Why my fdtd program dont work properly?

Hi, my fdtd program doesnt work properly. It is 4th order in space and 2nd order in time.

In 2th order in space my programworks correct.

L=100;
i=zeros(1,L);
v=zeros(1,L;

dz=0.01;
c=0.1;
l=250;

dt=dz*sqrt(l*c);

for n=1:2000
   t=n*dt;

      for k=1:L-1   
          if k==1
             v(k)=v(k)-dt/(c*dz)*(i(k+1)-i(k));
          elseif k==L-1
              v(k)=v(k)-dt/(c*dz)*(i(k+1)-i(k));

          else
              v(k)=v(k)-(((6/7)*dt)/(c*24*dz))*(i(k-1)-(27*i(k))+(27*i(k+1))-i(k+2));
          end
      end

      v(1)=sin(2*pi*t);

      v(st)=0;          

      for k=2:L
          if k==2
              i(k)=i(k)-dt/(l*dz)*(v(k)-v(k-1));
          elseif k==st
              i(k)=i(k)-dt/(l*dz)*(v(k)-v(k-1));
          else
              i(k)=i(k)-(((6/7)*dt)/(l*24*dz))*(v(k-2)-(27*v(k-1))+(27*v(k))-v(k+1));
          end
      end

      plot(v);
      axis([1 L -3 3]);

      frame = getframe;

end

Anyone knows whats wrong?

Anyone have a 4th order in space fdtd code? I searched and I didnt find anything.

Look at this url to see the results (in the red circle u can see the error): http://www.edaboard.com/attachments/95550d1377973325-.jpg

[Walter Roberson]

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