of the sequence is in).

    the trouble with saying (1,1,1,....,1,0,0,0) is an infinite sequence is: which natural numbers are the pre-images of the last 3 terms?

    we cannot decide, so the sequence is ill-defined (you can't count up to infinity, and then count 3 more, infinity + 3 is NOT a natural number).
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  10. #10
    Senior Member bugatti79's Avatar
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    Re: Why are these not elements of W

    Quote Originally Posted by Deveno View Post
    are these printed lecture notes? is there more text surrounding these statements? context is important. a symbol like:

    (1,1,1,....,1,0,0,0) could mean many things.

    the presence of a right parenthesis suggests this is not an infinite sequence, as infinite sequences do not terminate.

    even if "what was meant" was: an infinite number of 1's and then 3 zeros, that is still not an infinite sequence.

    an infinite sequence is just a function \mathbb{N} \to X where X is some set (the set the terms of the sequence is in).

    the trouble with saying (1,1,1,....,1,0,0,0) is an infinite sequence is: which natural numbers are the pre-images of the last 3 terms?

    we cannot decide, so the sequence is ill-defined (you can't count up to infinity, and then count 3 more, infinity + 3 is NOT a natural number).
    Sorry guys. Looks like I have a typo. It should be (1,1,1....0,0,0......) So this is?
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    Re: Why are these not elements of W

    we still have a problem determining WHEN the entries turn from 1's to 0. assuming this is NOT a "sequence of sequences", it would appear that we have an element of W, but it's not a very precise way of indicating it. to specify a sequence, you should be able (in theory, at least) specify every term.

    for example, in your last example, the explicit function was:

    f(n) = n, n ≤ 1000
    f(n) = 0, n > 1000

    with (1,1,1,....,0,0,0,....) how can we say what the 1000th term is, and-more importantly for deciding whether or not we have an element of W-how can we say for sure that there is some finite positive integer k, for which f(n) = 0 for all n > k?

    it's not "good enough" to say, well, eventually, every term is 0, unles you specifically say: after THIS number, every term is 0. this goes back to the same problem we have before: we can't just put in "infinite ones" and then say we have "zeros after". that's not a well-defined function on the natural numbers.

    so again, what is MEANT by (1,1,1,...,0,0,0,.....). i am not going to be sure it's actually an infinite sequence until i know what's in the first "...." (finite? all ones? something i don't know what the heck?).
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  12. #12
    Senior Member bugatti79's Avatar
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    Re: Why are these not elements of W

    Quote Originally Posted by Deveno View Post
    so again, what is MEANT by (1,1,1,...,0,0,0,.....). i am not going to be sure it's actually an infinite sequence until i know what's in the first "...." (finite? all ones? something i don't know what the heck?).
    ok, this was written under the notes....' a billion '1's that represents the first"..." Sorry, I overlooked this piece of information that is obviously important. So I am guessing then that it is a subset because the'1's is finite ie a billion of them. Correct?
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  13. #13
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    Re: Why are these not elements of W

    yes, a billion is a finite number.
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