A Turing degree a is said to isolate another degree b in the r.e. degrees if and only if a < b and there is no r.e. degree c with a < c < b. In this case b is called an isolated degree and a an isolating degree. There are of course analogous deenitions for any subsets A and B of !. If a isolates b, it follows from the Sacks density theorem that at least one of the two must fail to be r.e. When ...

It is well-known that the Plücker relations generate the ideal of relations of the maximal minors of a generic m× n matrix. In this paper we discuss the relations of t-minors for t < min(m,n). We will exhibit minimal relations in degrees 2 (non-Plücker in general) and 3, and give some evidence for our conjecture that we have found the generating system of the ideal of relations. The approach is...

The path of the rook is a curve γ (possibly, self-intersecting) inside an 8 × 8square. The length of γ should be at least 64 since all squares are visited. Next, I assumed that the rook turned each time by 90 degrees (and never 180). Then the total turn of γ is nπ/2, where n is the number of moves. Thus I traded the original problem for the following one: What is the smallest total turn of a cu...

In this paper, characterizations of the degree to which a mapping $mathcal{T} : L^{X}longrightarrow M$ is an $(L, M)$-fuzzy topology are studied in detail.What is more, the degree to which an $L$-subset is an $L$-open set with respect to $mathcal{T}$ is introduced.Based on that, the degrees to which a mapping $f: Xlongrightarrow Y$ is continuous,open, closed or a quotient mapping with respect t...

Introduction. According to a classical theorem of Post, the arithmetical sets may be obtained by the following construction : Step 0: "take" all the r.e. sets. Step n+1 : "add" all sets which are r.e. in sets "taken" at a previous stage. Moreover, this construction is intimately related to the Kleene arithmetical hierarchy, defined in terms of the number and quality of alternating numberquantif...

A set $S subseteq V(G)$ is a semitotal dominating set of a graph $G$ if it is a dominating set of $G$ andevery vertex in $S$ is within distance 2 of another vertex of $S$. Thesemitotal domination number $gamma_{t2}(G)$ is the minimumcardinality of a semitotal dominating set of $G$.We show that the semitotal domination problem isAPX-complete for bounded-degree graphs, and the semitotal dominatio...

Π1 classes are important to the logical analysis of many parts of mathematics. The Π1 classes form a lattice. As with the lattice of computably enumerable sets, it is natural to explore the relationship between this lattice and the Turing degrees. We focus on an analog of maximality, or more precisely, hyperhypersimplicity, namely the notion of a thin class. We prove a number of results relatin...