BROWSE ALPHABETICALLY LEVEL:    Elementary    Advanced    Both INCLUDE TOPICS:    Basic Math    Algebra    Analysis    Biography    Calculus    Comp Sci    Discrete    Economics    Foundations    Geometry    Graph Thry    History    Number Thry    Phys Sci    Statistics    Topology    Trigonometry countable – disjoint countable   A set is countable if it is finite, or if it is infinite and bijective to the set of natural numbers (finite ordinals), i.e., if there exists a complete one-to-one mapping of the set in question onto the set N. Sets that are both countable and infinite are sometimes called denumerable. Georg Cantor proved that sets may be uncountably infinite, for example the set of real numbers. Related MiniText: Infinity -- You Can't Get There From Here... countable chain condition   See chain condition. countably infinite   See countable cover   Topology: a collection of sets which contains a given set. If the sets in the covering collection are open sets, the cover is called an open cover.Partially ordered sets: If x and y are elements of a partially ordered set such that x y, and such that there is no z such that x z y, then we say that x covers y.Graph Theory: An edge or vertex of a graph is said to cover (verb) those vertices or edges, respectively, that it is incident on. A set of edges or vertices is said to cover any vertex or edge covered by any element of that set. A set of edges or vertices that covers all the vertices or edges, respectively, of the graph is called a cover (noun), usually with a specification of whether it consists of edges or vertices. See edge cover, vertex cover. cross product   See vector product. cub set   Closed, unbounded set. See closed set. D-system   Set Theory: A family A of sets is called a D-system (also called a quasi-disjoint family) if and only if there is a fixed set r, called the root of the D-system, such that for any two elements x and y of A, x y = r. It is a theorem that every uncountable family of finite sets has a sub-family which forms a D-system.Cf. almost disjoint. Dandelin’s Spheres    ARTICLE   A proof by the 17th century French mathematician Germinal Dandelin of the equivalence of the plane-geometry and conic-section definitions of the ellipse, parabola, and hyperbola. See the article for a full exposition. Dedekind cut   A partition of the set of rational numbers into two subsets A and B, such that every element of A is less than every element of B. The set of real numbers can be formally defined as the set of all Dedekind cuts; e.g., we can take the real number “square root of 2” to be the cutArithmetic on real numbers is then defined in terms of operations on cuts. For example, if (A1, B1) and (A2, B2) are Dedekind cuts defining real numbers x and y, then we define the inequality x < y to mean that there is an element of A2 that is not an element of A1, and we define x + y to be the real number corresponding to the Dedekind cut (A, B) where the elements of A are all those rational numbers obtained by adding an element of A1 to an element of A2.The set of irrational numbers can then be defined as the set of all real numbers that cannot be represented as a ratio of integers. definite integral   See: integral. degenerate conic   A conic section in which the intersecting plane passes through the vertex. Related article: Conics degree   Geometry: A unit of measure of angles, denoted by “ ° ”. A complete circle contains 360°, a straight line 180°, and a right angle 90°. Compare radians.Algebra: The degree of a polynomial is the highest sum of exponents appearing in any term with a non-zero coefficient.Graph Theory: The degree of a vertex is the number of edges incident on that vertex, counting loops twice. The degree of an edge is an unordered pair of the degrees of its two ends. dense   Given a space X and a subset A of X, we say that A is dense in X if the intersection of every open set of X with A is non-empty. E.g., the rational numbers are dense in the real numbers. dense linear order   A set X is said to be a dense linear order if it is a linear order under a relation “ < ” and such that for all distinct x and z with x < z, there is a y in X such that x < y and y < z. The canonical example of a dense linear order is the set Q of rational numbers. Related MiniText: Number -- What Is How Many? denumerable   A set is denumerable if it is infinite and countable. derivative   For a function f of a single real variable x, the derivative is defined as the limit of the difference quotientprovided this limit exists. In practice, the derivative is interpreted as the instantaneous rate of change of the function at x. Graphically, the derivative returns the slope of the tangent at x.Cf. differentiation rules. derived set   Given a set X, the derived set of X is the set of accumulation points of X. The second derived set is the derived set of the derived set, and so on.Cf. Cantor-Bendixson Theorem. diameter   Geometry: A diameter of a circle (or sphere) is a line containing the center and with endpoints on the perimeter (resp. surface).Analysis: Given a set X in a metric space, the diameter of X is the supremum of the distances between all pairs of points of X.Graph Theory: The diameter of a given graph G is the maximum, over all pairs of vertices u, v of G that are in the same connected component of G, of the distance between u and v. In other words, it is the greatest distance between two vertices on the graph. differentiable   A function is differentiable at a point of its domain if its derivative exists at that point. A function is said to be (simply) differentiable if its derivative exists at all points of its domain. differentiation rule    ARTICLE   A rule permitting easy differentiation of functions having certain forms. See the article for a complete description.Cf. derivative. disjoint   Two sets are disjoint if they have empty intersection. countable – disjoint
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