Friday, 25 October 2013

linear algebra - Matrix Logarithms are Not Unique

In my ODE class, we proved that if exp(L) = exp(L') then the eigenvalues are congruent mod 2πi. Here L, L' are two nxn matrices. I wanted to know if something more precise was true.



In a way, we should expect that matrix logs are multiple valued since this is the case in $mathbb{C}$. log(re) = log r + iθ + 2πik with $k in mathbb{Z}$. In this way we can construct an branched infinite cover of the complex plane.



We'll define multiplcity mod 2πi of an eigenvalue λ to be the number of eigenvalues congruent to λ mod 2πi up to multiplicity. If exp(L) = exp(L') are the spectra of L and L' the same including multiplicity mod 2πi?



To put this another way, I could imagine two 5x5 matrices exp(L) = exp(L') where



  • the spectrum of L is (λ1, λ1, λ2, λ2 + 2πi, λ2 + 4πi)

while



  • the spectrum of L' is (λ1, λ1, λ1, λ2 + 2πi, λ2 + 4πi)

Here the multiplicities mod 2πi are different. One would be (2,3) while the other would be (3,2). Could exp(L) = exp(L') in this case?

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