Determinant Functions in 3x3 Matrices for A, B, and I

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Homework Help Overview

The discussion revolves around the properties of functions defined on 3x3 matrices, specifically examining whether certain functions, such as f(A) = a11a22a33, g(A) = a11a12a13, and h(A) = 1, qualify as determinant functions. The original poster seeks to understand the criteria that differentiate these functions from true determinants.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants explore whether the function f(A) satisfies the properties of a determinant function and discuss the need for counterexamples to demonstrate that it does not. There are inquiries about what constitutes a suitable reference for proving these properties and the challenges of understanding determinants in the context of variables versus numbers.

Discussion Status

The discussion is ongoing, with participants providing insights into the requirements for a function to be considered a determinant. Some suggest that demonstrating the failure of f(A) to meet the determinant conditions through counterexamples may be a productive approach. There is an acknowledgment of the confusion surrounding the application of these concepts to variable matrices.

Contextual Notes

Participants note the importance of satisfying specific conditions that define determinant functions, and there is mention of using numerical examples to clarify the concepts being discussed. The original poster expresses difficulty in understanding the transition from numerical to variable-based determinants.

FourierX
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Homework Statement



Let A be a 3 x 3 matrix. Consider the function f(A) = a11a22a33; g(A) = a11 a12 a13 and h(A) =1. Show that each of these is not a determinant function.

Homework Equations



det(I) = 1
det(B) = det(A) (if B is obtained from adding a mutiple of one row of A to another row)
det(B) = - det(A) (if B is obtained from interchange two rows)
det(B) = m det(A) (if B obtained from A by multiplying a row of A by the number m)

The Attempt at a Solution


isn't f(A) = a11a22a33 a determinant function ?
 
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Can you show that all four conditions are satisfied for f(.)?
 
what to take as a reference, though?
 
I don't understand what you mean by reference. You should be able to show that for any arbitrary matrix A= aij, if you do the things stated in the four conditions, f(.) behaves as a determinant function.

To prove that f(.) is not a determinant function, a counterexample will suffice. Trying to prove that f() is a determinant function might help you come up with this counter-example.
 
I understand determinants with numbers. but its confusing with variables. Can you give me a simple example please?
 
An example please...
 
Pick A be the identity matrix. Interchange two rows. Have you tried any numerical examples at all?
 

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