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Functions that are not integrable in terms of elementary functions.

  1. Sep 27, 2009 #1
    I was doing my differential equations homework. I had to solve y'' -4y = (e^(2x))/x.

    While doing this I ran into an integral[tex]\int\frac{e^{4x}}{4x}dx[/tex]. I tried integrating my times but I couldn't; my guess is that this cannot be integrated in terms of elementary functions but I'm not sure.

    Is there a theorem or Algorithm for knowing if a function is integrable in terms of elementary functions or not ?

    If so, can someone tell me the theorem ?

    And in my case is my function integrable in a finite number of elementary functions ?
     
  2. jcsd
  3. Sep 27, 2009 #2

    Hurkyl

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    I believe there is a theorem, although it's not particularly simple.

    As a practical matter, I've found mathematica a good enough test.
     
  4. Sep 27, 2009 #3
    Yeah, I saw the answer given by mathematica after my post.

    But do you have a link to the theorem É
    I would like to see it even though, I may understand little of it.
     
  5. Sep 27, 2009 #4
    R. Risch, The problem of integration in finite terms, Trans. Amer. Math. Soc , 139 (1969), 167-189. Mathematical Reviews (MathSciNet): MR38:5759

    KASPER T. (1980): "Integration in Finite Terms: the Liouville Theory", Mathematics Magazine 53 pp 195 - 201.

    Maxwell Rosenlicht, Pacific Journal of Mathematics 54 (1968) pp 153 - 161 and 65 (1976) pp 485 - 492.

    http://en.wikipedia.org/wiki/Risch_algorithm
     
  6. Sep 27, 2009 #5
    Thanks.
     
  7. Nov 18, 2009 #6
    Maybe you could expand e^{4x} in its Taylor series expansion. and then look at the integral of (e^{4x}-1)/x+1/x. In the first term there will be a cancellation of a power of x so it will be a polynomial integration. and the second gives ln(x). But depending on your integration bound there might be an issue with ln(x) (it blows up at x=0).
     
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