clear all; nx=50; ny=30; hx=pi/nx; x=linspace(0,pi,nx+1); y=linspace(0,pi,ny+1); x_plus_h=x+hx.*ones(1,nx+1); x_minus_h=x-hx.*ones(1,nx+1); for i=1:nx+1 for j=1:ny+1 f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2); end; end; [xx,yy]=meshgrid(x,y); surf(xx,yy,f_xx); title('partial derivative w.r.t.xx') , and the other new document,f9, function l=f9(x,y) l=exp(sin(x-y)); and then the system said have error, ??? Input argument "x" is undefined. Error in ==> f9 at 2 l=exp(sin(x-y)); Error in ==> partial_derivxx at 11 f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2); where should i corrected to make it work? thanks anyone can help