function dx=onda(x,t)
    n=length(x)/2;
    u=[0;x(1:n)];
    v=[0;x(n+1:2*n)];
    du=zeros(u);
    dv=zeros(v);
    for i=2:n
        du(i)=v(i);
        dv(i)=n^2*(u(i-1)-2*u(i)+u(i+1))
    end
    du(n+1)=v(n+1);
    dv(n+1)=n^2*(2*u(n)-2*u(n+1));
    dx=[du(2:n+1);dv(2:n+1)];    
endfunction
