Minimal nil-transformations of class two (Q1261904)

In the article, the author continues investigations of H. Furstenberg and E. Lesigne of nil-transformations and ergodic sums of the form \({1\over N}\sum^ N_{n=0} T^ n f T^{2n}g T^{3n}h\), in the direction of topologically characterizing nil-transformations of class 2. The role of enveloping groups of corresponding flows is discussed. By the way, an explicit representation for the enveloping groups of nil- transformations of class 2 is obtained.