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Math Help - sum of alternating series

  1. #1
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    sum of alternating series

    Hi,
    I am trying to prone that the sum of the following series is log3.

    1+1/2-2/3+1/4+1/5-2/6+1/7+1/8-2/9+1/10+1/11-2/12...

    Thank's in advance.
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    Re: sum of alternating series

    Hey hedi.

    Hint: Look at the taylor series expanded around a = 1 (i.e. taylor series for log(1+x)).
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    Re: sum of alternating series

    This gives the harmonic alternating series,it is not so helpfull here.
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    Re: sum of alternating series

    Can you collect terms together to show that both are the same series?
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    Re: sum of alternating series

    Quote Originally Posted by hedi View Post
    Hi,
    I am trying to prone that the sum of the following series is log3.

    1+1/2-2/3+1/4+1/5-2/6+1/7+1/8-2/9+1/10+1/11-2/12...

    Thank's in advance.
    I think this is the continued fraction form of the natural log. Take a look at the wiki page on continued fractions here or here
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  6. #6
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    Re: sum of alternating series

    Quote Originally Posted by hedi View Post
    Hi,
    I am trying to prone that the sum of the following series is log3.

    1+1/2-2/3+1/4+1/5-2/6+1/7+1/8-2/9+1/10+1/11-2/12...

    Thank's in advance.
    Let H_n be the n-th Harmonic number (the sum of the first n terms of the Harmonic series). Consider partial sums of three terms of your series at a time. Show that S_n = H_{3n}-H_n gives the correct partial sums for your series. So, your series is given by \lim_{n \to \infty}S_n.

    Next, use common inequalities for the Harmonic numbers. For example, Young (1991) showed that \dfrac{1}{2n+2} < H_n - \ln n - \gamma < \dfrac{1}{2n}. So,

    \dfrac{1}{6n+2} < H_{3n} - \ln (3n) - \gamma < \dfrac{1}{6n} and -\dfrac{1}{2n} < \gamma + \ln n - H_n < -\dfrac{1}{2n+2}

    Adding these inequalities together gives:

    \dfrac{1}{6n+2} - \dfrac{1}{2n} < H_{3n} + \gamma + \ln n - \ln (3n) - \gamma - H_n < \dfrac{1}{6n} - \dfrac{1}{2n+2}

    Simplifying gives:

    -\dfrac{2n+1}{2n(3n+1)} < S_n - \ln 3 < \dfrac{1-2n}{6n(n+1)}

    Finally, use the Squeeze Theorem to show that \lim_{n \to \infty} (S_n - \ln 3) = 0, so \lim_{n \to \infty} S_n = \ln 3
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    Re: sum of alternating series

    Btw, to show that S_n = H_{3n} - H_n, you can write your series as follows:

    \begin{align*}1+\frac{1}{2} -\frac{2}{3} +\cdots + \frac{1}{3n-2} + \frac{1}{3n-1} - \frac{2}{3n} & = \sum_{k= 1}^n \left(\dfrac{1}{3k-2} + \dfrac{1}{3k-1} -\dfrac{2}{3k}\right) \\ & = \sum_{k= 1}^n \left(\dfrac{1}{3k-2} + \dfrac{1}{3k-1} + \dfrac{1}{3k} -\dfrac{3}{3k}\right) \\ & = \sum_{k= 1}^n \left(\dfrac{1}{3k-2} + \dfrac{1}{3k-1} + \dfrac{1}{3k}\right) -\sum_{k=1}^n\dfrac{1}{k} \\ & = \sum_{k=1}^{3n}\dfrac{1}{k} - \sum_{k=1}^n \dfrac{1}{k} \\ & = H_{3n} - H_n\end{align*}
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    Re: sum of alternating series

    Thanks to both of you so much.
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    Re: sum of alternating series

    I'm curious what course (if any) this would be learned in. I've had a fair bit of math and never ran into a detailed analysis of the Harmonic series like this.

    Is this something you just naturally come across post-grad in the course of solving other problems?
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    Re: sum of alternating series

    Quote Originally Posted by romsek View Post
    I'm curious what course (if any) this would be learned in. I've had a fair bit of math and never ran into a detailed analysis of the Harmonic series like this.

    Is this something you just naturally come across post-grad in the course of solving other problems?
    Are you asking hedi or me? If you are asking me, then this comes about from examining Euler's constant, which is \gamma = \lim_{n \to \infty} H_n - \ln n. Young was showing how quickly the sequence H_n - \ln n converges to \gamma.
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    Re: sum of alternating series

    This question comes from a course in problem solving.
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    Re: sum of alternating series

    Actually, I just thought of a more direct way to show that the series converges to \ln 3. By definition, \gamma := \lim_{n \to \infty} (H_n - \ln n), so

    \begin{align*}0 & = \gamma - \gamma \\ & = \lim_{n \to \infty} \left(H_{3n} - \ln (3n) \right) - \lim_{n \to \infty} \left( H_n - \ln n \right) \\ & = \lim_{n \to \infty} \left( H_{3n} - H_n - \ln 3 \right)\end{align*}
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    Re: sum of alternating series

    This is not an alternating series...
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