## Does it really take 10¹⁴¹ years to compute the determinant of a 100×100 matrix?

Well, it depends on how you compute it. If you compute it by using the definition of determinant directly, in fact it can take more than 10¹⁴¹ years, as we’ll see.${}^\dagger$ Let’s recall the definition first. The determinant of a matrix $\mathbf A$ is $$\text{det}(\mathbf A) = \sum\limits_p \sigma(p) a_{1p_1} a_{2p_2} \dots a_{np_n},$$ where the sum is taken over the $n!$ permutations $p=(p_1,p_2,\dots,p_n)$ of the numbers $1,2,\dots,n$. The total number of multiplications in this definition are $n! (n-1)$. Based on my…

## Reflectors should be your second nature

Reflectors are a class of matrices that are not introduced in all linear algebra textbooks. However, the book of Carl D. Meyer uses this class of matrices heavily for various fundamental results. Indeed, the book uses reflectors for theoretical reasons, such as proving the existence of fundamental transformations like SVD, QR, Triangulation, or Hessenberg decompositions (more to come below), as well as applications, such as the implementation of the QR algorithm via the Householder transformation or solution of large-scale linear systems…

## What is the point of Cauchy-Schwarz, Minkowski and Hölder inequalities?

These three inequalities often tend to appear as a package in many textbooks about real analysis, signal processing or linear algebra. It is good to know the main reason that we see this package all the time, and to separate the role of each of these three fundamental inequalities. The overall reason is that, these inequalities are the key to generalize the facts about vectors in 2D/3D spaces and the Euclidean norm to higher dimensional spaces and (non-Euclidean) $p$-norms. Each of…