Diagonalisability problem and others

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SUMMARY

The discussion centers on two problems related to linear algebra involving real matrices. The first problem establishes that if A is a real matrix, then the product AtA is diagonalizable, which can be proven by demonstrating that AtA is symmetric. The second problem involves calculating the maximum and minimum values of ||Ax|| on the unit sphere ||x|| = 1, where x is a 3x1 vector. Participants provided insights on matrix properties and the necessary conditions for vector dimensions.

PREREQUISITES
  • Understanding of linear algebra concepts, specifically matrix operations.
  • Familiarity with properties of symmetric matrices and diagonalization.
  • Knowledge of vector norms and their geometric interpretations.
  • Experience with matrix multiplication and dimensions of matrices.
NEXT STEPS
  • Study the properties of symmetric matrices and their diagonalizability.
  • Learn about the spectral theorem and its applications in linear algebra.
  • Explore optimization techniques for quadratic forms, particularly in relation to eigenvalues.
  • Investigate the geometric interpretation of matrix transformations on unit spheres.
USEFUL FOR

Students and professionals in mathematics, particularly those studying linear algebra, matrix theory, and optimization techniques. This discussion is also beneficial for anyone tackling similar homework problems in advanced mathematics courses.

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Homework Statement


1.Prove that if A is a real matrix then At A is diagonalisable.

2. Given a known 3*3 matrix A, Calculate the maximum and minimum values of ||Ax|| on the sphere ||x|| = 1.


Homework Equations





The Attempt at a Solution


For the first problem, I'm thinking of proving that AtA is symmetric, but I'm not sure which properties to use.
For the second one, is X a 3*n matrix? Do I need to discuss n?
Any help is greatly appreciated!
 
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rainwyz0706 said:
For the first problem, I'm thinking of proving that AtA is symmetric, but I'm not sure which properties to use.
If B=A^TA, write down what B_{ij} and B_{ji} are in terms of the elements of A and show that they're equal.
 
Thanks a lot! I can't believe I missed it in the first place.
Could anyone give me some hints about problem 2?
 
For the second one, yes, x must be such a vector that the product Ax is meaningful, i.e. a 3x1 vector (matrix).
 
thx, I finished it!
 

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