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Do Alternating Series Have Limits?

  1. Apr 28, 2012 #1
    I would imagine that an alternating series that goes of to infinity doesn't have a limit because it keeps switching back and forth, but I can't find anything in my textbook about it. I just want to make sure that this is right.
  2. jcsd
  3. Apr 28, 2012 #2
    Some alternating series do have limits. For example:

    [tex]\sum_{n=1}^{+\infty} \frac{(-1)^n}{n}[/tex]

    has a limit (and it equals log(2)).

    Other series, like

    [tex]\sum_{n=1}^{+\infty} (-1)^n[/tex]

    have a too large oscillation to have a limit.
  4. Apr 28, 2012 #3
    Actually [itex]-\ln 2[/itex].
  5. Apr 28, 2012 #4
    Ah yes, thank you!!
  6. Apr 28, 2012 #5
    I'm still kinda confussed, so for example if I have the series:
    [tex]\sum_{n=1}^{+\infty} (-1)^n \frac{n}{n+1}[/tex]

    Is the limit nonexistant or does it equal 1?
  7. Apr 28, 2012 #6
    That limit doesn't exist since

    [tex]\lim_{n\rightarrow +\infty}{ \frac{(-1)^n n}{n+1}}\neq 0[/tex]
  8. Apr 28, 2012 #7
    Okay. So in the previos example where the limit was equal -ln2 , do I have to solve for and indeterminate power to get that answer?
  9. Apr 28, 2012 #8
    No. You get that answer if you find the Taylor series for the logarithm.
  10. Apr 30, 2012 #9
    In general , an alternating series of the form [itex]\sum[/itex] (-1)k ak will converge if ak [itex]\rightarrow[/itex]0 as k[itex]\rightarrow[/itex]∞ and 0<ak+1≤ ak
  11. Apr 30, 2012 #10


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    If [itex]a_n[/itex] goes to 0 then the series [itex]\sum (-1)^na_n[/itex] converges.
    If it does not, then we can determine two subseries of [itex]\sum (-1)^n a_n[itex], one with n even, the other with n odd, that converge to two different limits. And so the series itself does not converge.
    Last edited by a moderator: May 1, 2012
  12. Apr 30, 2012 #11

  13. Apr 30, 2012 #12
    Not necessary for the following:
    Consider [itex]\sum[/itex]k=1 (-1)k+1 ak where ak= 1/k if k is odd and 1/k2 if k is even. It is possible to show that this alternating series diverge to +∞ although ak goes to zero.
    This counter example indicates the necessity of the condition 0<ak+1 ≤ ak for convergence to happens.
  14. Apr 30, 2012 #13


    Staff: Mentor

    First off, it's not clear to me whether you're asking about the limit of the terms in the series or the limit of the series itself.

    If you're asking about the limit of the terms, then the limit does not exist.
    $$\lim_{n \to \infty} (-1)^n \frac{n}{n + 1}\text{ does not exist}$$

    The reason is that for large n, successive terms oscillate between values close to 1 and -1, depending on whether n is even or odd, which affects the sign of (-1)n.

    If you're asking about the sum of the series, then there too the limit does not exist. The Nth Term Test for Divergence says that if the limit of the terms of the series is different from zero or doesn't exist, then the series diverges. Since I established that the limit of the terms of the series doesn't exist, this theorem says that the series diverges (does not converge).
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