Proving Positive Definite Scalar Product for $n \times n$ Matrices

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SUMMARY

The discussion focuses on proving that the scalar product defined as $\langle X, Y \rangle = tr(X Y^T)$ for $n \times n$ matrices is positive definite. Key properties to establish include $\langle X, X \rangle \geq 0$ and $\langle X, X \rangle > 0$ when $X \neq 0$. The proof hinges on understanding the trace operation and its properties, particularly in relation to matrix multiplication and the implications of matrix transposition.

PREREQUISITES
  • Understanding of matrix operations, specifically matrix multiplication and transposition.
  • Familiarity with the trace function and its properties for $n \times n$ matrices.
  • Knowledge of linear algebra concepts, particularly positive definiteness.
  • Basic experience with mathematical proofs in the context of linear algebra.
NEXT STEPS
  • Study the properties of the trace function in detail, focusing on its linearity and behavior under matrix multiplication.
  • Explore the concept of positive definite matrices and their characteristics in linear algebra.
  • Review examples of scalar products in vector spaces to understand their geometric interpretations.
  • Practice proving properties of scalar products using specific examples of matrices.
USEFUL FOR

Mathematicians, students of linear algebra, and anyone interested in the properties of matrices and scalar products will benefit from this discussion.

rputra
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Consider $X, Y$ as $n \times n$ matrices, I am given this definition of scalar product:
$$\langle X, Y \rangle = tr(X Y^T),$$
and I need to prove that it is positive definite scalar product. Of several properties I need to prove, two of them are
(1) $\langle X, X\rangle \geq 0$ and
(2) $\langle X, X\rangle > 0$ if $X \neq 0.$
I am lost on proving these two properties, any help or hints to prove them would be very much appreciated. Thank you before hand for your time and effort.
 
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Hi Tarrant,

A good place to start would be with the definition of the trace for an $n\times n$ matrix. Do you recall what this was?
 

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