# An Allen-Cahn equation with continuation

Nick Trefethen, November 2010

(Chebfun example ode/AllenCahn.m)

The Allen-Cahn equation is a reaction-diffusion that arises in material science: u_tt = Eps*u"+u-u^3=, where Eps is a small parameter. Here as an ODE boundary-value problem we shall consider a steady-state version of this problem on the interval [0,10] with a sinusouidal forcing term:

Eps*u" + u - u^3 = sin(x), u(0) = 1, u(10) = -1.

If we try a very small value of Eps without a well-chosen initial guess, Chebfun will not converge. Instead let's begin by solving the problem with the rather large initial guess Eps = 1:

Eps = 1; dom = [0,10]; x = chebfun('x',dom); f = sin(x); cheboppref('plotting',0.01) N = chebop(@(u) Eps*diff(u,2)+u-u.^3,dom,1,-1); tic, u = N\f; t = toc; LW = 'linewidth'; lw = 2; FS = 'fontsize'; fs = 14; close, plot(u,LW,lw) s = 'Eps = %5.1e length(u) = %d time = %3.1f secs'; title(sprintf(s,Eps,length(u),t),FS,fs)

We now progressively reduce Eps to get sharper and sharper solutions. We use a simple continuation method, in which the initial guess for each iteration is the previous solution.

Epsvec = [.5 .2 .1 .03 .009 .003]; for j = 1:length(Epsvec) close all Eps = Epsvec(j); N = chebop(@(u) Eps*diff(u,2)+u-u.^3,dom,1,-1); N.guess = u; tic, u = N\f; t = toc; close, plot(u,LW,lw) title(sprintf(s,Eps,length(u),t),FS,fs), snapnow end