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Does $x(t) = \exp\left(\frac{t}{t-1}\right)\cdot\theta(1-t)$ solve $\dot{x}\cdot(1-t)^2+x=0,\,\,x(0)=1$ with $\theta(t)$ the standard unitary step/Heaviside function

$$\theta(t) := \begin{cases} 0 & x\leq 0 \\ 1 & x > 0. \end{cases}$$

I know beforehand that $y(t) =\exp\left(\frac{t}{t-1}\right)$ solves $\dot{y}\cdot(1-t)^2+y=0,\,\,y(0)=1$, and I want to know if the same differential equation sustained as a finite-duration solution the cropped version $x(t)$.

If not, please explain which differential equation will be solved by $x(t)$ on the whole real line.

My attempts

Through Wolfram-Alpha I take the derivative of $x(t)$ and it looks it match the differential equation, except at one zero-measure point where a Dirac's Delta function rises, but because this term has the form $f(1-t)\delta(1-t)$ when multiplied by $(1-t)^2$ on the diff. eq., I believe lies under the property $x\cdot\delta(x) = 0$ so indeed $x(t)$ is solving the differential equation.

Since this system is not an autonoums ODE, it is different from the problems analyzed by V. T. Haimo: Finite Time Differential Equations and Finite Time Controllers, so I don't know how to apply here was is shown on the papers. Maybe someone with a better background on differential equations could explain how to figure out if indeed the differential equation stand a finite duration solution (these are the only related papers I have found so far).

But given I already know that $y(t)$ solves the irrestricted diff. eq., which also has the trivial solution $y(t)=0$, I believe that since $x(t)$ achieves zero "smoothly" there are no reasons to discard it as a solution... which is quite interestimg thinking about Uniqueness of solutions which should stand every Lipschitz ODE, so I guess the differential equation is not a Lipschitz ODE please confirm this explaining why.

Joako
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    I have recommended some books to look at to help you clarify all the very similar questions you are asking about dynamical systems. To this list, I am adding the book by Khalil. "Nonlinear systems". – KBS Mar 23 '22 at 18:00
  • Thanks for commenting. I have found it and I starting to review it, but it is not an easy read... so far, I search for finite duration solutions and I didn't find anything related (which is the topic I am interested in - see tag [tag:finite-duration])... Are there other different names to refer to these "finite duration solutions" I am not aware of? since it looks like they are wide known, but I can´t find related info anywhere I look (Wikipedia, Google, MSE, among others) – Joako Mar 24 '22 at 01:25
  • Start first with understanding dynamical systems in general. Then, look at systems having solutions that converge in finite time. Your posts show that you are very familiar with dynamical systems theory. So, this should be fixed first. – KBS Mar 24 '22 at 01:48
  • @KBS Could you visit this question please? I would like to know if in the recommended book I will find the tools to solve this other question. – Joako Mar 24 '22 at 03:30
  • I am sorry but I have replied to many of your questions, and you keep asking the same type of questions without really trying to understand dynamical systems better. I am not going to repeat the same things again and again. – KBS Mar 24 '22 at 07:56

1 Answers1

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Your equation is not an ODE at $t=1$, so as ODE it only has solutions for $t<1$ and for $t>1$.

However, as all these solutions have limit $0$ at $t=1$, you can combine any solution left of $t=1$ with any solution right of $t=1$ to get a continuous function that solves the given equation.

But no such combination is "The" solution of this equation. Your case is also such a combination, with the zero solution for $t>1$.

Lutz Lehmann
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  • Thanks you very much for answering. I didn't thought before that a scalar differential equation without delays could not been an ODE: There is a name/classification for this kind of differential equations?... Also, since it doesn't stand Uniqueness, It is not a Lipschitz differential equation, right? – Joako Mar 24 '22 at 01:16
  • I am trying to use your observation so solve this other question, hope you can give it a chance. – Joako Mar 29 '22 at 04:13
  • In this another question I am asking how are called these kind of differential equations that stops of being ODE on one point... hope you can answer it too. Beforehand, thanks you very much. – Joako May 23 '22 at 22:49