# Eigenvalues of diagonal matrix with two rank-one perturbations

I would like to generalize this question to a diagonal matrix that has two rank-one updates which are both of a special form. In the previous question I asked why most of the eigenvalues of a diagonal $$n\times n$$ matrix D remain the same even if I perturb this matrix with a rank-one update $$bk^T$$, where b and k are n-dimensional vectors: $$D+bk^T$$ The eigenvalues of D are obviously its diagonal entries. Numerical evaluation showed that many of its eigenvalues remain the same even after adding the perturbation. The answer to this question gave a proof using Sylvester's determinant theorem and shows why rank-one updated diagonal matrices with eigenvalues of multiplicity k > 1 will have the exact same eigenvalues with multiplicity k-1. Instead of having a diagonal matrix with one rank-one update, I wondered whether a similar proof can be done for matrices that get two rank-one updates, i.e. $$D + bk^T + uv^T$$ where D is a $$n\times n$$ diagonal matrix, and b, k, u, v are n-dimensional vectors. Instead of arbitrary vectors as in the previous question, I take u and k to be of the form: $$(*,*,0,0,0,\cdots)$$ and v and b to be of the form: $$(0,0,*,*,*,\cdots)$$ The resulting matrix looks something like$$\begin{pmatrix} * & 0 & *&* & * & * & \cdots\\ 0 & * & *&* & * & * & \cdots\\ * & * & *&0 & 0 & 0 & \cdots\\ * & * & 0&* & 0 & 0 & \cdots\\ * & * & 0&0 & * & 0 & \cdots\\ * & * & 0&0 & 0 & * & \cdots\\ \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \ddots \end{pmatrix}$$ where the diagonal elements come from D and the first two rows/columns come from the rank-one updates $$uv^T$$ and $$bk^T$$ respectively.

My numerical calculations show that in this case the eigenvalues of higher multiplicity also remain the same, at least partly. I tried to repeat the above proof from the previous question with two rank-one perturbations but I did not succeed. Can somebody help me out?

Edit: It would also be nice to get an analytical understanding about why numerically the first two eigenvalues of D are affected the most by this perturbation.

• Let's assume that $\lambda$ appears $k$ times on the diagonal of $D$. Then $D - \lambda I$ has an $n - k$-dimensional image and a $k$-dimensional kernel. But then $D - \lambda I + b k^T + u v^T$ has at most an $n - k + 2$-dimensional image so it has at least a $k - 2$-dimensional kernel. This shows that if $\lambda$ is an eigenvalue of $D$ of multiplicity $k \geq 3$ then $\lambda$ is also an eigenvalue of $D + b k^T + u v^T$ with multiplicity $k - 2$. Feb 18 '20 at 0:22

Suppose that we have a matrix, $$A$$, with an eigenvalue $$\lambda$$ of multiplicity $$k$$. Then we have the property that $$(A-\lambda I)v=0$$ where $$v$$ can be any combination of vectors $$v_i$$ for $$i$$ between $$1$$ and $$k$$, each of which also satisfy this equation and that are orthogonal to each other.
If we perform a rank-1 update to $$A$$, and then we now have $$(A+bc^T-\lambda I)(v+w)=0$$ for some $$w$$. Expanding our second equation and substituting in our first equation, we get $$(A+bc^T-\lambda I)w +[c^T v]b=0$$ where $$c^T v$$ is a scalar. If $$c^T v=0$$, then $$w=0$$ and there is no alteration to the eigenvector. Note, however, that $$v = \sum \alpha_i v_i$$, and so $$c^T v = \sum \alpha_i [c^T v_i]$$.
But you have $$k-1$$ degrees of freedom to ensure that $$c^T v=0$$, which means that, after the rank-1 update, eigenvalue $$\lambda$$ has multiplicity at least $$k-1$$.
But this didn't rely on $$A$$ being diagonal. Which means, after a rank-1 update, we can perform another rank-1 update - that is, we can perform a rank-1 update on $$A+bc^T$$. And the result is that the eigenvalue of multiplicity $$k-1$$ becomes an eigenvalue of multiplicity $$k-2$$.