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Reversing Logarithmic Integral Function

  1. Apr 9, 2010 #1
    The logarithmic integral function, which is what you get by integrating 1/ln(x), is closely linked to prime numbers. It approximates the number of primes smaller than x. Heres an infinite series which describes the function:


    where [tex]\gamma[/tex] is the Euler–Mascheroni constant.

    This infinite series is a continuous function and maps x to li(x) on a 1-to-1 basis.
    Therefore, in theory there should be an inverse function of li, right? But I'm having difficulty finding it.

    Thanks for any help guys :)
  2. jcsd
  3. Apr 9, 2010 #2


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    As long as you skip values below, say, 2 to avoid the singularity there's an inverse. But I don't know of any nice form for it.

    If you want to calculate its value, that's simple enough to do with a series expansion, a calculation of li at that point, and a correction.
  4. Apr 9, 2010 #3
    I'm looking into this for a computer program actually, so I dont need it in mathematical notion - just something I can put into an algorithm :)

    Also if its easier with a finite limit on the series then thats ok. I just cant find or work out an inverse of it :S
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