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The concept of unit circle (the set of all vectors of norm 1) is different in different norms: for the 1-norm, the unit circle is a square oriented as a diamond; for the 2-norm (Euclidean norm), it is the well-known unit circle; while for the infinity norm, it is an axis-aligned square.
The case p = 2 yields the Frobenius norm, introduced before. The case p = ∞ yields the spectral norm, which is the operator norm induced by the vector 2-norm (see above). Finally, p = 1 yields the nuclear norm (also known as the trace norm, or the Ky Fan 'n'-norm [7]), defined as:
The Frobenius norm defined by ‖ ‖ = = = | | = = = {,} is self-dual, i.e., its dual norm is ‖ ‖ ′ = ‖ ‖.. The spectral norm, a special case of the induced norm when =, is defined by the maximum singular values of a matrix, that is, ‖ ‖ = (), has the nuclear norm as its dual norm, which is defined by ‖ ‖ ′ = (), for any matrix where () denote the singular values ...
By Dvoretzky's theorem, every finite-dimensional normed vector space has a high-dimensional subspace on which the norm is approximately Euclidean; the Euclidean norm is the only norm with this property. [24] It can be extended to infinite-dimensional vector spaces as the L 2 norm or L 2 distance. [25]
An isometry between two normed vector spaces is a linear map which preserves the norm (meaning ‖ ‖ = ‖ ‖ for all vectors ). Isometries are always continuous and injective . A surjective isometry between the normed vector spaces V {\displaystyle V} and W {\displaystyle W} is called an isometric isomorphism , and V {\displaystyle V} and W ...
For a real number, the -norm or -norm of is defined by ‖ ‖ = (| | + | | + + | |) /. The absolute value bars can be dropped when p {\displaystyle p} is a rational number with an even numerator in its reduced form, and x {\displaystyle x} is drawn from the set of real numbers, or one of its subsets.
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The perimeter of the square is the set of points in ℝ 2 where the sup norm equals a fixed positive constant. For example, points (2, 0), (2, 1), and (2, 2) lie along the perimeter of a square and belong to the set of vectors whose sup norm is 2.