Toronto Math Forum
APM3462015F => APM346Lectures => Web Bonus = Oct => Topic started by: Victor Ivrii on September 26, 2015, 12:46:10 PM

http://www.math.toronto.edu/courses/apm346h1/20159/PDEtextbook/Chapter2/S2.3.P.html#problem2.3.P.8 (http://www.math.toronto.edu/courses/apm346h1/20159/PDEtextbook/Chapter2/S2.3.P.html#problem2.3.P.8) Problem 8

I didn't really make it that far on this problem, but I thought I ought to post as far as I got in case anyone has any ideas.
Plugging $u(x,t)=\phi(xvt)$ into the given PDEs:
\begin{equation}
v^2\phi''\phi''+\phi2\phi^3=0\\
v^2\phi''\phi''\phi+2\phi^3=0
\end{equation}
And so we get
\begin{equation}
(v^21)\phi''=2\phi^3\phi\\
(v^21)\phi''=\phi2\phi^3
\end{equation}
This ODE looks very difficult  any ideas?

We look for solutions such that $u_{tt}u_{xx}=u(12u^2)=0$.
$u_{tt}u_{xx}=0$ implies that $u(x,t) = f(x \pm t)$ for some function $f$.
$u(12u^2)=0$ implies that either $u = 0$ or $u = \pm \frac{1}{\sqrt 2}$.
A kink may be described by
\begin{equation}
u(x,t) = \left\{\begin{array}{21}
&\pm \frac{1}{\sqrt 2} \qquad & x \ge t\\
& 0 & x < t \end{array} \right.
\end{equation}
A soliton may be described by
\begin{equation}
u(x,t) = \left\{\begin{array}{21}
&\pm \frac{1}{\sqrt 2} \qquad & x = t\\
& 0 & x \neq t \end{array} \right.
\end{equation}

We have 2nd order ODE with no explicit $x$. Could be reduced to 1st order by the standard $\phi'=\psi$, $\phi''=\psi'=\frac{d\psi}{d\phi}\phi'$.