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I Interval of existence and uniqueness of a separable 1st ODE

  1. Sep 28, 2016 #1
    Problem:
    y'=((x-1)/(x^2))*(y^2) , y(1)=1 . Find solutions satisfying the initial condition, and determine the intervals where they exist and where they are unique.

    Attempt at solution:
    Let f(x,y)=((x-1)/(x^2))*(y^2), which is continuous near any (x0,y0) provided x0≠0 so a solution with y(x0)=y0 is guaranteed to exist when x0≠0
    The partial derivative with respect to y of f(x,y) is 2y*((x-1)/(x^2)) which is continuous near any (x0,y0) provided x0≠0 so a solution with y(x0)=y0 is unique when x0≠0 (By Picard's Theorem)

    From this the interval in which the solutions are unique is x∈(-∞,0)∪(0,∞).

    Solving the differential equation and using the initial condition y(1)=1 we see that y(x)=x/(ln|x|+1). The interval of existence is 1/e < x < ∞

    Is this right, or...?

    Thanks for any help!
     
  2. jcsd
  3. Sep 28, 2016 #2

    Ssnow

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    Gold Member

    are you sure?

    because by separation of variables of ##y'=\frac{x-1}{x^2}y^2## we have ##\int \frac{1}{y^2}\,=\,\int \frac{1}{x}-\frac{1}{x^2}dx## that gives ##y=-\frac{1}{\ln{|x|}+\frac{1}{x}+c}## ...
     
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