Self-adjoint

In mathematics, an element x of a *-algebra is self-adjoint if $x^*=x$.

A collection C of elements of a star-algebra is self-adjoint if it is closed under the involution operation. For example, if $x^*=y$ then since $y^*=x^{**}=x$ in a star-algebra, the set {x,y} is a self-adjoint set even though x and y need not be self-adjoint elements.

In functional analysis, a linear operator A on a Hilbert space is called self-adjoint if it is equal to its own adjoint A* and that the domain of A is the same as that of A*. See self-adjoint operator for a detailed discussion. If the Hilbert space is finite-dimensional and an orthonormal basis has been chosen, then the operator A is self-adjoint if and only if the matrix describing A with respect to this basis is Hermitian, i.e. if it is equal to its own conjugate transpose. Hermitian matrices are also called self-adjoint.

In a dagger category, a morphism $f$ is called self-adjoint if $f=f^\dagger$; this is possible only for an endomorphism $f\colon A \to A$.

See also

• Symmetric matrix
• Self-adjoint operator
• Hermitian matrix