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Thread: Differential Equations

  1. #1
    Len is offline
    Mar 2008

    Differential Equations

    \frac{dv}{dt}=k v^{2/3} where k= 3^{2/3} (4 \pi)^{1/3}

    Why doesn't this equation satisfy the hypothesis of the uniqueness theorem?
    Last edited by mr fantastic; Mar 26th 2009 at 07:46 PM. Reason: Fixed the latex
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  2. #2
    MHF Contributor chisigma's Avatar
    Mar 2009
    near Piacenza (Italy)
    For semplicity let's suppose that k=1, so that the equation is...

    v'= v^{\frac{2}{3}} (1)

    Writing a generic first order equation in the form...

    v'= f(v,t) (2)

    ... with 'initial condition' v(t_{0})=v_{0}, it admits one and only one solution if and only if the so called 'Lipschitz conditions' are satisfied. One of these conditions requires that f(*,*) must have bounded partial derivatives in a 'small region' around (v_{0}, t_{0}) and that is not for (1) if v_{0}=0.

    The fact is self evident if we try to solve (1), for example, with the condition v(0)=0. Proceeding in the 'standard way' you find that 'the solution' is...

    v(t)=f(t)=(\frac{t}{3})^{3} (3)

    All seems to be 'ok' but... but there is a little problem because it is not difficult to verify that this family of functions...

    f_{\tau}(t)= (\frac {t-\tau}{3})^{3} \cdot U(t-\tau) (4)

    ... where \tau >0 and U(*) is the so called 'step function' also satisfies the (1) with 'initial condition' v(0)=0... just a little problem! ...

    Kind regards

    \chi \sigma
    Last edited by chisigma; Mar 27th 2009 at 02:35 AM.
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