# Help with nonlinear 1st order ODE

1. Jan 29, 2012

### X89codered89X

So I'm supposed to prove that
${x}^{.}(t) = x^{2}+ t^{2}$ with $x(0) = 0$ blows up before $t = 1$.

I'm not sure what method to use to solve I've tried setting up an integral such as $\int^{x(t)}_{x(0)} \frac{dx}{x^{2}+t^{2}} = \int^{t}_{0} dt$ but I didn't think I could do this since 't' is varying over time on the Left Hand side and I'm integrating with respect to x.

The only other clue I have is to use a comparison ODE, which was mentioned in class, in which would use a function, say... $g^{.}(t) =< x^{.}(t)$ which was easier to work with. If I were able to prove that the lesser function exploded before $t= 1$, then logically the greater one explodes. The thing is I don't know what function I would even chose to set this up. Any ideas?

..and Thank youuuuu.

Last edited: Jan 29, 2012
2. Jan 30, 2012

### JJacquelin

Hello !

The ODE is on the Riccati kind. First let x(t) = -y'/y
which is on the Bessel kind
y(t) = sqrt(t)*(c1*J(k,X)+c2*J(-k,X) )
J(k,X) is the Bessel function of order k
k=1/4
X=t²/2
k=1/4

3. Jan 30, 2012

### X89codered89X

Right OK!! can I then substitute that back in somehow to show that it explodes at t -> 1? I don't necessarily need to get it solved. Just know how it behaves.

4. Jan 30, 2012

### JJacquelin

From my results it doesn't "explodes" close to t=1, but at t=2.003147 which is the first root of the BesselJ function of order -1/4.

File size:
26.3 KB
Views:
75