format compact 
set(gcf,'Name','Campo eléctrico','NumberTitle','off','Position',[10,350,700,300],'Color',[1 1 1]);
clear
res
x = 0:1/(mx+1):1; 
y = 0:1/(mx+1):1;
[X Y] = meshgrid(x,y); 
k=(mx+2)*(mx+2);
for j=1:mx+2
   for i=1:mx+2
         p(i,j)=sol(k);
         k=k-1;
      end
end
surf(X,Y,p)
shading interp;
colorbar