Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Alternating Series Test

  1. Mar 18, 2007 #1
    1. The problem statement, all variables and given/known data
    Test the series for convergence or divergence using the Alternating Series test:

    1. sum of (-1)^n * n/(ln n)

    2. sum of [sin(n*pi/2)]/n!

    2. Relevant equations

    3. The attempt at a solution
    1. lim of n/(ln n) goes to infinity (as n->infinity), so it can't satisfy the Alternating Series test. Then if I take limit of the entire sum, I can't figure out what it comes to. I think the limit doesn't exist so the whole thing is divergent, but I'm not sure how to get it.

    2. No clue on this one, it's not in the "proper format" as the examples I've seen.

  2. jcsd
  3. Mar 19, 2007 #2

    Gib Z

    User Avatar
    Homework Helper

    1. See if the taylor series for ln x helps.

    2. Consider the terms of this series in pairs, n=0,1. What do you notice with the sin part?

    EDIT: More clues, because i want to be generous :D

    Whats zero divided by anything (other than zero)? You should be able to get that sum to now look like

    [tex]\frac{1}{\sqrt{2}} \sum_{n=0}^{\infty} \frac{1}{(2n+1)!}[/tex] Does that converge :)?
    Last edited: Mar 19, 2007
  4. Mar 19, 2007 #3


    User Avatar
    Science Advisor

    1. One of the first things you learn about series is that if an does NOT go to 0, then [itex]\Sigma a_n[/itex] does NOT converge.

    2. Is this an alternating series? What is the "alternating series test"?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook