Quick linear algebra determinant proof.

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SUMMARY

The determinant of a square symmetric matrix A (nxn) is definitively the product of its eigenvalues. This is established through spectral decomposition, where A can be expressed as A = QλQ', leading to the determinant |A| = |Q||Q'||λ|, which simplifies to |λ|, representing the product of the eigenvalues. The discussion clarifies that interchanging QλQ' to QQ'λ is incorrect, as it does not preserve the original matrix A.

PREREQUISITES
  • Understanding of spectral decomposition in linear algebra
  • Familiarity with symmetric matrices
  • Knowledge of eigenvalues and eigenvectors
  • Basic properties of determinants
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  • Study the spectral decomposition theorem in detail
  • Learn about the properties of symmetric matrices
  • Explore the relationship between eigenvalues and the characteristic polynomial
  • Investigate applications of determinants in linear transformations
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Students of linear algebra, mathematicians, and anyone studying matrix theory or eigenvalue problems will benefit from this discussion.

Kuma
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Homework Statement



If A is a square symmetric matrix nxn. Show that the determinant of A is the product of its eigenvalues.

Homework Equations





The Attempt at a Solution



From spectral decomp.

A = QλQ'
|A| = |QλQ'| = |QQ'λ| = |Q||Q'||λ| = |λ| = the product of its diagonals (eigenvalues).

The step I'm not 100% sure of is if I can interchange QλQ' to QQ'λ
 
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Kuma said:

Homework Statement



If A is a square symmetric matrix nxn. Show that the determinant of A is the product of its eigenvalues.

Homework Equations





The Attempt at a Solution



From spectral decomp.

A = QλQ'
|A| = |QλQ'| = |QQ'λ| = |Q||Q'||λ| = |λ| = the product of its diagonals (eigenvalues).

The step I'm not 100% sure of is if I can interchange QλQ' to QQ'λ

Hi Kuma! :smile:

No, you can't interchange QλQ' to QQ'λ.
This can be seen because then QQ'λ=Iλ=λ, but this would not match your original matrix A.

However, there is no need to interchange them.
You can take the same steps without interchanging them.
 
Kuma said:

Homework Statement



If A is a square symmetric matrix nxn. Show that the determinant of A is the product of its eigenvalues.

Homework Equations





The Attempt at a Solution



From spectral decomp.

A = QλQ'
|A| = |QλQ'| = |QQ'λ| = |Q||Q'||λ| = |λ| = the product of its diagonals (eigenvalues).

The step I'm not 100% sure of is if I can interchange QλQ' to QQ'λ

The constant in the characteristic equation of A equals the determinant of A. How is that constant related to the eigenvalues?

RGV
 

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