Evaluating the operator norms of matrices
New blog post on Freedom Math Dance
Let E and F be normed vector spaces, over the real or complex numbers, and let u :E→F be a linear map. The continuity of uu is proved to be equivalent to the existence of a real number cc such that ∥u(x)∥≤c∥x∥ for every x∈E, and the least such real number is called the operator norm of uu; we denote it by ∥u∥. It defines a norm on the linear space L(E;F) of continuous linear maps from E to F and as such is quite important. When E=F, it is also related to the spectrum of uu and is implicitly at the heart of criteria for the Gershgorin criterion for localization of eigenvalues.
However, even in the simplest cases of matrices, its explicit computation is not trivial at all, and as we'll see even less trivial than what is told in algebra classes, as I learned by browsing Wikipedia when I wanted to prepare a class on the topic.
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https://freedommathdance.blogspot.com/2024/04/evaluating-operator-norms-of-matrices.html
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