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Math Help - Sequence question

  1. #1
    s3a
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    Sequence question

    Question:
    "Show that the sequence defined by a_1 = 1 a_(n+1) = 3 - 1/(a_n)
    is increasing and a_n < 3 for all n. Deduce that {a_n} is convergent and find its limit."

    Answer (from the back of the book):
    1/2 * (3 + sqrt(5)

    My work/answer:
    I notice that it is increasing because as a_n becomes larger the fractional part approaches 0 and therefore 3 - 0 = 3 so the limit should be 3 but my answer does not agree with the back of the book. It also seems convergent to me because it is bounded between [2,3) and it is monotonic (increasing) because the larger the the number you put on the denominator of the fractional part, the less you are subtracting and therefore the larger the final number is.

    If someone could explain what I am doing wrong, I would really appreciate it!
    Thanks in advance!
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  2. #2
    Super Member Anonymous1's Avatar
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    Quote Originally Posted by s3a View Post
    Question:
    "Show that the sequence defined by a_1 = 1 a_(n+1) = 3 - 1/(a_n)
    is increasing and a_n < 3 for all n. Deduce that {a_n} is convergent and find its limit."
    a_1 = 1

    a_{n+1} = 3 - \frac{1}{a_n}

    To show it is increasing use induction...

    a_2=  3 - \frac{1}{a_1} = 3 - 1 = 2.

    a_1 < a_2 there is your base case.

    Now show a_n \leq a_{n+1}

    Now to show it is bounded.

    a_n \leq a_{n+1} \Rightarrow a_n - (3 - \frac{1}{a_n}) \leq 0...

    Now, since it is increasing and bounded above, it has a limit. This is an important theorem in Real Analysis.
    Last edited by Anonymous1; April 4th 2010 at 03:28 PM.
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  3. #3
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    Use induction.
    Suppose that a_{N+1}\le a_N <3 then \frac{1}{a_{N+1}}\ge\frac{1}{a_N}>\frac{1}{3}.
    But that implies that 3-\frac{1}{a_{N+1}}\le 3-\frac{1}{a_N}< 3-\frac{1}{3}.
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