یوسف محمدی

In this paper, Kolmogorov-Sinai entropy is studied us- ing mathematical modeling of an observer \Theta. The relative entropy of a sub--algebra having finite atoms is defined and then the ergodic properties of relative semi-dynamical systems are inves- tigated. Also, a relative version of Kolmogorov-Sinai theorem is given. Finally, it is proved that the relative entropy of a relative -measure preserving transformations with respect to a relative sub-\Theta-algebra having nite atoms is affine.