### Author Topic: 9.2 problem set Q21?  (Read 1554 times)

#### Xin Wen

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##### 9.2 problem set Q21?
« on: November 24, 2018, 04:57:45 PM »
Can any one give me the idea about 9.2 problem set Q21?

Given that $x=\phi(t)$, $y=\psi(t)$ is a solution of the autonomous system

\frac{dx}{dt} = F(x,y), \frac{dy}{dt}=G(x,y)

for $\alpha < t < \beta$, show that $x=\Phi(t)=\phi(t-s), y=\Psi(t)=\psi(t-s)$ is a solution for $\alpha + s < t < \beta +s$ for any real number s.

#### Tianfangtong Zhang

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##### Re: 9.2 problem set Q21?
« Reply #1 on: November 24, 2018, 05:13:09 PM »
Since $x=\phi(t)$, $y=\psi(t)$ is a solution of the autonomous system

\frac{dx}{dt} = F(x,y), \frac{dy}{dt}=G(x,y)

for $\alpha < t < \beta$.

Then functions  $\Phi(t)=\phi(t-s)$ and $\Psi(t)=\psi(t-s)$ indicates same function $x=\phi(t)$ and $y=\psi(t)$ respectively (except time shift by a real number s)
Base on the definition of Autonomous system (on textbook page 509) "an autonomous system is one whose configuration, including physical parameters and external forces or effects, is independent of time."

Thus, $x = \Phi(t)=\phi(t-s)$, $y=\Psi(t)=\psi(t-s)$ is a solution for $\alpha +s < t < \beta$ +s for any real number s.