= Real spectrum criterion for a normal C-star element
{title2=$\sigma(x)\subset\mathbb R\Longrightarrow x=x^*$}
For normal $x$, the closed unital <C-star subalgebra> generated by $x,x^*$ is commutative. When the ambient spectrum is real, its complement is connected. Membership of the resolvent in this subalgebra is both open and closed on that complement, and holds at large modulus by the <Neumann series>. Thus the subalgebra spectrum is the same real set. Its isometric <Gelfand transform> takes $x$ to a real-valued function and $x^*$ to its conjugate, proving equality. Normality is necessary: the nonzero matrix $\begin{pmatrix}0&1\\0&0\end{pmatrix}$ has spectrum $\{0\}$ but is not self-adjoint.
Back to article page
