Why this property of the product of two matrices

Click For Summary

Discussion Overview

The discussion revolves around the properties of the product of two matrices, specifically focusing on a 2x5 matrix P and a 5x3 matrix B that results in a 2x3 zero matrix. Participants explore the implications of the determinant condition det(PPt) = det(BtB) = 7778 and seek to understand how to find another matrix B that maintains this property.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a specific matrix B that satisfies the condition det(PPt) = det(BtB) = 7778 and questions the meaning of this property and how to find another such B.
  • Another participant challenges the notion of B being a "solution," asking for clarification on what problem it solves.
  • A participant suggests that post-multiplying B by any 3x3 matrix C can yield another matrix D that also satisfies PD=0, with conditions on the determinant of D.
  • There is a discussion about whether the property is coincidental, with one participant expressing uncertainty about finding another B that fulfills the property.
  • Another participant asserts that the property is not by chance, citing the associative property of matrix multiplication to support their claim.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the property related to the determinant condition, with some questioning its significance and others asserting its validity. The discussion remains unresolved regarding the method to find another matrix B that meets the specified property.

Contextual Notes

Participants have not fully explored the assumptions underlying the determinant condition or the implications of matrix multiplication properties. There are also unresolved questions about the integer nature of the elements in the matrices discussed.

senmeis
Messages
77
Reaction score
3
Hello,

The product of a 2x5 matrix P and a 5x3 matrix B shall be a 2x3 zero matrix. P and B are all matrices of integers.

P = [6 2 -5 -6 1;3 6 1 -6 -5]

One possible B is [0 -4 0;3 0 0;-1 -1 3;2 -3 -2;1 1 3]

This solution B has a property: det(PPt) = det(BtB) = 7778

The question is: What does this property mean? How to get another B with this property?

Senmeis
 
Physics news on Phys.org
I really don't know how you got B or why you refer to it as a "solution". Solution to what problem? Just having the property that det(PPt) = det(BtB)?
 
Pb=0
 
Post-multiply B by any 3x3 matrix C, i.e., D=BC, and you'll have another 5x3 matrix D that satisfies PD=0. If C is unimodular (determinant = ±1), then det(D)=±det(B), so det(DTD)=det(BTB)=7778.

See if you can take this further to ensure that all elements of D are integers.
 
Last edited:
Yes, you are right, but I still don’t know if this property is by chance. More important, I can’t even get another B that fulfills this property.

Senmeis
 
No, it is not by chance. Matrix multiplication is associative: (PB)C = P(BC). If PB=0 then P(BC) must necessarily be zero as well.
 

Similar threads

  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 13 ·
Replies
13
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
13
Views
2K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K