ABSTRACT

2.1 In modern algebra the notion of “structure” is immediately linked to “morphism” because both of them are living symbiotically in the notion of category. Of course we do not wish to develope Category Theory here, but we remain faithful to some of its philosophy. Again the problem met in Chapter 1 appears, i.e. the choice between intuitive or axiomatic exposition; we will introduce the concept of map first in an intuitive way and then provide a more rigorous definition in the sequel (see Definition 3.4).