Proving the Frobenius Norm Identity for Matrices

iwan89
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Homework Statement



Prove ∥A∥F =√trace(ATA), for all A ∈ R m×n

Where T= transpose



Homework Equations





The Attempt at a Solution


I tried and i just can prove it by using numerical method. Is there anyway to prove the equation in a correct way?
 
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The product AT A worked out gives the sum of all entries squared.
 
Can you give me a head start? :( I am in total lost :(
 
iwan89 said:
Can you give me a head start? :( I am in total lost :(

Write trace(AA^T) out in index notation.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...

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