Let A be a Banach ∗-algebra
which has a faithful ∗-representation as bounded linear operators on a flilbert space.
It follows from Fuglede’s theorem concerning normal operators on a Hilbert space
that x∗y = yx∗ for all x,y in A where XX∗= X∗X and xy = yx. Other
commutativity properties in suitable Banach ∗-algebras A involving elements not
necessarily normal are considered.