Are Matrix Multiplication Rules the Same for Composing Linear Transformations?

Click For Summary

Discussion Overview

The discussion revolves around the rules governing matrix multiplication and their relationship to the composition of linear transformations. Participants explore the conditions under which these operations are defined, particularly focusing on the dimensional requirements for both matrices and transformations.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant notes that matrix multiplication is defined under specific dimensional conditions and questions whether this implies similar restrictions for composing linear transformations.
  • Another participant clarifies that for linear transformations f:V→W and g:X→Y, the composition g∘f is only meaningful if the dimensions match, specifically if W=X.
  • A further explanation is provided that if f:ℝ^K→ℝ^L and g:ℝ^J→ℝ^I, the composition g∘f is valid only if L=J, linking this to the matrix multiplication rule where the product of matrices is defined only if the relevant dimensions align.
  • A participant reiterates the dimensional requirements for matrix multiplication and suggests considering how the dimensions of linear transformations must align for their composition to be defined.

Areas of Agreement / Disagreement

Participants generally agree on the dimensional requirements for both matrix multiplication and the composition of linear transformations, although the discussion does not resolve whether there are any exceptions or nuances to these rules.

Contextual Notes

The discussion does not address potential exceptions to the dimensional requirements or explore cases where transformations might not be composable, leaving these aspects unresolved.

Who May Find This Useful

Students preparing for exams in linear algebra or related fields, particularly those focusing on matrix operations and linear transformations.

1MileCrash
Messages
1,338
Reaction score
41
So as I'm preparing for finals, I'm wondering:

The multiplication of two matrices is only defined under special circumstances regarding the dimensions of the matrices.

Doesn't that require that compositions of linear transformations are only defined in the same circumstances? I can't imagine not being able to not define a composition of linear transformations, can someone demonstrate this?
 
Physics news on Phys.org
If [itex]f:V\to W[/itex] and [itex]g:X\to Y[/itex] are linear transformations, it only makes sense to talk about the composition [itex]g\circ f[/itex] if [itex]W=X[/itex]. In particular, if [itex]f:\mathbb R^K \to \mathbb R^L[/itex] and [itex]g:\mathbb R^J \to \mathbb R^I[/itex] are linear transformations, it only makes sense to talk about the composition [itex]g\circ f[/itex] if [itex]\mathbb R^L=\mathbb R^J[/itex], i.e. if [itex]L=J.[/itex]

Phrasing the last point a different way now: If [itex]F[/itex] is an [itex]L\times K[/itex] matrix and [itex]G[/itex] is an [itex]I\times J[/itex] matrix, it only makes sense to talk about the matrix product [itex]GF[/itex] if [itex]L=J[/itex].

So the matrix dimension rule you learned is really there exactly because only certain functions can be composed. The expression [itex]g\circ f[/itex] only has meaning if the outputs of [itex]f[/itex] are valid inputs for [itex]g[/itex].
 
  • Like
Likes   Reactions: 1 person
1MileCrash said:
So as I'm preparing for finals, I'm wondering:

The multiplication of two matrices is only defined under special circumstances regarding the dimensions of the matrices.

Doesn't that require that compositions of linear transformations are only defined in the same circumstances? I can't imagine not being able to not define a composition of linear transformations, can someone demonstrate this?

If A is an m X n matrix, and B is an n X p matrix, then the product AB is defined, and will be an m X p matrix.

A linear transformation TA: Rn → Rm takes vectors from Rn and maps them to vectors in Rm. A matrix for TA will by m X n. Think about how TB would have to be defined (in terms of its domain and codomain) so that the composition TA ° TB would make sense. It might be helpful to use constants for the dimensions.
 
  • Like
Likes   Reactions: 1 person
Crystal clear, thanks you two.
 

Similar threads

  • · Replies 19 ·
Replies
19
Views
4K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 12 ·
Replies
12
Views
5K