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Euclidean space – Heine-Borel property
Euclidean space
A space satisfying the axioms of Euclidean geometry. Specifically, a space in which the parallel postulate holds; equivalently, a space in which the distance between points is given by a straight line.
Euler Polyhedron Formula
V - E + F = 2, where V is the number of vertices, E is the number of edges, and F is the number of faces of a polyhedron or a planar graph.
even function
A real-valued function y = f(x) is even if f(x) = f(–x) for all x in the domain of f. The graph of an even function in the Cartesian plane is symmetric with respect to the y-axis.
Cf. odd function.
exp
See exponential function.
exponential function
The function ex. We say that an expression is expontiated when it is used as an exponent on e.
More generally, any function which operates on its argument by placing it as an exponent may be called an exponential function. Example: f(x) = 2x.
Cf. Euler number.
extended real numbers
The set of real numbers together with a “point(s) at infinity.” If both a positive and a negative infinity are added, then the resulting space is order equivalent and topologically equivalent (i.e., homeomorphic) to the unit interval [0, 1]. If only a single point at infinity is added, the resulting space is topologically equivalent to the unit circle.
figure
General: A drawing, picture, or illustration, usually accompanying a text description. Also synonymous with digit or numeral.
Geometry: A graphical (visual) representation of points, lines, curves, surfaces, solids, or regions. The word “figure” may also be used to refer to the abstract object or set of points thus represented.
Cf. plane figure.
finite graph
A graph whose vertex and edge sets are finite. In all papers, theorems, conjectures, textbooks, discussions, and interactions of graph theory all graphs considered are always finite, unless explicitly specified otherwise.
Cf. infinite graph.
finite intersection property
A collection of sets has the finite intersection property if every finite subcollection has non-empty intersection.
finite measure
See measure.
function
Given a binary relation R on sets A and B, we say that the R is a function if each element of A is paired with at most one element of B. In this case we call A the domain set, and we call B the range set. If f is such a function and the ordered pair (x,y) is an element of the function, we typically write f(x) = y.
More generally, if R is an n-place relation, then R is a function of n-1 variables if each (n-1)-tuple is matched with at most one element of the range set (i.e., the nth set of the Cartesian product on which the relation is defined). If in addition there is at most one element of the domain set matched with any given element of the range set, then the function is called “one-to-one” or injective. If the function maps at least one element of the domain set to every element of its range set, then it is called “onto” or surjective. A function which is both injective and surjective is called bijective. Functions with range in the real numbers or complex numbers are called real-valued or complex-valued respectively.
Fundamental Theorem of Calculus
Intitively, that the integral and derivative are inverse operatorations on functions. The fundamental theorem is commonly expressed in either one of two formal guises:
Cf. Riemann integral.
geodesic
A curve on a space is a geodesic if, given any two points on the curve, the segment of the curve joining the points is the shortest such curve. In Euclidean space, geodesics are straight lines. On a sphere, the geodesics are the great circles of the sphere.
Graph Theory: A shortest path between two vertices is sometimes called a geodesic.
Cf. distance.
geometric series
An infinite series such that the ratio of consecutive terms is constant. Such a series may be denoted
where r is the common ratio. A geometric series converges if and only if the common ratio is less than 1, in which case the sum of the series is given by
Cf. arithmetic series, harmonic series.
Related article: Series
g.l.b.
Abbreviation for greatest lower bound.
graph
General: A picture or diagram representing one or more relationships among certain objects or quantities.
Functions: The collection of ordered tuples (x,y), consisting of domain elements x (possibly also tuples) and corresponding range elements y. Although informally we often mean the picture when we speak of the graph of a function, the graph should always be formally thought of as a set of tuples.
Graph Theory: A collection of nodes (also called vertices), together with a set of edges joining the nodes. Formally, a graph G is a set of vertices V, a set of edges E, and a symmetric relation between them specifying incidence. Every edge must be incident on exactly two (not necessarily distinct) vertices, called its ends. Two vertices incident on the same edge are called adjacent. Graphs are often represented pictorially, with the vertices being points (or small circles or squares or some such), and the edges being curves (often line segments), with the endpoints of each such curve being the ends of that edge in the graph.
Cf. Konigsberg Bridge Problem.
great circle
On a sphere, a great circle is a circle containing the endpoints of a diameter of the sphere, that is, a circle of maximal size. A geodesic on a sphere always lies on a great circle.
greatest lower bound
A lower bound which is greater than or equal to every other lower bound.
Cf. Least upper bound.
harmonic series
The infinite series whose terms are the reciprocals of the natural numbers
This series diverges, i.e., the sum does not exist.
Related article: Series
Hausdorff space
A topological space is Hausdorff if every pair of distinct points have disjoint neighborhoods. Two disjoint compact sets in a Hausdorff space have disjoint neighborhoods. A compact subset of a Hausdorff space is closed.
Heine-Borel property
Every closed and bounded subset of the real numbers is compact.
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