function dx=alambre(x,t)
    N=length(x)+1;
    dx=1/N;
    u=[100;x;x(N-1)];
    du=zeros(u);
    for i=2:N
        du(i)=(u(i-1)-2*u(i)+u(i+1))/dx^2;
    end
    dx=du(2:N);     
endfunction
