# Large-scale generalized eigenvalue problem with low rank LHS matrix

Assume that we have generalized eigenvalue problem:

$B^HB\textbf{x} = \lambda A\textbf{x}$

where $A$ is an nxn Hermitian sparse matrix (n is very large, so we do not have $A^{-1}$ but can solve using iterative methods) and full-rank, and $B$ is a 2xn matrix such that $B^HB$ is also nxn but only rank 2. Thus, we know that this problem can only have 2 non-zero eigenvalues. Is there any simple way for finding the two eigenpairs corresponding to nonzero eigenvalues by taking advantage of the very low rank of $B^HB$? Assume that we have the two eigenvectors of $B$.

If I am only interested in the eigenvector corresponding to the largest eigenvalue, is there a faster way of finding it than using simple power iteration on the transformed standard eigenvalue problem: $A^{-1}B^HB\textbf{x} = \lambda\textbf{x}$?

Thanks!

This answer is essentially a fix of the approach suggested by @WolfgangBangerth, as there is not enough space in the comments.

Starting from $$B^H B x = \lambda A x,$$ if we are interested in eigenpairs corresponding to nonzero eigenvalues, then we must have that $B^H B x$ lies in the range of $A$, and $Ax$ lies in the range of $B^H B$, which is to say that, since $A$ is invertible, $$B^H B x \in \mathrm{Range}(A) = \mathbf{C}^n,$$ and $$Ax \in \mathrm{Range}(B^H B) = \mathrm{span}(B^H).$$ Now, the first constraint is trivially satisfied, but we must ensure that $Ax \in \mathrm{span}(B^H)$, which is equivalent to the constraint $$x \in \mathrm{span}(A^{-1} B^H).$$ Then if the columns of a unitary matrix $Q$ span the columns of $A^{-1}B^{H}$, we have that $$x = Q Q^H x$$ for any eigenvector corresponding to a nonzero eigenvalue.

We are now ready to use the mechanism from Wolfgang's approach:

1. Compute $W := A^{-1} B^H$ through two (preconditioned) Krylov solves
2. Compute $[Q,R]=\mathrm{qr}(W)$
3. Form $K := (B Q)^H (B Q)$ and $M := Q^H (A Q)$
4. Solve the $2 \times 2$ eigenvalue problem $K U = M U \Lambda$
5. Form the interesting global eigenvectors, $Z := Q U$.
• Thanks, Jack! Are you sure you need to do QR? I don't think the two vectors comprising the W matrix need to necessarily be orthonormal? (A quick test shows that it works even if you solve $W^HB^HBW\textbf{y} = \lambda W^HAW\textbf{y}$ ) May 22 '12 at 2:08
• The QR decomposition for an $m \times n$ matrix, $m \ge n$, is $O(mn^2)$. In this case, $n=2$, so the cost is linear and should be dominated by the Krylov solves. I am skeptical of how this would work without a QR decomposition. May 22 '12 at 2:26
• $W=QR$, so substituting: $R^HQ^HB^BQR\textbf{y}=\lambda R^HQ^HAQR\textbf{y}$. Multiply both sides by $R^{-H}$ and substitute $\textbf{x}=R\textbf{y}$. You end up with $Q^HB^BBQ\textbf{x}=\lambda Q^HAQ\textbf{x}$ which has the same eigenvalues as if you just used W instead of Q. May 22 '12 at 2:34
• I can get the eigenvector by just doing: $\textbf{w}=W\textbf{y}$ which implicitly multiplies by R since $W=QR$. Just tried a quick test case and it seems to work, although as you said I think it would be trivial do QR as compared to doing the Krylov solves. May 22 '12 at 2:45
• Ah, good point! I would still rather work with $Q$ though, as the cost of computing it is insignificant, and it will be more numerically stable. May 22 '12 at 2:48

If $B$ is $2\times n$, then the only two non-trivial eigenvectors (i.e. the eigenvectors corresponding to the two non-zero eigenvalues) can be written as linear combinations of the vectors that form the two rows of $B$. Let's call these two vectors $b_1, b_2$ so that $B=\left[\begin{matrix}b_1^T\\b_2^T\end{matrix}\right]$.

Now, let $P \in {\mathbb R}^{2\times n}$ be the projector from ${\mathbb R}^n$ onto the two-dimensional space spanned by $b_1,b_2$. Since we are only interested in vectors in this space, we know that the two non-trivial eigenvectors must satisfy $x = P^TPx$. The eigenvalue problem can then be written as $$B^H B P^T P x = \lambda A P^T P x.$$ Even though this linear system has $n$ rows, it is really only a two-dimensional problem since we can only determine only two components of $x$. The remainder of the linear system is over-determined, but we can select the two independent equations by projecting onto the non-trivial subspace: $$P B^H B P^T P x = \lambda P A P^T P x.$$

In other words, you only have to solve the $2 \times 2$ eigenvalue problem $$P B^H B P^T y = \lambda (P A P^T) y.$$ This is easy to solve since the matrices involved are only $2\times 2$ and the matrix on the right can easily be computed using just two matrix-vector and two vector-vector products.

• I don't think your assumption that $x=P^T Px$ is valid for non-trivial $A$. May 21 '12 at 20:14
• I think that $P$ will need to be modified to span a space including the columns of $B^H$ and $A^{-1} B^H$, which is at most rank 4, and only requires two solves with $A$ to set up. May 21 '12 at 21:37
• Hmmm.. I think you only need the columns of $A^{-1}B^H$ actually, so if you take $P=A^{-1}B^H$ Wolfgang's approach will work. May 22 '12 at 1:14
• @Costis: I think you are right. I have a nice explanation of why which I will post as an answer, as there is not enough space here. May 22 '12 at 1:31
• I feel like the nomenclature is slightly unclear. $P$ cannot be a projector, because $P$ is neither square, nor idempotent. For the sake of clarity, the relevant (orthogonal) projector appears to be $P^{T}P$. Your explanation also seems to implicitly rely on knowing that the eigenvectors of $B^{H}B$ form an orthonormal basis, which is why an orthogonal projector is appropriate. May 22 '12 at 2:05