47 lines
1.2 KiB
Matlab
47 lines
1.2 KiB
Matlab
% solves x-a*x*a'=b for b (and then x) symmetrical
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function x=lyapunov_symm(a,b)
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n = size(b,1);
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if n == 1
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x=b/(1-a*a);
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return
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end
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x=zeros(n,n);
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[u,t]=schur(a);
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b=u'*b*u;
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for i=n:-1:2
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if t(i,i-1) == 0
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if i == n
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c = zeros(n,1);
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else
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c = t(1:i,:)*(x(:,i+1:end)*t(i,i+1:end)')+...
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t(i,i)*t(1:i,i+1:end)*x(i+1:end,i);
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end
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x(1:i,i)=(eye(i)-t(1:i,1:i)*t(i,i))\(b(1:i,i)+c);
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x(i,1:i-1)=x(1:i-1,i)';
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else
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if i == n
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c = zeros(n,1);
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c1 = zeros(n,1);
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else
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c = t(1:i,:)*(x(:,i+1:end)*t(i,i+1:end)')+...
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t(i,i)*t(1:i,i+1:end)*x(i+1:end,i)+...
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t(i,i-1)*t(1:i,i+1:end)*x(i+1:end,i-1);
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c1 = t(1:i,:)*(x(:,i+1:end)*t(i-1,i+1:end)')+...
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t(i-1,i-1)*t(1:i,i+1:end)*x(i+1:end,i-1)+...
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t(i-1,i)*t(1:i,i+1:end)*x(i+1:end,i);
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end
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z = [eye(i)-t(1:i,1:i)*t(i,i) -t(1:i,1:i)*t(i,i-1);...
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-t(1:i,1:i)*t(i-1,i) eye(i)-t(1:i,1:i)*t(i-1,i-1)]...
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\[b(1:i,i)+c;b(1:i,i-1)+c1];
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x(1:i,i) = z(1:i);
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x(1:i,i-1) = z(i+1:end);
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x(i,1:i-1)=x(1:i-1,i)';
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x(i-1,1:i-2)=x(1:i-2,i-1)';
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i = i - 1;
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end
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end
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if i == 2
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c = t(1,:)*(x(:,2:end)*t(1,2:end)')+t(1,1)*t(1,2:end)*x(2:end,1);
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x(1,1)=(b(1,1)+c)/(1-t(1,1)*t(1,1));
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end
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x=u*x*u'; |