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Math Help - Infinite series (1)

  1. #1
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    Infinite series (1)

    Prove that \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} is divergent.
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  2. #2
    Super Member Deadstar's Avatar
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    Does it diverge? According to maple it converges to 1.412025371...
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  3. #3
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    Quote Originally Posted by Deadstar View Post
    Does it diverge? According to maple it converges to 1.412025371...
    here's another reason why i don't trust maple! the series is divergent, as i said, and to prove it, write the series in the form \sum_{n=1}^{\infty} (-1)^{n-1}a_n, \ a_n \geq 0, first and then see that \lim_{n\to\infty} a_n \neq 0.
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    MHF Contributor chisigma's Avatar
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    The original series is...

    \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} (1)

    ... and it can be transformed in the form...

    \sum_{n=1}^{\infty} (-1)^{n-1} a_{n} (2)

    ... where \forall n is a_{n}>0 only changing the order of its terms. But (1) is not absolutely convergent and in this case changing the order of its terms can transform the series from convergent to divergent or vice-versa, so that ...

    Kind regards

    \chi \sigma
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  5. #5
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    Quote Originally Posted by chisigma View Post
    The original series is...

    \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} (1)

    ... and it can be transformed in the form...

    \sum_{n=1}^{\infty} (-1)^{n-1} a_{n} (2)

    ... where \forall n is a_{n}>0 only changing the order of its terms. But (1) is not absolutely convergent and in this case changing the order of its terms can transform the series from convergent to divergent or vice-versa, so that ...
    ... but if we keep the terms in the same order then we won't alter the convergence/divergence. Roughly speaking, the terms of the series have the same sign throughout any interval of the form e^k<n<e^{k+1}. The sum of the absolute values of the terms in that interval is \sum_{e^k<n<e^{k+1}}\frac1n \approx \int_{e^k}^{e^{k+1}}\frac1x\,dx = \ln(e^{k+1}) - \ln(e^k) = 1. So the series alternates between a batch of positive terms adding up to approximately1, followed by a batch of negative terms adding up to approximately 1. This oscillation means that it cannot converge.
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  6. #6
    MHF Contributor chisigma's Avatar
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    The series...

    \sum_{n=1}^{\infty} (-1)^{n-1}= 1-1+1-1+... (1)

    ... properly speaking is neither convergent nor divergent, it is indeterminated [for someone it converges to \frac{1}{2} but is not the case to discuss about that here...]. If we 'alterate' in some way [also in 'microscopic way'...] the (1), it can...

    a) 'explode' [diverge]
    b) 'implode' [converge]
    c) remain 'stable' [indeterminated]

    In the case...

    \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} (2)

    ... what of the hypothesis a),b), c) is 'true'?...

    Kind regards

    \chi \sigma
    Last edited by chisigma; May 26th 2009 at 08:12 PM. Reason: spelling
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  7. #7
    MHF Contributor chisigma's Avatar
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    Considering the identity...

    (-1)^{\lfloor \ln n \rfloor} = (-1)^{ \ln n - \{\ln n\}} = e^{- i\cdot \pi (\ln n - \{\ln n\})} = n^{-i\cdot \pi} \cdot e^{i\cdot \pi \cdot \{\ln n\}} (1)

    ... where the symbol \{*\} means 'fractional part of', the proposed series can be written as...

    \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} = \sum_{n=1}^{\infty} \frac{e^{i\cdot \pi \cdot \{\ln n\}}}{n^{1+i\cdot \pi}} (2)

    Now if we consider the sequence a_{n}= \frac{1}{n^{1+i\cdot \pi}} , it generates the series...

    \sum_{n=1}^{\infty} \frac{1}{n^{1+i\cdot \pi}} = \zeta(1+i\cdot \pi) (3)

    ... that is convergent since the function \zeta(*) is analytic in the whole complex plane with the only exception of the point s=1. So if the sequence a_{n} produces a convergent series, may be not hazarded suppose that the sequence \alpha_{n} = a_{n}\cdot e^{i\cdot \pi \cdot \{\ln n\}} produces also a convergent series. It is obvious that this conclusion must be supported in rigorous fashion... at first however seems to be not an absurd 'a priori'...

    Kind regards

    \chi \sigma
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  8. #8
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    Quote Originally Posted by chisigma View Post
    ...

    Now if we consider the sequence a_{n}= \frac{1}{n^{1+i\cdot \pi}} , it generates the series...

    \sum_{n=1}^{\infty} \frac{1}{n^{1+i\cdot \pi}} = \zeta(1+i\cdot \pi) (3)

    ... that is convergent since the function \zeta(*) is analytic in the whole complex plane with the only exception of the point s=1.
    The zeta function is analytic in \mathbb{C}\setminus\{1\}, but it is not equal to the sum of the series \textstyle\sum n^{-s} on the whole of that domain. In fact, the series only converges in the region \text{Re}\,s>1. To extend it beyond there, you have to use analytic continuation. The equation (3) is not correct because the left side does not converge.
    Last edited by Opalg; May 28th 2009 at 12:19 AM.
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  9. #9
    MHF Contributor chiph588@'s Avatar
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    Quote Originally Posted by chisigma View Post
    Considering the identity...

