Universal Characteristic Factors and Furstenberg Averages

Tamar Ziegler

Let X=(X^0,μ,T) be an ergodic measure preserving system. For a natural number k we consider the averages (*) 1/N \sum_{n=1}^N \prod_{j=1}^k f_j(T^{n a_j}x) where the functions f_j…