The aim of these notes is to indicate, using very simple examples, that not all results in ring theory can be derived from monoids and that there are results that deeply depend on the interplay between "+" and "·".
Mathematics Subject ClassificationIn a commutative unitary ring (R, +, ·), with 0 = 1, there are two binary operations, denoted by "+", addition, and by "·", multiplication, at work. So, in particular, we can consider R as an algebraic system closed under one of the binary operations, that is (R, +) or (R, ·). In both cases, R is at least a monoid. This state of affairs could lead one to think that perhaps it is sufficient to study monoids to get a handle on ring theory. The aim of these notes is to indicate, using elementary and easily accessible results available in literature, that not all results in ring theory can be derived separately from semigroups or monoids and that there are results that deeply depend on the interplay between "+" and "·". Following the custom, we choose to use the simplest examples that can be developed with a minimum of jargon to establish our thesis. We shall restrict our attention to integral domains and to results of multiplicative nature, as that is our area of interest. Indeed, we plan to show that there are results on integral domains that cannot be established for monoids, one way or another. Of course, to show that monoids cannot prove all the results on rings we have to have an idea of what monoids can do. For this, we start the paper with a review of the monoids. Our coverage of this topic will be tool specific, in that we shall concentrate more on what we need to establish our thesis. For this the best source we find in the field is Franz Halter-Koch's book [10] where, between the lines, further evidence to the aim of the present paper can be found.Before we start the review, we wish to point out that the material presented here is a sort of fact-presenting mission and it is not our intention to belittle monoids which have their own uses and play active roles in various contexts. Moreover, on the other side, it is known that monoid theory can produce results of ring-theoretical relevance which cannot be derived from the ring structure alone. For example, in a Noetherian domain, the monoid of elements coprime to the conductor forms a Krull monoid, whose arithmetic properties cannot be detected by purely ring-theoretical methods [10, Chapter 22].