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Proof by induction of a problem

  1. Feb 25, 2007 #1
    I'm working out a problem that requires me to proof a result by induction. I have worked out what I think is a correct proof, but I would like for somebody to look over it and give me feedback.
    Part 1: Give a reasonable definition of the symbol [tex]\sum_{k = m}^{m + n}{a}_{k}[/tex]
    I first define:
    \sum_{k = m}^{m}{a}_{k} = {a }_{ m}
    Then assuming I have defined [tex]\sum_{k = m}^{n}{a}_{k}[/tex] for a fixed n >= m, I further defined:
    [tex] \sum_{ k=m}^{n+1 } {a }_{k } =( \sum_{k=m }^{ n} { a}_{ k} )+ { a}_{n+1 } [/tex]

    Part 2 requires me to prove by induction that for all n >= 1, we have the assertion (call it A(n)):
    [tex]\sum_{k=n+1 }^{ 2n} \frac{1 }{k } = \sum_{ m=1}^{ 2n} \frac{ {(-1) }^{m+1 } }{m } [/tex]

    I approach this problem as I would have any proof by induction. The base case A(1) is true so I won't write it out here. Now, assuming the assertion is true for some k:
    [tex] \frac{ 1}{ k+1} + \frac{1 }{k+2 } +...+ \frac{ 1}{ 2k} = 1 - \frac{ 1}{2 } + \frac{1 }{3 } - \frac{ 1}{ 4} +...+ \frac{ {(-1) }^{2k+1 } }{ 2k} [/tex]

    where the last term on the RHS simplifies to [tex]- \frac{ 1}{2k } [/tex].
    I have to show that A(k+1) is true:
    (*)[tex]\frac{ 1}{ k+2} + \frac{1 }{k+3 } +...+ \frac{ 1}{ 2(k+1)} = 1 - \frac{ 1}{2 } + \frac{1 }{3 } - \frac{ 1}{ 4} +...+ \frac{ {(-1) }^{2(k+1)+1 } }{ 2(k+1)} [/tex]

    where the last term on the RHS simplifies to [tex]- \frac{1 }{ 2(k+1)} [/tex]
    Starting with A(k), I add [tex] \frac{1 }{ 2k+1} + \frac{ 1}{ 2k+2} - \frac{ 1}{ k+1} [/tex] to each side and obtain:

    [tex]\frac{ 1}{k+1 } + \frac{ 1}{k+2 } +...+ \frac{ 1}{2k } - \frac{1 }{k+1 } + \frac{1 }{ 2k+1} + \frac{1 }{2k+2 } =1- \frac{ 1}{ 2} + \frac{ 1}{ 3} - \frac{ 1}{ 4} +...- \frac{1 }{2k+2 } - \frac{1 }{ k+1} + \frac{ 1}{ 2k+1} + \frac{1 }{2k+2 }
    After subtracting out the two like terms on each side, the left hand side becomes the sum
    [tex] \sum_{ k=(n+1)+1}^{ 2(n+1)} \frac{1 }{k } [/tex]

    and the terms [tex] \frac{ 1}{ 2k+2}- \frac{ 1}{ k+1} [/tex] on the right simplify to [tex] \frac{ -1}{2(k+1) } [/tex], so the right side resembles the right side in (*). Therefore, the RHS becomes the sum [tex] \sum_{ m=1}^{2(n+1) } \frac{ {(-1) }^{m+1} }{2m } [/tex]

    I'm wasn't sure if I was supposed to use the definition of part 1 in part 2 and I didn't, but I'm wondering if that would have made things easier. Thanks.
    Last edited: Feb 25, 2007
  2. jcsd
  3. Feb 26, 2007 #2


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    Staff Emeritus
    Science Advisor

    Your teacher might not appreciate that! Saying something is true is not proving that it is true.

    In going form n to n+1, two things happen. First, since the sum starts at n+1 in the first case and (n+1)-1 in the second, you are missing the first term. Second, since the sum ends at 2n in the first case and 2(n+1)= 2n+ 2 in the second, you will have two new terms, 2n+1 and 2n+ 2. That is,
    [tex]\sum_{k= (n+1)+1}^{2(n+1)} \frac{1}{k}= \sum_{k= n+1}^{2n} \frac{1}{k}+ \frac{1}{2n+1}+ \frac{1}{2n+2}- \frac{1}{n+1}[/tex]
    Now, replace that sum by
    [tex]\sum_{m=1}^{2n} \frac{(-1)^m+1}{m}[/tex]
    and do the algebra.

    You might want to use the fact that
    [tex]\frac{1}{2n+2}- \frac{1}{n+1}= \frac{1- 2}{2n+2}= -\frac{1}{2n+2}[/tex]
  4. Feb 26, 2007 #3
    Thanks, your answer was helpful and much appreciated.
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