num=1; den=[1,0,0]; G=tf(num,den);
nW1=[0.5,2]; dW1=[1,1];
[a,b,c,d]=tf2ss(num,den); S=mksys(a,b,c,d);
TSS_=augtf(S,[nW1; dW1],[],[1,0,0; 0,0,100]);
C=hinf(TSS_);

p1=-0.1; a_shift=a-p1*eye(size(a));
S_shift=mksys(a_shift,b,c,d);
TSS_shift=augtf(S_shift,[nW1; dW1],[],[1,0,0; 0,0,100]);
ss_F_shift=hinf(TSS_shift);

[af_shift,bf,cf,df]=branch(ss_F_shift); 
af=af_shift+p1*eye(size(af_shift)); 
Gc=zpk(ss(af,bf,cf,df))

G_o=G*Gc; G_c=feedback(G_o,1);
t=0:.4:40; step(G_c,t), 
figure, ngrid('new'), nichols(G_o), axis([-360,0,-40,40])

epsilon=1e-4; den0=conv([1,epsilon],[1,epsilon]);
[a,b,c,d]=tf2ss(num,den0); S=mksys(a,b,c,d);
TSS_pert=augtf(S,[nW1; dW1],[],[1,0,0; 0,0,100]);
ss_F2=hinf(TSS_pert);

[af,bf,cf,df]=branch(ss_F2); 
Gc=zpk(ss(af,bf,cf,df))

G_o=G*Gc; G_c=feedback(G_o,1);
step(G_c), 
figure, ngrid('new'), nichols(G_o), axis([-360,0,-40,40])