Note on the number of monoids of order
We prove that if is an Abelian -group of length not exceeding and is its -projective subgroup for such that is countable, then is also -projective. This enlarges results of ours in (Arch. Math. (Brno), 2005, 2006 and 2007) as well as a classical result due to Wallace (J. Algebra, 1971).
The class of semigroups satisfying semimedial laws is studied. These semigroups are called semimedial semigroups. A connection between semimedial semigroups, trimedial semigroups and exponential semigroups is presented. It is proved that the class of strongly semimedial semigroups coincides with the class of trimedial semigroups and the class of dimedial semigroups is identical with the class of exponential semigroups.
We show that if the average number of (nonnormal) Sylow subgroups of a finite group is less than then is solvable or . This generalizes an earlier result by the third author.