Published online by Cambridge University Press: 20 November 2018
In this note the Hilbert spaces under consideration are complex, and the operators referred to are bounded, linear operators. If is a Hilbert space, then the algebra of all operators on
is denoted by
.
It is known (1) that if is any Hilbert space, then the class of commutators on
, i.e., the class of all operators that can be written in the form PQ — QP for some
, can be exactly described. A similar problem is that of characterizing all operators on
that can be written in the form
for some
.
This research was supported in part by the National Science Foundation. At the time the paper was written, the second author was an Alfred P. Sloan Research Fellow.