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Math Help - Existence of Solutions to ODEs

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    Existence of Solutions to ODEs

    Say you have a non-linear ODE or a system of ODEs. How do you know that a solutions in elementary functions doesn't exist? In other words how can you tell if the equation cannot be solved explicitly?
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    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by fobos3 View Post
    Say you have a non-linear ODE or a system of ODEs. How do you know that a solutions in elementary functions doesn't exist? In other words how can you tell if the equation cannot be solved explicitly?
    The short answer: you can't. That's what mathematicians are for. Otherwise you could computerise the lot.
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    Quote Originally Posted by Matt Westwood View Post
    The short answer: you can't. That's what mathematicians are for. Otherwise you could computerise the lot.
    Then are there any theorems about existence of solutions in elementary functions?
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    Super Member Matt Westwood's Avatar
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    Quote Originally Posted by fobos3 View Post
    Then are there any theorems about existence of solutions in elementary functions?
    Certain types of ODEs are known to have solution techniques. The conditions under which certain ODEs can be solved are discussed in most decent text books on the subject, e.g.

    * First order separable
    * First order homogeneous
    * Second order linear

    ... and so on. The subject is too vast to go into in this medium.

    See, for example:
    Picard's Existence Theorem - ProofWiki
    which gives the conditions under which a 1st-order ODE even has a solution, but that doesn't help you actually find such a solution, or whether it exists in elementary functions.

    Also check out the Method of Frobenius (GIYF) for a particular approach to 2nd order equations for when you aren't expecting the solution to have anything to do with elementary functions.

    And if your ODE is really difficult to solve, you can always use numerical methods.

    But all the above can be found in the introduction of most decent texts on the subject.
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