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Change of variables in differential equation

  1. Jan 31, 2010 #1
    1. The problem statement, all variables and given/known data

    I have to transform the following equation using variables [tex](u,v,w(u,v))=(yz-x,xz-y, xy-z)[/tex]:

    [tex](xy+z)\frac{\partial z}{\partial x}+(1-y^2)\frac{\partial z}{\partial y}=x+yz.[/tex]


    2. Relevant equations
    chain rule: [tex]
    \frac{dw}{dx} = \frac{\partial w}{\partial u} \frac{\partial du}{\partial dx} + \frac{\partial w}{\partial v} \frac{\partial dv}{\partial dx}
    [/tex]


    3. The attempt at a solution
    Using the chain rule and the product rule:

    [tex]\frac{\partial w}{\partial x}=\frac{\partial z}{\partial x}(y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v})-\frac{\partial w}{\partial u}+z\frac{\partial w}{\partial v}[/tex]

    and a similar expression for [tex]\frac{\partial w}{\partial y}[/tex].
    On the other hand

    [tex]w=xy-z[/tex], so

    [tex]\frac{\partial w}{\partial x}=y-\frac{\partial z}{\partial x}[/tex]

    (and similar for [tex]\frac{\partial w}{\partial y}[/tex]), so

    [tex]\frac{\partial z}{\partial x}(y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v})-\frac{\partial w}{\partial u}+z\frac{\partial w}{\partial v}=y-\frac{\partial z}{\partial x}[/tex] and therefore:

    [tex]\frac{\partial z}{\partial x}[y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1]=y+\frac{\partial w}{\partial u}-z\frac{\partial w}{\partial v}[/tex] and

    [tex]\frac{\partial z}{\partial y}[y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1]=x+\frac{\partial w}{\partial v}-z\frac{\partial w}{\partial u}.[/tex]

    After multiplying the given equation by

    [tex][y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1][/tex]

    and writing

    [tex]\frac{\partial z}{\partial x}[y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1][/tex] as [tex]y+\frac{\partial w}{\partial u}-z\frac{\partial w}{\partial v}[/tex] I get

    [tex]\frac{\partial w}{\partial v}(1-x^2-y^2-z^2-2xyz)=0.[/tex]

    If
    [tex][y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1]\neq 0[/tex]

    this equation is equivalent to the given one. But what if [tex][y\frac{\partial w}{\partial u}+x\frac{\partial w}{\partial v}+1]=0[/tex]?.

    I'm kind of stuck at this point, cause in the first case

    [tex]\frac{\partial w}{\partial v}=0[/tex] or [tex](1-x^2-y^2-z^2-2xyz)=0[/tex].

    I was thinking of a way to change [tex](x,y,z)[/tex] to [tex](u,v,w)[/tex] in the last equation, but without success. I also have no idea what to do in the second case. So what should I do now? Is this a correct method to solve a problem like this?
     
  2. jcsd
  3. Jan 31, 2010 #2
    I've found the answer to this problem in a book by B.P. Demidowich - it says
    [tex]
    \frac{\partial w}{\partial v}=0
    [/tex]
    So this means that I need a proof that
    [tex]
    (1-x^2-y^2-z^2-2xyz)\neq 0
    [/tex], or do I still miss something?

    [tex]
    (1-x^2-y^2-z^2-2xyz)=(xy+z)z+(1-y^2)(-1)+x(x+yz)
    [/tex]
    looks similar to
    [tex]
    (xy+z)\frac{\partial z}{\partial x}+(1-y^2)\frac{\partial z}{\partial y}=x+yz.
    [/tex] but I don't have other ideas.
     
    Last edited: Jan 31, 2010
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