Invariance properties of random vectors and stochastic processes based on the zonoid concept

Two integrable random vectors and in IRd are said to be zonoid equivalent if, for each uÎ IRd, the scalar products ,u and *,u have the same first absolute moments. The paper analyses stochastic processes whose finite-dimensional distributions are zonoid equivalent with respect to time shift (zonoid stationarity) and permutation of time moments (swap-invariance). While the first concept is weaker than the stationarity, the second one is a weakening of the exchangeability property. It is shown that nonetheless the ergodic theorem holds for swap invariant sequences and the limits are characterized