Group theory? This solution doesn't make sense....

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SUMMARY

The discussion centers on the relationship between matrices A and B in the context of group theory and diagonalization. It establishes that if matrix A is proportional to the identity matrix, then A is always diagonal. The key conclusion is that while B is presumed to be diagonalizable, the transformation that diagonalizes B also maintains A's diagonal form if A and B commute. This highlights the importance of commutativity in matrix diagonalization.

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  • Basic knowledge of similarity transformations
  • Concept of identity matrices in linear algebra
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Case 2:
I get that D = c I means A must also be proportional to I but how does that mean B must be diagonal?

Question:
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Answers:
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It does not show that B is diagonalizable. B is presumed to be such. The point is that there is a transform that makes B diagonal and also makes A diagonal IFF A and B commute. The particular case of A proportional to the identity matrix means that A is always diagonal. Because every similarity transform on the identity matrix simply gives back the identity matrix. So, in this case, any similarity transform that makes B diagonal will leave A unchanged and still diagonal.
 
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DEvens said:
It does not show that B is diagonalizable. B is presumed to be such. The point is that there is a transform that makes B diagonal and also makes A diagonal IFF A and B commute. The particular case of A proportional to the identity matrix means that A is always diagonal. Because every similarity transform on the identity matrix simply gives back the identity matrix. So, in this case, any similarity transform that makes B diagonal will leave A unchanged and still diagonal.
THANKS! :D
 

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