We discuss selected unsolved problems on multiple mixing, structures of self-joinings, and the local rank of automorphisms of a probability space. For an account of the spectrum and entropy of the dynamical system that we use, see, for example, I. P. Cornfeld, S. V. Fomin, and Ya. G. Sinai, Ergodic Theory, Springer, 1982.
V. V. Ryzhikov (Sun,) studied this question.