Having Trouble with this DEQ

  1. I am trying to solve

    [tex]x^2y'' + xy' = 0[/tex]

    I know that two solutions that work, by inspection are

    [tex]y_1 = c_1[/tex]

    and

    [tex]y_2 = c_2ln(x)[/tex]

    where [tex]c_1[/tex] and [tex]c_2[/tex] are just arbitrary constants.

    However I was hoping I would be able to find a more systematic way to solve it. I can get the constant solution by the method of Frobenius, but when I tried to get the natural log solution using the Wronskian, I wasn't able to get it.

    I ended up with the second solution being the integral of a Gaussian or something of that sort. I know the Wronskian method is always supposed to work for DEQ's of this type. Where am I going wrong? Can someone show me how to use the Wronskian and the constant solution to get the natural log solution?

    (Also why do my LaTeX expressions look funny)

    Thanks for the help
     
  2. jcsd
  3. HallsofIvy

    HallsofIvy 40,203
    Staff Emeritus
    Science Advisor

    Since y itself does not appear in that equation, there is a pretty standard method. Let u= y'. Then the equation becomes x2u'+ xu= 0 which is separable:
    x2u'= -xu or (1/u)du= -(1/x)dx. Integrating, ln(u)= -ln(x)+ C or dy/dx= u= c/x (where c= eC) dy= (c/x)dx gives y= cln(x)+ D.

    This also happens to be an "Euler-type" or "equi-potential" equation- the power of x in each term is equal to the order of the derivative. The change of variable, u= ln(x), so that x= eu, makes that an equation with constant coefficients: dy/dx= (dy/du)(du/dx)= (1/x)dy/du and d2y/dx2= d/dx(dy/dx)= d/dx((1/x)dy/du)= (1/x)d/du((1/x)dy/du)= (1/x)((1/x)d2y/du2)-(1/x2dy/dx.
    The equation becomes d2y/dx2- dy/du+ dy/du= d2/du2= 0. Integrating once y'(u)= C. Integrating again y(u)= Cu+ D. Since u= ln(x), y(x)= Cln(x)+ D.
     
  4. tiny-tim

    tiny-tim 26,055
    Science Advisor
    Homework Helper

    Hi dsr39! :smile:

    Do you mean in the line "where [tex]c_1[/tex] and [tex]c_2[/tex] are just arbitrary constants."?

    Because when it's in the middle of a text line, you need to use "itex" (for "inline latex") instead of "tex" …

    where [itex]c_1[/itex] and [itex]c_2[/itex] are just arbitrary constants. :wink:
     
  5. hmm... What if the equation is (x^2)*y"[x]+x*y'[x]-y[x] = 0?

    How different would the solution be?
     
  6. lurflurf

    lurflurf 2,322
    Homework Helper

    x^2*y''+x*y'=0
    x*y''+y'=0
    (x*y')'=0
    x*y'=C1
    y'=C1/x
    y=C1*log(x)+C2
     
  7. Not sure if u are replying to my question or the original poster's question.
    What if there is an additional y[x] term in the equation? i.e.

    (x^2)*y"[x]+x*y'[x]-y[x] = 0
     
  8. We could try guessing solutions.

    Try y = x. This works.

    Indeed, y = cx works, for all constants c.

    Does y = cx + d work? not really, since you have that 'd' left over.

    So far, we can be sure that the general solution will contain a "cx" term.

    Similarly, one can see that a/x works too; try it out.

    So I'm guessing the general solution is

    y(x) = ax + b/x

    Notice that it has two degrees of freedom, as required for a 2nd order ODE.
     
  9. lurflurf

    lurflurf 2,322
    Homework Helper

    (x^2)*y"+x*y'-y = 0
    [(x^2)*y'-x*y]'=0
    let y=u/x
    [(x^2)*y'-x*y]'=0->x*u''-u'=0
    u'=2x*C2
    u=C1+(x^2)*C2
    y=u/x=C1/x+x*C2
     
    Last edited: Mar 4, 2009
  10. Nice.

    Does it come out to be what I got?
     
  11. lurflurf

    lurflurf 2,322
    Homework Helper

    ^Now that I fixed my error they match.
     
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