Wielandt theorem for unital normed algebras
Theorem. (Wielandt (1949)) Let be a normed unital algebra (with unit ). If then .
Proof.
Assume there are such that . Then for all we have
We prove this by induction over . It holds for by assumption
. Assume it is valid for . Then and
From this identity it follows that
It follows that for all which is impossible.∎
Corollary. The identity operator on a Hilbert space cannot be expressed as a commutator of two bounded linear operators in .
Remark. The above can be understood as a version of the uncertainty principle in one dimension. Let . Let be with , the coordinate operator and the momentum operator with . It follows that
According to the corollary can never be the case.