Proving B^2=B & Finding det(B) | Linear Algebra Exercises

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Discussion Overview

The discussion revolves around proving properties related to a matrix \( B \) where \( B^2 = B \) and exploring the implications for its determinant. Participants also examine the case where the transpose of \( B \) equals its inverse, seeking to determine the determinant in that scenario. The scope includes mathematical reasoning and problem-solving in linear algebra.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant asks for help proving that either \( \text{det}(B) = 1 \) or \( B \) is singular given \( B^2 = B \).
  • Another participant suggests using the property \( \text{det}(A \cdot B) = \text{det}(A) \cdot \text{det}(B) \) to approach the proof.
  • A participant emphasizes that every matrix satisfies a polynomial, which relates to its eigenvalues and singularity.
  • Further clarification is requested on the steps to derive the determinant implications from the equations \( B^2 = B \) and the determinant properties.
  • Participants express frustration with the proofs and seek more detailed guidance on the steps involved.

Areas of Agreement / Disagreement

There is no consensus on the proofs for the problems presented. Participants express varying levels of understanding and seek clarification on the steps required to reach a solution.

Contextual Notes

Some participants indicate uncertainty about the application of determinant properties and the polynomial satisfied by the matrix, which may affect their ability to complete the proofs.

hola
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B is a n*n matrix

1. Let B^2 =B. Prove that either det(B) =1 or B is singular.
2. If Transpose(B) = B^-1 , what is det(B)?


Sorry I am asking, but I can't figure them out! I'd really like to improve my linear algebra skills.
Thanks!
 
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The first is really easy.

HINT:What is [itex]\mbox{det} \ A\cdot B[/itex] equal to...?(A,B matrices n*n).Then take A=B and recover the result u were supposed to prove.

For the second HINT:multiply to the left by B and use the "det" property u used for 1.

Daniel.
 
I don't understand, more depth please?
 
Alright.Take two n*n matrices A and B.Each of them has a determinant.Question:what is the product of their determinants...?

[tex]\mbox{det} \ A\cdot B =...?[/tex]

Daniel.
 
det(a*b) = det(a)det(b)

So for #1, I'm still stuck.
What do I do?
 
the point is that every matrix satisfies a polynomial, and that polynomial tells you rthe eigenvaleus, which tell you whetehr it is singualr or not.


now a polynomial satisfied by a matrix such that A^2 = A would be?
 
hola said:
det(a*b) = det(a)det(b)

So for #1, I'm still stuck.
What do I do?

Perfect,then u must agree that

[tex]\mbox{det} \ B^{2}=\left(\mbox{det} \ B\right)^{2}[/tex] (1)

And now apply "det" on the equation

[tex]B^{2}=B[/tex](2)

and use (1) to get a quadratic algebraic eq. in [itex]\mbox{det} \ B[/itex].


Daniel.
 
dextercioby, I didn't quite get your each of your steps in the last post.

Could you help me in writing the full proof for 1 and 2?
Aaargh! I feel so frustrated. I should have take regular linear algebra instead of the honors one. I suck at proofs.


I'm really sorry, but I need to solve these problems, but can't get them. Thanks!
 

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