1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Odd Factoria (1x3x5x )

  1. Aug 6, 2012 #1
    1. The problem statement, all variables and given/known data

    The question is stated in the picture attached.

    3. The attempt at a solution

    I used a slightly different method, by making use of recurrence relations through integration by parts:

    Jn = (2/2n+1) Jn+1

    Jn = [1x3x5x....]/[2n] √∏

    = [2n-1]! / [2n-1 * (n-1)! ] = (2n)! / (2n * n!)

    But the answer had 4n instead at the bottom... Not sure where I went wrong.

    Attached Files:

  2. jcsd
  3. Aug 6, 2012 #2


    User Avatar
    Science Advisor

    Your working is mostly correct unscientific, you just left behind a factor of [itex]2^n[/itex] on the denominator.

    Where you had Jn = [1x3x5x....(2n-1)]/[2n] √∏, this is correct.

    Next you can replace the "odd factorial" with,

    [tex](2n-1)(2n-3) \ldots 1 = \frac{(2n-1)!}{2^{n-1} (n-1)!} = \frac{(2n)!}{2^{n} (n)!}[/tex]
    Last edited: Aug 6, 2012
  4. Aug 6, 2012 #3

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    [tex]1 \cdot 3 \cdot 5 \cdot \cdots \cdot (2m+1) = \frac{(2m+1)!}{2 \cdot 4 \cdot \cdots \cdot 2m} = \frac{(2m+1)!}{2^m m!}.[/tex]
    Thus [tex] \frac{1 \cdot 3 \cdot 5 \cdot \cdots \cdot (2m+1) }{2^m} = \frac{(2m+1)!}{4^m m!}.[/tex]

  5. Aug 6, 2012 #4
    Oh damn! I forgot about the 2n below initially! Can't believe I missed it despite checking numerous times. Sorry, I will check my working more thoroughly next time!
  6. Aug 6, 2012 #5
    Also, are there any links on this site that teaches one how to use latex? I think it'll make my workings clearer.
  7. Aug 6, 2012 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

  8. Aug 7, 2012 #7
    Splitting the integral up and setting [itex]x=\sqrt{t}[/itex] yields the integral
    [tex]2\int^{\infty}_{0}\frac{t^n e^{-t}\,dt}{2\sqrt{t}}[/tex]
    which gives
    [tex]\int^{\infty}_{0}t^{n-1/2} e^{-t}\,dt = \Gamma(n+1/2)[/tex]
    The Lagrange duplication formula can be applied here:
    For integer n, this simplifies to
    which is equivalent to
    This gives the result in the paper. Usually, I try to work integrals that have a power of a variable and the exponential function in it using the Gamma function. It worked in this case as well.
    Last edited: Aug 7, 2012
  9. Aug 7, 2012 #8
    I seem to have problems with logging into my main account: unscientific

    It always says that I've tried to log in too many times..
  10. Aug 7, 2012 #9
    I know I shouldn't be posting here but I have problems with my both accounts (unscientific and this) - It keeps saying that I've tried to log in too many times and can't try for 15mins..

    This question is about the green function..

    I'm wondering what could be wrong with my working..

    I'm getting (d2x/dt2) + x = 0

    instead of f(t)...

    Attached Files:

  11. Aug 7, 2012 #10
    I've attached the solutions..I got nearly the same answer just with an additional term of


    Unless f(∏) = 0, which they never stated anywhere...

    Attached Files:

  12. Aug 7, 2012 #11

    Ray Vickson

    User Avatar
    Science Advisor
    Homework Helper

    I had that response for a while this morning, too: it said I had exceeded the number of incorrect log-in attempts (not true: 0 is not greater than 4 or whatever), and to wait 15 minutes and try again. It still did not work after waiting 45 minutes, nor after waiting 2 hours. However, it is now working again, obviously.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook