function [W,Z]=julia2(X,Y,a,n_iter)
Z=X+i*Y;  W=zeros(size(X));
for k=1:n_iter, 
   Z=(2*Z.^3+1)./(3*Z.^2); 
   i0=find(abs(Z)>a); W(i0)=k; Z(i0)=NaN;
end
i0=find(W==0); W(i0)=NaN;