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Math Help - Isocline methods

  1. #1
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    Isocline methods

    I have the equation \frac{dx}{dt} = x^2 - t^2 - 1

    I have started to draw the isoclines and found them to be Hyperbolas i have then drawn the direction plot and cant really see what to do from there i have worked out that the line x = -t fits but cannot see any other solutions.

    Thank you for you help

    ViperRobK
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  2. #2
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    Re: Isocline methods

    do you graph the isoclines with a calculator? often they involve powers of e so it seems tricky to do it in your head
    an interesting thing to do is to predict the behavior of the solution as t goes to infinity. the isoclines are slope lines where dy/dt=m=-a(t)y+b(t) for instance. another example of isoclines would be circles for: y'=-y/(x^2+y^2) where the circles become flattened out into the null cline m=y=0
    Last edited by mathlover10; September 10th 2013 at 12:43 AM.
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  3. #3
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    Re: Isocline methods

    The solutions of the ODE are :
    x = -t +exp(-tē)/(c-sqrt(pi/4)*erf(t))
    If t tends to infinity, exp(-tē) tends to 0. So, the asymptotic line is x = -t.
    If abs(c) < sqrt(pi/4) there is a particular value of t=a so that (c-sqrt(pi/4)*erf(a))=0 and then x tends to infinity. In these cases there is a second asymptotic line t=a.
    As a consequence, in some cases (depending on c), the isoclines look like, but are not exactly hyperbolas.
    In any case, never they are hyperbolas.
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  4. #4
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    Re: Isocline methods

    how do you do the denominator erf function? so this is a linear Riccati equation that can always be reduced to a 2nd order linear ODE? x'=qo(t)+qo(t)x+qo(t)x^2
    Last edited by mathlover10; September 11th 2013 at 02:09 AM.
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  5. #5
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    Re: Isocline methods

    The linear Riccati equation can be reduced to a 2nd order linear ODE :
    x'=P(t)+Q(t)x+R(t)x^2
    Let : x = -y' / (R*y) Where y(t) is the new unknown function.
    Last edited by JJacquelin; September 11th 2013 at 05:31 AM.
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  6. #6
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    Re: Isocline methods

    if q2 is nonzero then you can make a substitution v=yq2, S=q2q0, R=q1+(q2'/q2) which satisfies a Riccati, v'=v^2+R(x)v+S(x)=(yq2)'=q0q2+(q1+q2'/q2)v+v^2
    with double substitution v=-u'/u, u now satisfies a linear 2nd ODE:
    u''-R(x)u'+S(x)u=0, v'=-(u'/u)'=-(u''/u)+v^2, u''/u=v^2-v'=-S+Ru'/u, therefore we have reduced the Ricatti to an equation
    u''+Su-Ru'=0, solving this equation will lead to a solution y=-u'/(q2u) You can find this proof in texts, but what was the original motivation behind the invention of the substitutions?
    Last edited by mathlover10; September 12th 2013 at 02:45 AM.
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  7. #7
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    Re: Isocline methods

    do you plot these isoclines by hand somehow? it can be done for circles, but I'm not sure how to do it for more complex functions
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