Can a Non-Self-Adjoint Element Have a Real Spectrum?

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In summary, we discussed the properties of a C^*-algebra, specifically the fact that if x\in X is self-adjoint, then its spectrum is real, \sigma(x)\subset\mathbb{R}. We also considered the possibility of the converse being true, but it was determined that there are cases in which x\in X has a real spectrum, but still x^*\neq x. Additionally, we discussed the process of subscribing to a thread to receive e-mail notifications.
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jostpuur
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Let [tex]X[/tex] be a [tex]C^*[/tex]-algebra. I know that if [tex]x\in X[/tex] is self-adjoint, then its spectrum is real, [tex]\sigma(x)\subset\mathbb{R}[/tex]. I haven't seen a claim about the converse, but it seems difficult to come up with a counter example for it. My question is, that is it possible, that some [tex]x\in X[/tex] has a real spectrum, but still [tex]x^*\neq x[/tex]?
 
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  • #2
jostpuur said:
My question is, that is it possible, that some [tex]x\in X[/tex] has a real spectrum, but still [tex]x^*\neq x[/tex]?
Yes it is. Take for instance the 2x2 matrix (so [itex]X=M_2(\mathbb{C})[/itex])

[tex]x = \begin{pmatrix}a & 1 \\ 0 & b\end{pmatrix},[/tex]

where a and b are any real numbers. The spectrum of x is {a,b} but x is not selfadjoint.
 
  • #3
I see.

(hmhmhmh... I didn't receive mail notification of your response...)
 
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You will only get e-mail notification if you "subscribe" to a thread. To do that, clilck on "Thread Tools" at the top of the thread, then click on "Subscribe to this Thread".
 
  • #5
I am facing the same problem (see https://www.physicsforums.com/showthread.php?t=257751)

On the internet I found a reference, however I don't have acces to it:
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0736116"

In general you can not say anything about the eigenvalues of a real (unsymmetric) matrix. However, if you can write your matrix as a product of matrices then analyzing them you may say something about the eigenvalues of the big matrix.

I put here two articles, maybe you will find them usefull.
 

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HallsofIvy said:
You will only get e-mail notification if you "subscribe" to a thread. To do that, clilck on "Thread Tools" at the top of the thread, then click on "Subscribe to this Thread".

But isn't the subscribing automatic, so that one has to unsubscribe a thread if one doesn't want notifications. I didn't do anything with thread tools, and I got the notification of your post now.

There is a non-zero probability for the possibility, that I casually destroyed the first notification without later remembering it. I cannot know it for sure, of course... I was merely mentioning the remark anyway.
 
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When you initially join this forum you are offered the option of automatic "subscription" or not. I chose not because I don't want an e-mail everytime someone responds to one of the threads I responded to. I can't delete all those e-mails AND respond to questions!
 

FAQ: Can a Non-Self-Adjoint Element Have a Real Spectrum?

What is a real spectrum?

A real spectrum refers to the set of all possible values that a physical system can take on. In mathematics, it is the collection of all eigenvalues of a linear operator.

What is the difference between a real spectrum and a self-adjoint spectrum?

A self-adjoint spectrum refers to the set of eigenvalues of a self-adjoint operator, which is a special type of linear operator that is symmetric with respect to a certain inner product. In contrast, a real spectrum can include eigenvalues of both self-adjoint and non-self-adjoint operators.

Why is the concept of real spectrum important in physics?

The real spectrum is important in physics because it helps us understand the possible states and behaviors of physical systems. By studying the eigenvalues and eigenvectors of a given operator, we can make predictions and analyze the dynamics of a system.

Can an operator have a real spectrum but not be self-adjoint?

Yes, an operator can have a real spectrum but not be self-adjoint. This means that the operator is not symmetric with respect to a certain inner product, but its eigenvalues are all real numbers.

How is the real spectrum related to the complex spectrum?

The real spectrum is a subset of the complex spectrum, which includes all complex eigenvalues of a linear operator. The complex spectrum provides a more complete picture of the possible states and behaviors of a system, but the real spectrum is often more relevant in physical applications.

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