I have a question on the particular case of the Cauchy theorem for a first order differential equati

Laylah Mora

Laylah Mora

Answered question

2022-05-19

I have a question on the particular case of the Cauchy theorem for a first order differential equation with separable variables.
y ( x ) = a ( x ) b ( y ( x ) )
If I impose:
a ( x ) continuous in a interval I
b ( y ( x ) ) continuous and with continuous derivative in a interval J
Can I say that there is only one solution in all the interval I that satisfies the condition of passage through a point ( x 0 , y 0 ) I × J?
Or, instead of the continuity of the derivative of b ( y ( x ) ), should I impose that the derivative of b ( y ( x ) ) is limited on J?
Can anyone suggest me the correct conditions to impose in this particular case?
Thanks in adivce

Answer & Explanation

bgu999dq

bgu999dq

Beginner2022-05-20Added 9 answers

Your conditions ensure local existence and uniqueness. However, there is no condition that forces the solution to stay inside the interval J.
As a classical if overused example take y = y 2 which falls in your class. Solutions blow up in finite time, so you get all, solutions leaving J, and solutions not existing on the whole of I.

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