Discovering the Image of a Linear Map: Methods and Examples

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Discussion Overview

The discussion revolves around methods for determining the image of a linear map, including theoretical approaches and practical examples. Participants explore various techniques related to linear algebra, particularly in the context of finite and infinite dimensional spaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks a general method for determining the image of a linear map and suggests that examples might be helpful.
  • Another participant proposes applying the linear map to the basis vectors of the domain space to obtain a spanning set for the image, which can then be reduced to find a basis.
  • A suggestion is made to use matrix representation, where column reduction of the matrix can yield a basis for the image, and if only row reduction is known, transposing the matrix can be an alternative approach.
  • It is noted that proving the image is the whole codomain can be done by showing the rank of the map equals the dimension of the codomain, particularly if the dimensions of the domain and codomain are the same.
  • A participant expresses gratitude and mentions their ongoing learning process in linear algebra, indicating a developing understanding of the topic.
  • Concerns are raised about the complexity of determining the image of linear maps in infinite dimensions, referencing a theorem related to linear differential operators and smooth functions.

Areas of Agreement / Disagreement

Participants present multiple approaches and techniques for determining the image of a linear map, indicating a lack of consensus on a single method. The discussion includes both finite and infinite dimensional contexts, highlighting differing perspectives on complexity and applicability.

Contextual Notes

Some participants mention specific techniques that may depend on the context, such as the dimensionality of the spaces involved and the nature of the linear map. There is also an acknowledgment of the challenges posed by infinite dimensional spaces.

Marin
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Hi all!

Does anyone know a general method for determining the image of a lin map?

I´m aware of the definition if im, but how could I determine it. Maybe it would be useful to show this on some examples :)
 
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What exactly do you mean by "determining the image". Apply the linear map to each of the basis vectors of a the domain space gives you a set that spans the image. You can reduce that to get a basis for the image.

If you are looking at a matrix, you can "column reduce" the matrix and and the columns of the reduced matrix are a basis for the image. If you have only learned "row reduction", swap rows for columns (the "transpose") and row reduce. The rows of the reduced matrix form a basis for the image.
 
Most computational linear algebra questions are best approached by first formulating the question in terms of matrices.

e.g. HallsofIvy suggests to capture the notion of image somehow via the column space of an appropriate matrix.
 
a standard technique for proving the image is the whole codomain is to show the rank of the map equals the dimension of the codomain. e.g. if the domain and codomain have the same dimension, then it suffices to show the map is injective.
 
thanks to all of you!

I think it`s all getting somehow clearer to me :)

(sorry for the inexactly asked question, I started my linear algebra course about a month ago and I`m still getting used to the level of abstraction it requires)
 
In general it is not trivial to determine the image of a linear map especially in infinite dimensions. e.g. the main theorem of ordinary differential equations says certain linear differential operators acting on smooth functions, have as image the space of all smooth functions.
 

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