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ellipse – extended real numbers

ellipse   The locus of points in the plane, the sum of whose distances from two fixed points, called the foci, remains constant.

Like the hyperbola and parabola, the ellipse is a conic section.

Related article: Conics

empty set   The unique set having no elements, generally denoted by a circle with a forward slash through it, or by an empty pair of braces.

epimorphism   A morphism f from X to Y is called an epimorphism when it is surjective, that is, when to each element y of Y there corresponds an x in X such that f(x) = y.
Cf. monomorphism.

equicontinuous   Let C[a,b] denote the space of all continuous functions on the real interval [a,b]. A subset S of C[0,1] is called equicontinuous at x in [a,b] if for any e greater than zero there is some d greater than zero such that for every function f in S we have:

equilateral triangle   A triangle with equal sides and equal angles.

equipotent   Two sets are equipotent if there exists a function between them that is bijective, that is, which is “one-to-one” and “onto.”
Cf. cardinal.

equivalence relation   An equivalence relation on a set X is a binary relation on X that is reflexive, symmetric and transitive. An equivalence relation on X gives rise to (and is determined by) a partition on X.
Cf. congruence relation.

 self portrait
Escher, Maurits Cornelis
(born 1898)   Dutch graphic artist whose works exhibit many mathematical concepts, including tesselations, infinity, and polyhedra.

Related MiniText: Mathematical Art of M.C. Escher

Euclidean geometry   The theory of geometry arising from the five postulates (axioms) of Euclid:
1. That any two points determine a line;
2. That any line may be extended indefinitely;
3. That a circle may be drawn with any center and radius;
4. That all right angles are equal to one another;
5. That, if a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side on which the angles are less than two right angles.
Euclidean geometry is characterized by the 5th postulate, also called the “parallel’s postulate,” which may be restated as; given a line and a point not on the line, exactly one line may be drawn through the given point parallel to the given line. Theories of geometry which reject the 5th postulate form the subject of non-Euclidean geometry. Euclid recorded his theory of geometry in his 13-book opus titled Elements, c. 300 b.c., and his theory was considered the only “true” theory of geometry until the 18th century, when non-Euclidean geometry was discovered independently by Nikolai Lobachevsky, Janos Bolyai, and Karl Friedrich Gauss. Euclid’s axioms were found to be deficient in the 19th century, and Euclidean geometry was re-axiomatized by David Hilbert with the addition of a between-ness axiom, and the relegation of the concepts of point and line, which Euclid had defined, to the status of undefined terms.

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.

Euclid number   See perfect number.

Euler number   The transcendental number e, approximately 2.71828.... It may be defined as the limit

or as the infinite sum

The function ex is called the exponential function, and has the property that it is its own derivative.

Euler phi function   Given a natural number n, this function returns the number of integers between 1 and n which are relatively prime to n.

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.

existential quantifier

exp

exponent   Also called index, a number or expression applying to another number or expression, called the base, and indicating that the base is to be “raised to the power of” the exponent. For integers, this corresponds to the operation of repeated self-multiplication the number of times specified by the exponent.

The exponent is written to the right of, and superscripted to, the base.
Cf. rational exponent, laws of exponents.

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.

ellipse – extended real numbers

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