clear all; close all;
% Grid:
L = 20;
N = 2^8;
kappa = 1;
x2 = linspace(-L/2,L/2,N+1);
x = x2(1:N);
dx = x(2) - x(1);
%
e1 = ones(N,1); e2 = -2*e1;
A = 1/dx^2*spdiags([e1 e2 e1],[-1 0 1],N,N);
A(1,N) = 1; A(N,1) = 1;
tspan = [0:1:20];
%
u0 = sin(x);
[t,u1] = ode45('heatrhs',tspan,u0,[],A,dx);
figure(1), 
waterfall(x,t,u1);