Abstract. We introduce the notion of positive implicative ordered filters in implicative semigroups. We show that every positive implicative ordered filter is both an ordered filter and an implicative ordered filter. We give examples that an ordered filter (an implicative ordered filter) may not be a positive implicative ordered filter. We also give equivalent conditions of positive implicative ordered filters. Finally we establish the extension property for positive implicative ordered filters.Keywords and phrases. Implicative semigroup, ordered filter, (positive) implicative ordered filter.2000 Mathematics Subject Classification. Primary 20M12, 06F05, 06A06, 06A12.
Introduction.The notions of implicative semigroup and ordered filter were introduced by Chan and Shum [3]. The first is a generalization of implicative semilattice (see Nemitz [6] and Blyth [2]) and has a close relation with implication in mathematical logic and set theoretic difference (see Birkhoff [1] and Curry [4]). For the general development of implicative semilattice theory the ordered filters play an important role which is shown by Nemitz [7]. Motivated by this, Chan and Shum [3] established some elementary properties, and constructed quotient structure of implicative semigroups via ordered filters. Jun, Meng, and Xin [6] discussed ordered filters of implicative semigroups. Also Jun [6] stated implicative ordered filters of implicative semigroups. For a deep study of implicative semigroups it is undoubtedly necessary to establish more complete theory of ordered filters. In this paper, we introduce the notion of positive implicative ordered filters in implicative semigroups. We show that every positive implicative ordered filter is both an ordered filter and an implicative ordered filter. We give examples that an ordered filter (an implicative ordered filter) may not be a positive implicative ordered filter. We also give equivalent conditions of positive implicative ordered filters. Finally we establish the extension property for positive implicative ordered filters.We recall some definitions and results. By a negatively partially ordered semigroup (briefly, n.p.o. semigroup) we mean a set S with a partial ordering "≤" and a binary operation "·" such that for all x, y, z ∈ S, we have:(1) (x · y) · z = x · (y · z), (2) x ≤ y implies x · z ≤ y · z and z · x ≤ z · y, (3) x · y ≤ x and x · y ≤ y.