Q:

Special Woodbury identity

I wish to understand linear transformations of the type $M = \left(\alpha I_n+ D^TD\right)^{-1}$
where $\alpha \neq 0$ and $D$ is a (full-rank) $k$ by $n$ matrix. ($k<n$)
My understanding is that though $D^TD$ is singular and thus does not have an inverse, $\left(\alpha I_n+ D^TD\right)^{-1}$ for $\alpha \ll 1$ could be a "good" approximation for it. (or am I mistaken?)
Ideally I would like to relate the eigenvectors of $M$ to my original $D$ (probably dependent also on $\alpha$) (see footnote [1])
So my question is: Is it possible to compute the eigenvalues and eigenvectors of this matrix $M$? 
Is there any interesting fact that you know about these types of matrices (thinking them of linear transformations)
I tried to use Woodbury identity to arrive at an expression 
$$M = \left(\alpha I_n+ D^TD\right)^{-1}\\
=\alpha^{-1}I_n - \alpha^{-1}D^T\left(I_k+\alpha^{-1}DD^T\right)^{-1}D\alpha^{-1}\\
=\alpha^{-1}I_n - \alpha^{-1}D^T\left(\alpha I_k+DD^T\right)^{-1}D\\
= \alpha^{-1}I_n - \alpha^{-1}D^T\left(\alpha^{-1}I_k - \alpha^{-1}D\left(\alpha I_n+D^TD\right)^{-1}D^T\right)D\\
= \alpha^{-1}I_n - \alpha^{-2}D^TD + \alpha^{-2}D^TD\left(\alpha I_n+D^TD\right)^{-1}D^TD\\
= \alpha^{-1}I_n - \alpha^{-2}D^TD + \alpha^{-2}D^TDMD^TD$$
and got to an implicit equation for $M$ but could not solve it.
Any help is appreciated either in finding a closed-form solution or geometrical interpretations of the effect of $M$
[1] Some initial experimentation with simple $D$ lead me to believe that the rows of $D$ are eigenvectors for $M$ is this possible?

A:

The eigenvalues of $\alpha I + D^TD$ can be written as $\alpha + \lambda_j$,
where $\lambda_j$ are the eigenvalues of $D^TD$, of which at least $n-k$ are zero. The eigenvectors of $D^TD$ and $\alpha I+ D^TD$ can be computed by the right-singular vectors of $D$.
The matrix $(\alpha I+D^TD)^{-1}D^T$ is an approximation of the inverse $D$ in the following sense:
$\lim_{\alpha \searrow0}(\alpha I+D^TD)^{-1}D^T =D^\dagger$, where $D^\dagger$ is the Moore-Penrose pseudo-inverse of $D$, see Moore-Penrose pseudo-inverse.
The matrix $(\alpha I+D^TD)^{-1}D^T$ also appears if you want to solve a least-square problem $\|Dx -y\|_2\to 0$ by Tikhonov regularization, where for $\alpha>0$ the problem
$$
\min \frac12\|Dx-y\|_2^2 + \frac\alpha2\|x\|_2^2
$$
is solved. Then $x$ solves this problem if and only if
$$
(\alpha I + D^TD)x = D^Ty \quad \Leftrightarrow \quad x = (\alpha I + D^TD)^{-1}D^Ty.
$$

