Friday, July 15, 2022

Left Matrix Inverses in R

The following question popped up on OR Stack Exchange: given an $m\times n$ matrix $A$ with $m > n,$ how can one find all left inverses of $A$ in R? The author mentions that dimensions in their case would be around 200x10.

A left inverse is a matrix $B\in \mathbb{R}^{n\times m}$ such that $B A = I.$ In order for a left inverse to exist, $A$ must have full column rank. If it does not, $Ax = 0$ for some nonzero $x\in \mathbb{R}^n,$ in which case $$x = Ix = (BA)x = B(Ax) = B\cdot 0 = 0,$$ a contradiction.

Henceforth we assume that $A$ has full column rank, in which case the left null space $\mathcal{N} \subseteq \mathbb{R}^m$ will have dimension $m-n.$ Now suppose that $C\in \mathbb{R}^{n\times m}$ has the property that every row of $C$ belongs to $\mathcal{N}.$ Then $CA = 0$ and so $(B+C)A = BA+0 = I,$ making $B+C$ another left inverse of $A.$ That means $A$ has an uncountably infinite number of left inverses. Conversely, if $DA=I$ then the rows of $D-B$ belong to $\mathcal{N}.$ So the set of left inverses of $A$ can be fully characterized by any individual left inverse and a basis for the left null space of $A.$

Getting this information in R is remarkably easy. There are multiple ways to compute a left inverse, but if you have the pracma library loaded, then the function pracma::pinv() can be used to compute the Moore-Penrose pseudoinverse, which is a left inverse of $A.$ To get a basis for the left null space of $A,$ we apply the function pracma::nullspace() to $A^T,$ which computes a basis for the right null space of the transpose of $A,$ and then transpose the results.

I have a small R notebook that demonstrates this on a random 200x10 matrix.

No comments:

Post a Comment

Due to intermittent spamming, comments are being moderated. If this is your first time commenting on the blog, please read the Ground Rules for Comments. In particular, if you want to ask an operations research-related question not relevant to this post, consider asking it on Operations Research Stack Exchange.