A CHARACTERIZATION OF THE n-ARY MANY-SORTED CLOSURE OPERATORS AND A MANY-SORTED TARSKI IRREDUNDANT BASIS THEOREM

摘要

A theorem of single-sorted algebra states that, for a closure space (A, J) and a natural number n, the closure operator J on the set A is n-ary if and only if there exists a single-sorted signature ∑ and a ∑-algebra A such that every operation of A is of an arity ≤ n and J = Sg_A, where Sg_A is the subalgebra generating operator on A determined by A. On the other hand, a theorem of Tarski asserts that if J is an n-ary closure operator on a set A with n ≥ 2, then, for every i, j ∈ IrB(j4, J), where lrB(A, J) is the set of all natural numbers which have the property of being the cardinality of an irredundant basis (≡ minimal generating set) of A with respect to J, if i < j and {i +1,..., j - 1} fl IrB(v4, J) = ø, then j — i ≤ n — 1. In this article we state and prove the many-sorted counterparts of the above theorems. But, we remark, regarding the first one under an additional condition: the uniformity of the manv-sorted closure ODerator.