    (-1)^{\lfloor \ln n \rfloor} = (-1)^{ \ln n - \{\ln n\}} = e^{- i\cdot \pi (\ln n - \{\ln n\})} = n^{-i\cdot \pi} \cdot e^{i\cdot \pi \cdot \{\ln n\}} (1)

    ... where the symbol \{*\} means 'fractional part of', the proposed series can be written as...

    \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} = \sum_{n=1}^{\infty} \frac{e^{i\cdot \pi \cdot \{\ln n\}}}{n^{1+i\cdot \pi}} (2)

    Now if we consider the sequence a_{n}= \frac{1}{n^{1+i\cdot \pi}} , it generates the series...

    \sum_{n=1}^{\infty} \frac{1}{n^{1+i\cdot \pi}} = \zeta(1+i\cdot \pi) (3)

    ... that is convergent since the function \zeta(*) is analytic in the whole complex plane with the only exception of the point s=1. So if the sequence a_{n} produces a convergent series, may be not hazarded suppose that the sequence \alpha_{n} = a_{n}\cdot e^{i\cdot \pi \cdot \{\ln n\}} produces also a convergent series. It is obvious that this conclusion must be supported in rigorous fashion... at first however seems to be not an absurd 'a priori'...

    Kind regards

    \chi \sigma
    \zeta(1+i\cdot \pi)=\sum_{n=1}^{\infty} \frac{1}{n^{1+i\cdot \pi}} does diverge, but \sum_{n=1}^{\infty} \frac{1}{n^{1+i\cdot \pi}}\neq\infty . This sum oscillates forever, it's kind of analogous to  \sum_{n=0}^\infty (-1)^n in that sense.
    This can be shown by a use of partial summation, if you are familiar.
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  10. #10
    MHF Contributor chiph588@'s Avatar
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    Quote Originally Posted by NonCommAlg View Post
    Prove that \sum_{n=1}^{\infty} \frac{(-1)^{\lfloor \ln n \rfloor}}{n} is divergent.
    Let's consider  \log_2(x) instead.

    It's easy to see the sum is the same as  \sum_{n=1}^\infty (-1)^na_n , where  a_n=\frac{1}{2^n}+\frac{1}{2^n+1}+\cdots+\frac{1}{  2^{n+1}-1} .

    The conclusion then follows, but isn't this technically changing the order of summation since we're not evaluating the sum in the same order?
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  11. #11
    Grand Panjandrum
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    Quote Originally Posted by chisigma View Post
    The series...

    \sum_{n=1}^{\infty} (-1)^{n-1}= 1-1+1-1+... (1)

    ... properly speaking is neither convergent nor divergent, it is indeterminated [for someone it converges to \frac{1}{2} but is not the case to discuss about that here...].
    A series is convergent if the sequence of partial sums converges, otherwise it is divergent. You may invent a distinction between divergent and indeterminate if you wish you are now using non-standard terminology.

    CB
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  12. #12
    Grand Panjandrum
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    Quote Originally Posted by Opalg View Post
    ... but if we keep the terms in the same order then we won't alter the convergence/divergence. Roughly speaking, the terms of the series have the same sign throughout any interval of the form e^k<n<e^{k+1}. The sum of the absolute values of the terms in that interval is \sum_{e^k<n<e^{k+1}}\frac1n \approx \int_{e^k}^{e^{k+1}}\frac1x\,dx = \ln(e^{k+1}) - \ln(e^k) = 1. So the series alternates between a batch of positive terms adding up to approximately1, followed by a batch of negative terms adding up to approximately –1. This oscillation means that it cannot converge.
    What Opalg has shown here is that the sequence of partial sums of the series (in its original order) has a non-convergent subsequence, and so cannot converge. There is no reordering of the sequence involved.

    CB
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  13. #13
    MHF Contributor chisigma's Avatar
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    Quote Originally Posted by CaptainBlack View Post
    A series is convergent if the sequence of partial sums converges, otherwise it is divergent. You may invent a distinction between divergent and indeterminate if you wish you are now using non-standard terminology.

    CB
    The following is from 'T. J. A. Bromwich An introduction to the theory of infinite series [London: Macmillan, 1908]' ...



    The [british] term oscillatory is much better than the [italic] indeterminate to indicate a bounded sequence that has no finite limit, so that I'll be glad to use it in the future ...

    Kind regards

    \chi \sigma
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  14. #14
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    Quote Originally Posted by chisigma View Post
    The following is from 'T. J. A. Bromwich An introduction to the theory of infinite series [London: Macmillan, 1908]' ...



    The [british] term oscillatory is much better than the [italic] indeterminate to indicate a bounded sequence that has no finite limit, so that I'll be glad to use it in the future ...

    Kind regards

    \chi \sigma
    In the 102 years since Bromwich wrote his book, the definition of divergence used throughout the English-speaking world has long since stabilised on the one given in Bromwich's footnote. In other words, a sequence diverges if it does not converge.
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  15. #15
    MHF Contributor chiph588@'s Avatar
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    Quote Originally Posted by CaptainBlack View Post
    What Opalg has shown here is that the sequence of partial sums of the series (in its original order) has a non-convergent subsequence, and so cannot converge. There is no reordering of the sequence involved.

    CB
    I follow his argument, but I was trying something different. Basically I'm asking if what I did is valid, because it feels like I'm changing the order of summation.
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