How to Prove an Inequality Involving a Hermitian Negative Definite Matrix?

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SUMMARY

The discussion centers on proving the inequality \((x, Qx) \leq a(x, x)\), where \(Q\) is a Hermitian negative definite matrix and \(a\) represents the maximum eigenvalue of \(Q\). Participants suggest diagonalizing \(Q\) as a potential method for proving this inequality. The use of the Euclidean inner product \((.,.)\) is emphasized in the context of the proof.

PREREQUISITES
  • Understanding of Hermitian matrices and their properties
  • Knowledge of eigenvalues and eigenvectors
  • Familiarity with the concept of negative definiteness
  • Proficiency in linear algebra, particularly inner product spaces
NEXT STEPS
  • Study the properties of Hermitian matrices in detail
  • Learn about diagonalization techniques for matrices
  • Research the implications of negative definite matrices on eigenvalues
  • Explore proofs involving inner product spaces and inequalities
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Mathematicians, students of linear algebra, and anyone involved in theoretical physics or engineering requiring a solid understanding of matrix inequalities and properties of Hermitian matrices.

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Let x be in R^n and Q in Mat(R,n) where Q is hermitian and negative definite. Let (.,.) be the usual euclidian inner product.

I need to prove the following inequality:

(x,Qx) <= a(x,x)

where "a" is the maximum eigenvalue of Q.

Any idea?
 
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Maybe try to diagonalize Q?
 

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