OAMs. In view of the physical inequivalence of the two types of OAMs, i.e. the dynamical OAMs and the (generalized) canonical OAMs, one might expect that they obey different evolution equations. However, the past researches indicate that they do obey the same evolution equation at least at the 1-loop level [@JTH96]-[@Teryaev98]. The reason of this somewhat mysterious observation need explanation.

The plan of the paper is as follows. To make the paper self-contained, we briefly summarize, in sect.II, the current status of the nucleon spin decomposition problem from our own viewpoint. Next in sect.III, we clarify the physical meaning of the potential angular momentum, which characterizes the difference between the two types of OAMs, and consequently the difference between the two inequivalent decompositions of the nucleon spin. Sect.V is devoted to the discussion on the relation between the two different decompositions of the nucleon momentum. It will be shown that the two decompositions lead to the same evolution equation at least for the longitudinal momentum fractions of quarks and gluons, thereby predicting the same asymptotic limits for them. The relation between the evolution equations for the quark and gluon OAMs corresponding to the two different decompositions of the nucleon spin 