Linear Algebra: Find Matrix Determinant w/o Evaluating Directly

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SUMMARY

The discussion focuses on finding the determinant of a symmetric matrix without direct evaluation. The matrix in question is structured as follows: a b b b, b a b b, b b a b, b b b a. Key insights include leveraging the properties of eigenvalues, recognizing that the eigenvalues are roots of a polynomial that cancels the matrix, and using the product of these eigenvalues to determine the determinant. Special cases such as when a equals b, or when either a or b is zero, provide trivial solutions that simplify the problem.

PREREQUISITES
  • Understanding of symmetric matrices
  • Knowledge of eigenvalues and eigenvectors
  • Familiarity with polynomial equations
  • Basic linear algebra concepts
NEXT STEPS
  • Study the properties of symmetric matrices and their diagonalizability
  • Learn how to compute eigenvalues using characteristic polynomials
  • Explore matrix factorization techniques for determinants
  • Investigate special cases of determinants in linear algebra
USEFUL FOR

This discussion is beneficial for students and professionals in mathematics, particularly those studying linear algebra, as well as educators looking for effective methods to teach determinant calculations without direct evaluation.

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Homework Statement


Find the determinant of the matrix using identities without evaluating the determinants directly:

a b b b
b a b b
b b a b
b b b a

The Attempt at a Solution


I tried getting it into a triangular matrix but halfway through it got too complicated and it has to be simpler than what I think it is. The matrix is symmetric but I don't know how that relates to the determinant.
 
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I don't know if you are supposed to know this way but let's try :
if a and b are real, a symmetric matrix is diagonalisable
You know that the eigenvalues are roots of any polynom that cancel your matrix. For a start, compute the square of you matrix and reexpress it in terms of the indentity matrix and your original one. This identity gies you a polynom that cancel your matrix. Its roots are eigenvalues, and their product is the determinant. Take care of the multiplicity of the eigenvalues, that's all !

You can also have guidance from basic aspects : you know trivial answers for special cases : a=b, a=0 or b=0 etc ... this should come then !

Last possibility (but also not very elegant) : trying linear combination of lines and you will find a nice factorization appearing...
 

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