Commutativity up to a factor for bounded and unbounded operators
  In this paper, we further investigate the problem of commutativity up to a factor (or $\lambda$-commutativity) in the setting of bounded and unbounded linear operators in a complex Hilbert space. The results are based on a new approach to the problem. We finish the paper by a conjecture on the commutativity of self-adjoint operators. 
Ch\'erifa Chellali and Mohammed Hichem Mortad
math.FA math.OA