Remarks on steady rotation about a fixed axis in relativity

The relativistic motion of a medium with respect to an inertial frame is described by the four-velocity field from which natural generalizations of the nonrelativistic concepts of angular velocity and rate of strain can be derived in a covariant manner. These quantities can be used to characterize special motions, e.g. those that are volume-preserving, those with constant angular velocity or with vanishing rate of strain. In this paper the simple case of plane steady motion about a fixed axis with arbitrary speed-distance law v(r), is considered. (Auth.)