Sunday, 30 December 2007

Normal operators and it's spectrum in C*-algebras

If $A$ is a C*-algebra and $n$ is a normal element of $A$, then we have: (By Gelfand duality for example.)



$operatorname{spec}( |N| ) = | operatorname{spec}(N) | := left{ | lambda | ; lambda in operatorname{spec}(N) right}$



where we define: $|n|:=(n^*n)^{1/2}$. My question is, does the converse also hold?



That is if $ain A$ and for $r>0$:



$left{ r e^{it} : t in [0, 2pi[ right} cap operatorname{spec}(a)$ is not empty if and only if $rin operatorname{spec}(|a|)$



implies that $a$ is normal. (Possibly some exceptions made for the zero-element) Or bluntly speaking if the mapping $a$ to $a^*a$ does not create any "new" (or removes any "old") elements in the spectrum then $a$ is normal.



For example if $e$ is an idempotent in $A$, then $e$ is a projection if and only if $||e||=1$. Hence if $e$ is a non-projection idempotent we have $left{0,1right} = operatorname{spec}(e) subsetneq operatorname{spec}(|e|)$, since $||e||>1$ and by the spectral radius $operatorname{spec}(e^*e)$ contains an element strictly bigger that one.



Clearly if p is a projection, then p=|p|.

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