Why are vectors written like this: ||w||

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Discussion Overview

The discussion revolves around the notation used for representing the norm of vectors, specifically the use of double vertical lines (||w||) compared to single vertical lines (|w|). Participants explore the implications, origins, and conventions of this notation within the context of mathematical representation of vector magnitudes.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants note that the symbol || \cdot || is commonly used to represent the norm function on vectors.
  • Others suggest that there is no profound logic behind the choice of double lines, viewing it as a chosen symbol.
  • It is mentioned that single lines are also used for absolute values and determinants, and that the notation may have originated from these uses.
  • One participant argues that while ||\text{whatever}|| is a well-agreed upon symbol for the Euclidean norm, there are infinitely many ways to define the length of a vector, all of which are norms.
  • Another participant emphasizes that the notation ||w|| specifically denotes the magnitude of a vector, likening it to the absolute value of a number.

Areas of Agreement / Disagreement

Participants express differing views on the significance and origins of the notation, indicating that there is no consensus on whether the double lines have a deeper meaning or are simply a matter of convention.

Contextual Notes

Some limitations include the potential confusion between the notation for vector magnitudes and absolute values, as well as the varying definitions of norms that may not be universally agreed upon.

Femme_physics
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Is there a point to the double lines?
 
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The symbol [itex]|| \cdot ||[/itex] (where the argument is placed where the dot is) is commonly used to represent the norm function on vectors.
 
There's no profound logic behind it then, right? Just the chosen symbol eh?
 
It's often written with single lines: |w|. Single lines also denote the absolute value of a real number, and the absolute value (also called the norm, magnitude or modulus) of a complex number. Maybe the notation started life there and spread to vectors.
 
Rasalhague said:
It's often written with single lines: |w|.

That symbol is used for absolute value and the determinant of a matrix. While it is sometimes used to denote a norm as well, but doing so is generally considered to be bad form.

Back to the OP, then: [itex]||\text{whatever}||[/itex] just a symbol with a (somewhat) well-agreed upon meaning. The typical meaning is the Euclidean norm. There are however many ways (an infinite number of ways) to define the length of a vector. All are norms.
 
First, vectors aren't written like that! The "magnitude" of a vector is written like that. And the symbol is intended to look like "absolute value" because the magnitude of a vector is similar to and is used like the absolute value of a number.
 

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