Hi. First off, sorry for the not so descriptive title. If one of you finds a better tilte I will edit it.(adsbygoogle = window.adsbygoogle || []).push({});

We have the equation

\begin{equation}

\partial_{xx}\phi = -\phi + \phi^{3} + \epsilon \left(1- \phi^{2}\right)

\end{equation}

We will look for solutions satisfying

\begin{equation}

\phi(\pm \infty) = \pm 1 \qquad \partial_{x}\phi\big|_{x=\pm \infty} = 0

\end{equation}

Integrating once we find

\begin{equation}

\left(\partial_{x}\phi\right)^{2} = -\phi^{2} + \frac{\phi^{4}}{2} + 2\epsilon \left(\phi - \frac{\phi^{3}}{3}\right) + K

\end{equation}

Using the boundary conditions we find that no solution exists for [itex]\epsilon \neq 0[/itex]. For [itex]\epsilon=0[/itex] we find( assuming [itex]\phi(0) = 0[/itex])

\begin{equation}

\phi_{0} = \tanh\left( \frac{x}{\sqrt{2}} \right)

\end{equation}

Now lets assume [itex]\epsilon << 1[/itex] and expand [itex]\phi = \sum \epsilon^{j}\phi_{j}[/itex] imposing the boundary conditions

\begin{equation}

\phi_{0}(\pm \infty) = \pm 1 \qquad \phi_{j}(\pm \infty) = 0

\end{equation}

For j>0, and

\begin{equation}

\partial_{x}\phi_{i}\big|_{x=\pm \infty} = 0

\end{equation}

For all i.

Now what will the higher and higher order terms signify? And will this series ever converge towards anything( either stationary or periodic)? I worked out the expansion up to [itex]\phi_{5}[/itex] without getting any wiser( I can write down the solutions if anybody asks but it takes a bit of time.)

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# Convergence of perturbative solutions to a non-linear diff eq

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