In mathematics, an element of a *-algebra is called self-adjoint if it is the same as its adjoint (i.e. ).
Definition
Let be a *-algebra. An element is called self-adjoint if .[1]
The set of self-adjoint elements is referred to as .
A subset that is closed under the involution *, i.e. , is called self-adjoint.[2]
A special case of particular importance is the case where is a complete normed *-algebra, that satisfies the C*-identity (), which is called a C*-algebra.
Especially in the older literature on *-algebras and C*-algebras, such elements are often called hermitian.[1] Because of that the notations , or for the set of self-adjoint elements are also sometimes used, even in the more recent literature.
Examples
- Each positive element of a C*-algebra is self-adjoint.[3]
- For each element of a *-algebra, the elements and are self-adjoint, since * is an involutive antiautomorphism.[4]
- For each element of a *-algebra, the real and imaginary parts and are self-adjoint, where denotes the imaginary unit.[1]
- If is a normal element of a C*-algebra , then for every real-valued function , which is continuous on the spectrum of , the continuous functional calculus defines a self-adjoint element .[5]
Criteria
Let be a *-algebra. Then:
- Let , then is self-adjoint, since . A similarly calculation yields that is also self-adjoint.[6]
- Let be the product of two self-adjoint elements . Then is self-adjoint if and commutate, since always holds.[1]
- If is a C*-algebra, then a normal element is self-adjoint if and only if its spectrum is real, i.e. .[5]
Properties
In *-algebras
Let be a *-algebra. Then:
- Each element can be uniquely decomposed into real and imaginary parts, i.e. there are uniquely determined elements , so that holds. Where and .[1]
- The set of self-adjoint elements is a real linear subspace of . From the previous property, it follows that is the direct sum of two real linear subspaces, i.e. .[7]
- If is self-adjoint, then is normal.[1]
- The *-algebra is called a hermitian *-algebra if every self-adjoint element has a real spectrum .[8]
In C*-algebras
Let be a C*-algebra and . Then:
- For the spectrum or holds, since is real and holds for the spectral radius, because is normal.[9]
- According to the continuous functional calculus, there exist uniquely determined positive elements , such that with . For the norm, holds.[10] The elements and are also referred to as the positive and negative parts. In addition, holds for the absolute value defined for every element .[11]
- For every and odd , there exists a uniquely determined that satisfies , i.e. a unique -th root, as can be shown with the continuous functional calculus.[12]
See also
Notes
References
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