“…The properties of amenable semigroups and invariant means on them can be found, for example, in [120], [121]; in particular, it is known that a commutative semigroup is amenable. We shall say that problem (6.6) is invariant, or more precisely, (Θ, T)-invariant, if, for any (x, y) G W and s G 5, there exists an element y' £Y with the properties y = r(s)y', (9(s) and in problem (6.6) there exists an extremal operator that is (Τ, Θ)-invariant.…”