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		Cauchy sequence – combination Cauchy sequence   A sequence (x1, x2, x3, ... ) of elements of a metric space X with metric d(x, y) is Cauchy if for any e greater than zero there is some natural number N such that In other words, in a Cauchy sequence, the elements eventually become “arbitrarily close together.” If the metric space X is closed, this condition is equivalent to the sequence being convergent. CH   See: continuum hypothesis. chain   If X is a partially ordered set, then a subset Y of X is called a chain if it is totally ordered, that is, if for any two elements a, b of Y, either a b or b a. Cf. antichain. chain condition   For a an infinite cardinal, a partial order P is said to have the a-chain condition if every antichain in P has cardinality not greater than a. If a = w, this is called the countable chain condition, or “c.c.c.” characteristic function   Given a subset E of a space X, the characteristic function cE is defined by cE(x) = 1 if x is in E, and cE(x) = 0 otherwise. All properties of sets and set operations may be expressed by means of characteristic functions. choice, axiom of   See: axiom of choice. chord   A straight line segment connecting two points on a curve or surface. circle   In a plane, the locus of all points equidistant from a given point, called the center. The general equation for a circle in the Cartesian plane is given by (x - h) 2 + (y - k) 2 = r 2, where r is the radius of the circle (distance from the center to the locus of points), and (h, k) are the coordinates of the center. The interior of a circle is referred to as an open disk. A circle is also a conic section; a special case of an ellipse in which the foci coincide. Related article: Conics circumference   Geometry: The distance around a circle in the plane, or around a great circle of a sphere. Graph Theory: The circumference of a graph G is defined as the length of the longest cycle of G. The circumference is ususally denoted by c(G), and is undefined if G has no cycles. class   See proper class. closed   General: A set is closed under an operation if applying the operation to its elements returns only elements in the set. For example, the set of integers is closed under addition, since adding two integers always gives another integer, but it is not closed under division, since dividing two integers may result in a non-integer. Geometry: A plane figure is closed if it consists of lines and/or curves that entirely enclose an area. Similarly, a figure in 3-dimensional space is called closed if it entirely encloses a volume. See the following listings for other uses of the word “closed” in mathematics. Topology: A set is topologically closed if it is not open. closed interval   An interval of the real number line (or any other totally ordered set) which includes its endpoints. An interval containing only one of its endpoints is called half-open. Cf. open interval. closed set   Topology: A subset E of a topological space X is closed if X - E (set difference) is open. In a metric space, E is closed if every convergent sequence in E converges in E; equivalently, if every accumulation point of E is in E. Set Theory: If a is a limit ordinal, then a set C contained in a is called closed if and only if for every limit ordinal b less than a, if C b is unbounded in b, then b C. C is called c.u.b. (“cub set” or “club set”) if and only if C is closed and unbounded in a. Cf. stationary set. closed set system   If X is a set (or proper class) and F is a family of subsets of X, then F is called a closed set system provided X is a member of F, and F is closed under arbitrary intersections. Cf. filter. closure   Topology: The closure of a subset E of a topological space is the smallest closed set containing E. It may also be expressed as the union of E with its accumulation points. If E is closed, then it is equal to its closure. Algebra: An algebraic closure of a field F is a field G containing F such that every polynomial with coefficients from F has a root in G. closure operator   If X is a set, then a function C from P(X) into P(X) (i.e., a function on the power set of X) is called a closure operator provided Y is contained in C(Y) for every subset Y of X, C(C(Y)) = C(Y) for every subset Y of X, and If Y and Z are both subsets of X, with Y a subset of Z, then C(Y) is a subset of C(Z). Closure operators induce closed set systems. closure property   A property of subsets of a set X is a closure property if X has the property and the intersection of any subsets of X having the property also has the property. club set   Closed, unbounded set. See closed set. coefficient   See polynomial. cofinality   A function f which maps an ordinal a into an ordinal b is said to map a cofinally if the range of f is not bounded in b. (I.e., for every b in b there is an a in a such that f(a) b.) The cofinality of an ordinal b is the least ordinal a such that there is a cofinal map of a into b. combination   A subselection of a set of r elements from a set of n elements. The number of such combinations, i.e., the number of ways in which r elements may be chosen from a set of n elements, is given by the formula This operation is also sometimes denoted by nC r, and is read “n choose r.” Cf. permutation, factorial.
		Cauchy sequence – combination

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