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Math Help - Proof ln is irrational

  1. #1
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    Proof ln is irrational

    I have been giving the challenge to proof the natural logarithm function is irrational. I'm trying to proof it by contradcition but I seem to be getting nowhere. Can someone hint me how to?

    Thank you.
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  2. #2
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    What do you mean by saying that the whole function is irrational?

    This short PDF document was the first in Google results, and it seems pretty good.
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    Sorry I am not a native speaker. I mean that ln isnt a rational function. Thanks for the document!
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    MHF Contributor chisigma's Avatar
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    Quote Originally Posted by gordo151091 View Post
    I have been giving the challenge to proof the natural logarithm function is irrational. I'm trying to proof it by contradcition but I seem to be getting nowhere. Can someone hint me how to?

    Thank you.
    A function f(x) is said to be 'rational' if it is computable with a finite number of 'elementary operations' +,-,* and \... all other functions are said to be 'irrational'...

    Kind regards

    \chi \sigma
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    A function f(x) is said to be 'rational' if it is computable with a finite number of 'elementary operations' +,-,* and \... all other functions are said to be 'irrational'...
    Thanks, this slipped my mind somehow.

    Then are we allowed to use irrational constants to build rational functions? I.e., if a function is a ratio of two polynomials, can the polynomials' coefficients be irrational? If no, then the document provided above is still sufficient because it shows that \ln n\notin\mathbb{Q} for n\in\mathbb{Z}.
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    MHF Contributor chisigma's Avatar
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    Quote Originally Posted by emakarov View Post
    Thanks, this slipped my mind somehow.

    Then are we allowed to use irrational constants to build rational functions?... i.e., if a function is a ratio of two polynomials, can the polynomials' coefficients be irrational?...
    From the 'theoretical' point of view, if You have a computer capable to menage irrational numbers [i.e. it has a memory of 'infinite dimension' and an infinite 'speed of computation'...], the ratio of two polynomials can be computed with a finite sequence of elementary operations...

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    \chi \sigma
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    I am asking whether rational functions can use irrational constants with respect to the OP's problem. If irrational numbers are not allowed, then the problem has been solved. Otherwise, more thought is needed, but I don't want to go there if it's not necessary.
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    Here is discussion of rational functional.
    Use the discussion tab at the top of that page.
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    Assume \ln x = \frac{a_{n}x^{n} + ... + a_{1}x + a_{0}}{b_{m}x^{m} + ... + b_{1}x + b_{0}} then x = e^{\frac{a_{n}x^{n} + ... + a_{0}}{b_{m}x^{m}+...+b_{0}}.

    See how this yields a contradiction?
    Last edited by ragnar; February 26th 2011 at 08:31 PM. Reason: Typo
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  10. #10
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    Er... This proves all cases where b_{0} \ne 0. Not totally sure what to do with the rest of the cases right now... I'll write back about this later.
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