C..3 逆反復法

% eigen2.m --- inverse iteration method
format long
a=[1 1 1;1 2 2;1 2 3]
eigen=eig(a)
x=ones(3,1);
maxiter=30;
r=zeros(maxiter,1);
[L u p]=lu(a);
shift=1;
for k=1:maxiter
    y=u\(L\(p*x));
    x=y/norm(y);
    r(k)=x'*(a*x);
end
r
error=r-min(eigen)
clf
semilogy(1:maxiter,abs(error))