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The Exponential of an infinite sum

  1. Oct 24, 2011 #1
    I have a problem that arises in quantum field theory. It involves a problem in combinitorics and about the theory of connected graphs.

    Essentially, I am trying to prove an identity involving an exponential of an infinite series with the ways to decompose an integer into the sum of integers.

    [tex]\exp\left({\sum_{n=1}^\infty a_n}\right) = \sum_{n=0}^\infty \sum_{partitions} \prod_{i=1}^n \frac{a_i^{n_i}}{n_i ! }[/tex]

    Where the partition is over the ways to write n as a sum of integers.

    http://en.wikipedia.org/wiki/Partition_(number_theory)

    [tex]\sum_{i=1}^n in_i = n[/tex]

    If you expand out the first few terms of the right hand side, it looks good.

    n=0 (I'm taking it to be 1 just to avoid confusion)

    n=1

    [tex] a_1 [/tex]

    n=2

    [tex] a_2 + \frac{1}{2!}a_i^2 [/tex]

    cummulative

    [tex] a_1 + \frac{1}{2!}a_1^2 + a_2 [/tex]

    n=3

    [tex] a_3 + a_1 a_2 + \frac{1}{3!} a_1^3 [/tex]

    cummulative

    [tex] a_1 + \frac{1}{2!}a_1^2 + \frac{1}{3!}a_1^3+ a_2 + a_1 a_2 + a_3[/tex]

    n=4

    [tex] a_4 + a_1 a_3 +\frac{1}{2!} a_2^2 + \frac{1}{2!} a_1^2 a_2 + \frac{1}{4!} a_1^4[/tex]

    cummulative

    [tex] a_1 + \frac{1}{2!}a_1^2 + \frac{1}{3!}a_1^3 + \frac{1}{4!}a_1^4 + a_2 + \frac{1}{2!} a_2^2 + a_1 a_2 + \frac{1}{2!}a_1^2 a_2 + a_3 + a_1 a_3 + a_4[/tex]

    As you can see, this seems to be systematically giving us the terms of the exponential.

    Any thoughts on how to prove this generally?
     
    Last edited: Oct 24, 2011
  2. jcsd
  3. Oct 24, 2011 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    Suggestion: this is a math question. Why not move it there?
     
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