In the mathematical formula, a pair of double vertical lines represent:

If two stand together ||, the "or" in the logical OR operator.

There are unknowns in the two vertical lines, indicating normality.

X and y are vectors, and sometimes double vertical lines are used to distinguish them from the absolute values of numbers. ||| X-Y ||| is the root sign of the sum of squares of components after vector division.

The general double vertical line refers to the measurement between the elements x and y of a metric space.

Specifically, the earliest metric spaces are real number sets, N-dimensional Euclidean spaces and so on.

Extended data:

Different types of specifications:

1, 1- norm: ║ A ║ 1 = Max {∑| AI 1 |, ∑| AI 2 |, ..., ∑| ain |} (column norm, sum of absolute values of elements in each column.

2,2-norm: ║ a ║ 2 = (max {λi (A'A)}) 1/2 (spectral norm, that is, the square root of λIλm, the largest eigenvalue of A' a, where A' is the transposed matrix of A).

3.∞- norm: ║ A ║∞ = Max {∑|a 1j|, ∑| A2J |, ..., ∑| ANN |} (row norm, the maximum sum of absolute values of elements in each row) (where ∑| a1)

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