# Fixed point equation to solve Burgers' equation IVP

Using the equation $$u \equiv u ( x , t ) = u _ { 0 } ( x - t u ( x , t ) )$$ to compute $$u \left( T , x _ { j } \right)$$ for the Burgers equation, where the Burgers equation is $$u _ { t } + \left( \frac { 1 } { 2 } u ^ { 2 } \right) _ { x } = 0$$ with $$u ( 0 , x ) = u _ { 0 } ( x )$$ and initial conditions $$u _ { 0 } ( x ) = \sin ( \pi x )$$

I think we have to use the fixed point equation $$y = y + c \left( y - u _ { 0 } \left( x _ { j } - T y \right) \right)$$ for a suitable $$c$$.

I don't know how to go about this and I have to implement this in Matlab as well. How to proceed? Thanks.

## 1 Answer

Indeed, the solution deduced from the method of characteristics satisfies $$u = u_0(x - ut)$$, to be solved for each point $$(x,t)$$ of interests. One approach is fixed-point iterations:

x = linspace(0,1,50);
u = x;
t = 0.2;
for i=1:length(x)
fun = @(u) sin(pi*(x(i)-u*t));
u0 = 0;
err = abs(fun(u0)-u0);
n = 0;
while (err>1e-10)&&(n<100)
u0 = fun(u0);
err = abs(fun(u0)-u0);
n = n+1;
end
u(i) = u0;
end
plot(x,u);
xlabel('x');
ylabel('u');
title(strcat('t=',num2str(t)));


with the following output: A native Matlab method based on a root-finding algorithm is provided here.