Banach-Mazur distance

In the mathematical study of functional analysis, the Banach–Mazur distance is a way to define a distance on the set Q(n) of n-dimensional normed spaces. With this distance, the set of isometry classes of n-dimensional normed spaces becomes a compact metric space, called the Banach–Mazur compactum.

If X and Y are two finite-dimensional normed spaces with the same dimension, let GL(X,Y) denote the collection of all linear isomorphisms T : X → Y. With ||T|| we denote the operator norm of such a linear map — the maximum factor by which it "lengthens" vectors. The Banach–Mazur distance between X and Y is defined by

We have δ(X, Y) = 0 if and only if the spaces X and Y are isometrically isomorphic. Equipped with the metric δ, the space of isometry classes of n-dimensional normed spaces becomes a compact metric space, called the Banach–Mazur compactum.

Many authors prefer to work with the multiplicative Banach–Mazur distance

for which d(X, Z) ≤ d(X, Y) d(Y, Z) and d(X, X) = 1.

F. John's theorem on the maximal ellipsoid contained in a convex body gives the estimate:

where ℓ_n² denotes Rⁿ with the Euclidean norm (see the article on L^p spaces). From this it follows that d(X, Y) ≤ n for all X, Y ∈ Q(n). However, for the classical spaces, this upper bound for the diameter of Q(n) is far from being approached. For example, the distance between ℓ_n¹ and ℓ_n^∞ is (only) of order n^1/2 (up to a multiplicative constant independent from the dimension n).

A major achievement in the direction of estimating the diameter of Q(n) is due to E. Gluskin, who proved in 1981 that the (multiplicative) diameter of the Banach–Mazur compactum is bounded below by c n, for some universal c > 0.

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