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Limits of sequences involving factorial statements!

  1. Mar 26, 2016 #1
    1. The problem statement, all variables and given/known data
    I have to determine whether or not the following sequence is convergent, and if it is convergent, I have to find the limit.

    an = (-2)n / (n!)

    In solving this problem, I am not allowed to use any form or variation of the Ratio Test.
    2. The attempt at a solution
    I was thinking that I could reasonably compare the above sequence to bn = (1/2)(-2)n/(3n-2), like so:

    (1/2)(-2)n/(3n-2) <= (-2)n / (n!) <= (1/2)(2)n/(3n-2)

    By manipulating the bn statement, I can make a geometric sequence:

    bn = (1/2)(-2)2(-2/3)n-2

    I know this converges since r = -2/3, and the absolute value of r is less than 1. Taking the limit of the sequence (bn) as n goes to infinity, I find that the value of the sequence approaches zero. Then, since both bn and its absolute value converge to zero, I can say that an converges and its limit is zero as well, by the Squeeze Theorem.

    I honestly have no idea if this is right or not. My professor gave the class this problem as part of a practice test, and we weren't given an answer key. The closest problems I can find in my textbook all use the Ratio Test in their solutions, so they're no help. The corresponding test is on Monday, and we will not be going over any solutions to the practice test beforehand, so I'd really like to make sure my thought process is correct.
     
  2. jcsd
  3. Mar 26, 2016 #2

    LCKurtz

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    Ratio test?? There is no ratio test for sequences. Or is this the nth term of a series? It matters and students often confuse the two. Are you testing a sequence or a series for convergence?

    Inequalities like that with alternating signs don't work. And factorials grow faster then exponentials, so even with absolute values, that first inequality wouldn't work for large n.
     
  4. Mar 26, 2016 #3
    I'm testing a sequence for convergence. I know that factorials grow faster than exponentials but simply stating that isn't enough.
     
  5. Mar 26, 2016 #4

    LCKurtz

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    Try looking at the absolute value and write out the factors of numerator and denominator and see if that gives you any ideas.
     
  6. Mar 26, 2016 #5
    I've already tried that. It's just 2 * 2 * 2..... to infinity in the numerator and an infinitely long factorial in the denominator. Doesn't tell me anything.
     
  7. Mar 26, 2016 #6

    LCKurtz

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    Do it for ##a_n##, not ##a_\infty##, whatever that might mean.
     
  8. Mar 26, 2016 #7
    Same thing, only the factorial ends in (n-1) * n and 2 is multiplied by itself n times. I still don't have a clue what that tells me.
     
  9. Mar 26, 2016 #8

    LCKurtz

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    Does it look like this:$$
    \frac{2\cdot 2 \cdot 2 \cdots 2\cdot 2}{1\cdot 2 \cdot 3 \cdots n-1\cdot n}$$They don't quite line up on screen, but you have a bunch of fractions there you can think about.
     
  10. Mar 26, 2016 #9
    Yes, that's how it should look. Again, I'm not sure what it is I'm supposed to be seeing here.
     
  11. Mar 26, 2016 #10

    LCKurtz

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    You are trying to show it goes to zero. What if you group it like this:$$
    \frac{2\cdot 2}{1\cdot 2}\left(\frac{2}{3}\frac 2 4 \cdots \frac 2{n-1}\right)\frac 2 n$$Can you see any useful overestimates that would still go to zero?
     
  12. Mar 26, 2016 #11
    I know that 23/2n goes to zero. Would that be a valid overestimate?
     
  13. Mar 26, 2016 #12

    LCKurtz

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    Not sure why you would write ##\frac{2^3}{2n}## instead of ##\frac 4 n##, but as to whether it is a valid overestimate, you tell me. Can you give cohesive steps or argument why ##|a_n|\le \frac 4 n##? That is what your teacher would want on an exam. You have to say why...
     
  14. Mar 26, 2016 #13
    That's the problem. I don't have any idea how to do that. At all. All we were told was that we had to know how to do these kinds of problems. It was never taught in class.
     
  15. Mar 26, 2016 #14

    LCKurtz

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    Well, you asked me whether ##\frac 4 n## was a valid overestimate. You must have had some reason to think it was. How did you get that and why did you think it might be an overestimate? You had some reason. Just tell me.
     
  16. Mar 26, 2016 #15
    Well, my thinking is that n! becomes infinitely large. That said, I think that since n! is in the numerator, I should look for a value in the numerator which is comparatively less than n! and also becomes infinitely large. However, I'm unsure how the numerator is established. Also, why did you divide up the fractions the way you did?
     
    Last edited: Mar 26, 2016
  17. Mar 26, 2016 #16

    LCKurtz

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    I don't think that tells me how you came up with ##\frac 4 n##.

    That last question is the key. What do you notice about the fractions in the parentheses? What would be a nice overestimate for them?
     
  18. Mar 26, 2016 #17
    The numerator is constant, and the denominator is continuously increasing. I'm grasping at straws at this point, but could an overestimate be 2/n?
     
  19. Mar 26, 2016 #18

    LCKurtz

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    You need to quit thinking about things increasing or n changing. We are looking at$$
    |a_n|=\frac{2\cdot 2}{1\cdot 2}\left(\frac{2}{3}\frac 2 4 \cdots \frac 2{n-1}\right)\frac 2 n$$for fixed n. We might wonder later about what happens to it if ##n## is large, but right now we are looking at this expression. I asked you if you notice anything about the fractions in the parentheses. You haven't answered that.

    Earlier in this thread, you somehow came up with ##\frac 4 n## as a possible overestimate for this expression. There is a good reason to do that but you haven't told me how you got it.
     
  20. Mar 26, 2016 #19
    Forgive me if I sound irritated, but the whole reason I asked this question is I HAVE NO CLUE WHAT I'M DOING. I chose 4/n because it was what resulted from the terms on either side of the parentheses. I don't notice anything about the fractions in parentheses.
     
  21. Mar 26, 2016 #20

    LCKurtz

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    It's not a good thing to have "no clue" with a test coming Monday, is it. I have led you to a critical step in this problem and I'm trying to get you to think about it. I'm going to ask again for you to look at those fractions. You can't not notice anything about them. Are they complex numbers (no), are they negative (no), are they irrational numbers (no), are they positive (yes) etc.

    So instead of telling me you have no clue, I want you to tell me what you do know about those fractions in addition to the fact that they are positive. Write several of them down if you have to. Look at them.
     
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