Question

Show that if g = i uvt is nonsingular, then g~l has the form i fluvt . Give a formula for /3.

Answer 1

Thought Samrat Mukhopadhyay"s answer is of course in principle correct, it provides no justification for the stated multiplicities of the eigenvalues. In what follows, I have tried to explain, amongst other things, just how these multiplicities arise.

If

$u = 0 \;\; \text{or} \;\; v = 0 \tag{1}$

we have

$uv^T = 0 \tag{2}$

and

$v^Tu = 0; \tag{3}$

then

$\det(I + uv^T) = \det(I) = 1 = 1 + v^Tu; \tag{4}$

thus we may assume

$u \ne 0 \ne v; \tag{5}$

note that

$(uv^T)u = u(v^Tu) = (v^Tu)u, \tag{6}$

which shows that $v^Tu$ is an eigenvalue of $uv^T$ with eigenvector $u$; thus the multiplicity of $v^Tu$ is at least $1$; we will now attempt to find out just when it is precisely $1$, and also to find the other eigenvalues and their multiplicities.

We consider the map

$v^T(\cdot): \Bbb R^n \to \Bbb R \tag{7}$

given by

$v^T(x) = v^Tx; \tag{8}$

it is manifestly a linear functional on $\Bbb R^n$, that is, $v^T(\cdot) \in \Bbb R^{n*}$, the dual vector space to $\Bbb R^n$. We note that $v \ne 0$ implies

$v^T(v) = v^Tv \ne 0 \tag{9}$

as well; thus $0 \ne v^T(\cdot) \in \Bbb R^{n*}$; as such, $\ker v^T(\cdot) \subset \Bbb R^n$ is a subspace with

$\dim (\ker v^T(\cdot)) = n - 1. \tag{10}$

Now suppose that

$v^T(u) = v^Tu \ne 0; \tag{11}$

then

$u \notin \ker v^T(\cdot), \tag{12}$

and since

$uv^T(x) = 0 = 0x \tag{13}$

for $0 \ne x \in \ker v^T(\cdot)$, it follows from (6), (10) and (12) that $uv^T$ has a one dimensional-eigenspace $\operatorname{span} \langle u \rangle$ associated with $v^Tu$, and an $n - 1$ dimensional eigenspace $\ker{v^T}$ associated with eigenvalue $0$; thus the multiplicity of $v^Tu$ is $1$ and that of $0$ is $n - 1$. If, on the other hand,

$v^T(u) = v^Tu = 0, \tag{14}$

that is,

$u \in \ker(v^T(\cdot)), \tag{15}$

then we further note that

$uv^T(v) = u(v^Tv) = (v^Tv)u \in \ker v^T(\cdot), \tag{16}$

whence

$(uv^T)^2(v) = (uv^T)(uv^T)(v) = uv^T((v^Tv)(u)) = (v^Tv) u(v^Tu) = 0. \tag{17}$

Now by virtue of the facts that $v \notin \ker(v^T)$ (cf. (9)) and $\dim (\ker v^T) = n - 1$, we see that

$\Bbb R^n = \ker v^T + \operatorname{span}(\langle v \rangle); \tag{18}$

clearly, $(uv^T)^2 = 0$ on $\ker v^T(\cdot)$, and by what we have just seen, $(uv^T)^2 = 0$ on $\operatorname{span}(\langle v \rangle)$; thus

$(uv^T)^2 = 0 \tag{19}$

when (14) applies. But (19) implies that every eigenvalue of $uv^T$ must vanish; in fact, from (6), $u$, and hence $(v^Tv)u$, are eigenvectors of $uv^T$ with eigenvalue $0$; whence by (16), $v$ is a generalized eigenvector of $uv^T$ for eigenvalue $0$. In any event, the above considerations show that $0$ is an eigenvalue of $uv^T$ with multiplicity $n$ provided (14) binds.

We bring all these observations together to reach the desired conclusion. The case (1) is first and most easily dispensed with, as has been done in (4). The case (5) breaks into the two sub-cases (11) and (14); in (11), $uv^T$ has eigenvalues $v^Tu$ of multiplicity one and $0$ of multiplicity $n - 1$; thus $I + uv^T$ has eigenvalues $1 + v^Tu$ of multiplicity one and $1$ of multiplicity $n - 1$; using the fact that the determinant of a matrix is the product of its eigenvalues, we find

$\det(I + uv^T) = 1^{n - 1} (1 + v^Tu) = 1 + v^Tu; \tag{20}$

as for case (14), $uv^T$ has only eigenvalue $0$ of multiplicity $n$; thus $1 + uv^T$ has only eigevalue $1$ of multiplicity $n$; then

$\det(I + uv^T) = 1^n = 1 = 1 + v^Tu, \tag{21}$

by (14).

Thus we see that, in all cases,

$\det(I + uv^T) = 1 + v^Tu, \tag{22}$

as was to be shown. QED.