function [bz,az]=zmapping(bZ,aZ,Nz,Dz)
bzord=(length(bZ)-1)*(length(Nz)-1);
azord=(length(aZ)-1)*(length(Dz)-1);
bz=zeros(1,bzord+1);
for k=0:bzord
pln=[1];
   for l=0:k-1
       pln=conv(pln,Nz);
end
pld=[1];
for l=0:bzord-k-1
pld=conv(pld,Dz);
end
bz=bz+bZ(k+1)*conv(pln,pld);
end
az=zeros(1,azord+1);
for k=0:azord
pln=[1];
for l=0:k-1
pln=conv(pln,Nz);
end
pld=[1];
for l=0:azord-k-1
pld=conv(pld,Dz);
end
az=az+aZ(k+1)*conv(pln,pld);
end
az1=az(1);az=az/az1;bz=bz\az1;