r-entropy, equipartition, and Ornstein’s isomorphism theorem inRn
Department of MathematicsUniversity of California
Cite this article as:
Feldman, J. Israel J. Math. (1980) 36: 321. doi:10.1007/BF02762054
A new approach is given to the entropy of a probability-preserving group action (in the context ofZ and ofRn), by defining an approximate “r-entropy”, 0<r<1, and lettingr → 0. If the usual entropy may be described as the growth rate of the number of essential names, then ther-entropy is the growth rate of the number of essential “groups of names” of width≦r, in an appropriate sense. The approach is especially useful for actions of continuous groups. We apply these techniques to state and prove a “second order” equipartition theorem forZm ×Rn and to give a “natural” proof of Ornstein’s isomorphism theorem for Bernoulli actions ofZm ×Rn, as well as a characterization of such actions which seems to be the appropriate generalization of “finitely determined”.