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function F=stabil(A,B,alfa)
//
//returns F such that A+B*F is stable if (A,B) is stabilizable.
//Assignable poles are set to alfa(1),alfa(2),...
//If (A,B) is not stabilizable a warning is displayed
//and assignable poles are set to alfa(1),alfa(2),...
// Default value for alfa is -1.
//
//K=stabil(Sys,alfa,Beta) returns K, a compensentor for Sys
//such that (A,B)-controllable eigenvalues are set to
//alfa and (C,A)-observable eigenvalues are set to Beta.
// All assignable closed loop poles (which are given by the
//eigenvalues of Aclosed=h_cl(Sys,K) are set to alfa(i)'s
//and Beta(j)'s.
//
//Example:
// Sys=ssrand(2,2,5,list('st',2,3,3));
// A=Sys(2);B=Sys(3); F=stabil(A,B);
// spec(A)
//2 controllable modes 2 unstable uncontrollable modes
// and one stable uncontrollable mode
//spec(A+B*F)
//the two controllable modes are set to -1.
//
// Copyright INRIA
[LHS,RHS]=argn(0)
if typeof(A)~='state-space' then
[ns,nc,U,sl]=st_ility(syslin('c',A,B,[]));
[nx,nx]=size(A);[nn,nu]=size(B);
if ns<nx then
warning('system not stabilizable (or detectable)=>stabilizing the stabilizable part');
end
if RHS==2 then
alfa=-ones(1,nx);
end
if prod(size(alfa))==1 then
alfa=alfa*ones(1,nx);
end
An=U'*A*U;Bn=U'*B;
k=size(alfa,'*');
if k < nc then
alfa=[alfa,-ones(1,nc-k)];
end
F=-ppol(An(1:nc,1:nc),Bn(1:nc,:),alfa(1:nc));
F=[F,zeros(nu,nx-nc)]*U';
else
//F=stabil(Sys,alfa,Beta);
Sys=A;[A1,B1,C1,D1]=abcd(Sys);
if RHS==1 then al=-1;be=-1;end
if RHS==2 then al=B;be=-1;end
if RHS==3 then al=B;be=alfa;end
F=stabil(A1,B1,al);
G=stabil(A1',C1',be);G=G';
F=obscont(Sys,F,G);
end
endfunction
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