Interval of existence and uniqueness of a separable 1st ODE

In summary, the problem involves finding solutions to the differential equation y'=((x-1)/(x^2))*(y^2) with the initial condition y(1)=1. The solutions exist on the interval x∈(-∞,0)∪(0,∞) and are unique on x∈(-∞,0)∪(0,∞). By solving the differential equation and using the initial condition, the solution is y(x) = x/(ln|x|+1). However, there may be a mistake as another solution is obtained through separation of variables, y(x) = -1/(ln|x|+1/x+c). Further clarification is needed to determine the correct solution.
  • #1
Apothem
39
0
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!
 
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  • #2
Apothem said:
(1)=1 we see that y(x)=x/(ln|x|+1)

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}## ...
 

Related to Interval of existence and uniqueness of a separable 1st ODE

1. What is an interval of existence for a separable 1st ODE?

The interval of existence refers to the range of values for the independent variable where a solution to the separable 1st ODE exists. This interval can be determined by solving the ODE and checking the domain of the solution.

2. How is the interval of existence determined for a separable 1st ODE?

The interval of existence can be found by solving the ODE and checking for any restrictions on the independent variable. It is important to note that the interval may differ for different initial conditions.

3. What is the uniqueness of a solution for a separable 1st ODE?

The uniqueness of a solution refers to the fact that there is only one possible solution for a given separable 1st ODE, given a specific set of initial conditions. This means that the solution is not affected by any changes in the initial conditions or other parameters.

4. Can a separable 1st ODE have multiple solutions?

No, a separable 1st ODE can only have one unique solution, as it is a well-defined mathematical problem with a unique solution for a given set of initial conditions. However, different initial conditions may result in different solutions.

5. What happens if the interval of existence is infinite for a separable 1st ODE?

If the interval of existence is infinite, it means that the solution to the separable 1st ODE exists for all values of the independent variable. This can happen when the ODE is a simple exponential function, for example.

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