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function [S]=tf2des(G,flag)
//[S]=tf2des(G)
// Transfer function to descriptor form: S=list('d',A,B,C,D,E)
// E*dx=A*x+B*u
// y=C*x+D*u
// with flag="withD" a maximal rank D matrix is returned
//!
// Copyright INRIA
[LHS,RHS]=argn(0);
if RHS==1 then flag=[];end
if RHS==2&flag<>"withD" then warning("tf2des: unknown flag");end
Num=G(2);Den=G(3);
%s=poly(0,varn(Den));
[n,m]=size(Num);
pol=zeros(n,m);pro=pol;
// Pro = strictly proper part of G
// Pol = polynomial part of G.
for l=1:n,
for k=1:m,
denlk=Den(l,k);
[r,q]=pdiv(Num(l,k)+0*%s,denlk+0*%s);
pol(l,k)=q;
pro(l,k)=r/denlk;
end;
end;
sp=tf2ss(pro);
D=zeros(Num);
if flag=="withD" then
D=coeff(pol,0);pol=pol-D;
end;
spol=tf2ss(horner(pol,1/%s)/%s);
[n1,n1]=size(sp(2));
[n2,n2]=size(spol(2));
A=[sp(2),0*ones(n1,n2);
0*ones(n2,n1),eye(n2,n2)];
E=[eye(n1,n1),0*ones(n1,n2);
0*ones(n2,n1),spol(2)];
B=[sp(3);
spol(3)];
C=[sp(4),-spol(4)];
S=tlist(['des','A','B','C','D','E'],A,B,C,D,E)
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