The logic
RM and its basic fragments (always with implication) are considered here as entire
consequence relations, rather than as sets of theorems. A new observation made here is that the disjunction of
RM is definable in terms of its other
positive propositional connectives, unlike that of
R. The basic fragments of
RM therefore fall naturally into two classes, according to whether disjunction is or is not definable. In the equivalent quasivariety semantics of these fragments, which consist of subreducts of Sugihara algebras, this corresponds to a distinction between strong and weak congruence properties. The distinction is explored here. A result of Avron is used to provide a local deduction-detachment theorem for the fragments without disjunction. Together with results of Soboci

ski, Parks and Meyer (which concern theorems only), this leads to axiomatizations of these
entire fragments — not merely their theorems. These axiomatizations then form the basis of a proof that
all of the basic fragments of
RM with implication are finitely axiomatized consequence relations.
Keywords Sugihara algebra - mingle - RM - fragment - relevance logic
Special issue of Studia Logica:
Algebraic Theory of Quasivarieties
Presented by
M. E. Adams, K. V. Adaricheva, W. Dziobiak, and A. V. Kravchenko