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obtuse – Platonic solid
obtuse
An angle is called obtuse if it is greater than a right angle, that is, if its measure is greater than 90° (p/2 radians). A triangle is called obtuse if one of its angles is obtuse.
 Cf. acute.
octahedron
A polyhedron having eight faces.
The faces of a regular octahedron are congruent, equilateral triangles.
Cf. Platonic solid, polyhedron.
open disk
The interior of a circle.
Cf. neighborhood, disk.
open interval
An interval of the real number line (or any other totally ordered set) which does not include its endpoints. An interval containing only one of its endpoints is called half-open.
Cf. closed interval.
ordered pair
An ordered tuple (a,b), the first element of which is called the abscissa, and the second element the ordinate, and for which (a,b) = (b,a) if and only if a = b. Functions, graphs of functions, and binary relations are represented as sets of ordered pairs. In standard set theory, the ordered pair (a,b) is defined to be the set { {a}, {a,b} }.
Cf. flat pair.
order of operations
As a matter of convention, in any given expression involving arithmetic and/or algebraic operations, operations within parentheses (or other grouping symbols) are evaluated first. Within this constraint, exponentiation precedes multiplication and division, and the latter precede addition and subtraction. Within these constraints, operations are evaluated from left to right.
order relation
A relation R on a set S is an order relation exactly if it is reflexive, transitive and antisymmetric. Order relations are usually denoted by “ < ” or “  ”.
Cf. partial order, total order
ordinal
The class of ordinals is defined by:- 0 is an ordinal;
- if a is an ordinal, then a + 1 = a union {a}) is an ordinal;
- if A is a collection of ordinals, then union(A) is an ordinal;
- nothing else is an ordinal.
The class of ordinals is transitive, and is a well-founded, linear ordering. An ordinal of the form a + 1 is called a successor ordinal, and is otherwise called a limit ordinal.
Cf. Von Neumann Heirarchy.
ordinate
The second element of an ordered pair.
Cf. abscissa.
parabola
The locus of points in the plane whose distances from a fixed point, called the focus, and a fixed line, called the directrix, are equal.
Like the ellipse and hyperbola, the parabola is a conic section. See the related article for a full exposition.
Related article: Conics
paradox
A seemingly 'necessary' contradiction or absurdity. A paradox arising logically out of formal axioms is called an antinomy. True paradoxes may be broadly classified as paradoxes of logic, paradoxes of infinity, paradoxes of knowledge, paradoxes of language, and paradoxes of self-reference. See:