What is the Infinity Norm & Why Use It?

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    Infinity Norm
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Discussion Overview

The discussion revolves around the concept of the infinity norm in mathematics, specifically its definition and the rationale behind using the maximum value of vector components as its measure. Participants explore the theoretical implications and connections to other norms.

Discussion Character

  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the definition of the infinity norm, seeking clarity on why it is defined as the maximum of the absolute values of vector components.
  • Another participant affirms that the infinity norm is positive and satisfies the triangle inequality, but does not directly address the initial question about the maximum definition.
  • A different participant reiterates the inquiry about the use of "max" in the definition and suggests a connection to the limit of other norms as the parameter approaches infinity.
  • Further elaboration is provided on the relationship between the infinity norm and other norms, explaining how taking the limit as p approaches infinity leads to the maximum value.
  • A later reply indicates that the initial question was understood and appreciated, confirming the connection to the concept of limits in the context of norms.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the definition of the infinity norm, with some seeking clarification and others providing explanations. The discussion does not reach a consensus on the initial inquiry, as it remains exploratory.

Contextual Notes

The discussion includes assumptions about the reader's familiarity with norms and mathematical concepts, which may not be universally shared. There are also unresolved aspects regarding the intuitive understanding of the infinity norm's definition.

alecrimi
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Hi
I was wondering about the meaning of the infinity norm
|| x ||_\inf= max\{|x_1|, |x_2|...|x_n| \}

if a norm is a function that assigns a strictly positive length or size to all vectors in a vector space, why do we assign the maximum (or sup) as the value of this norm ?
It must be a very basic or obvious answer because I cannot find in any text.
 
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I'm not sure of exactly what you're asking alerimi.

It is positive (and zero only when the entire vector is identically zero). It's very easy to show that it satisfies the triangle inequality. Exactly what problem do you have with it's use as a norm?
 
why is it defined by "max" ?
 
alecrimi said:
why is it defined by "max" ?

Do you mean in the sense of limit of the sequence one-norm, two-norm etc?

Ok I think that must be what you're asking, what is the connection between infinity in the name and maximum in the definition.

There are a family of norms,

|| x ||_1 = |x_1| + |x_2| + ... |x_n|
|| x ||_2 = \left( |x_1|^2 + |x_2|^2 + ... |x_n|^2 \right)^{\frac{1}{2}}
|| x ||_p = \left( |x_1|^p + |x_2|^p + ... |x_n|^p \right)^{\frac{1}{p}}

Imagine that you take the limit as p goes to infinity. Factor out the largest |x_i| and then the i-th element becomes unity, while the all other elements are (magnitude) less than one. Think about what happens as you take the p-th power of each element now (as p -> infinity). That's an oversimplification, but hopefully it let's you see how it works.
 
Last edited:
yes, it was that. I thought it was something like that. Thanks
 

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