function u=exact(x,y)
r=sqrt(x.^2+y.^2); th=atan2(y,x);
%u=1+r.*cos(th);
u=2.5+r.*(-0.1*cos(th)+0.5*sin(th))-0.5*r.^2.*cos(2*th)+r.^4.*cos(4*th);
%u=r.*(cos(th)+3*sin(th))-r.^3.*sin(3*th);
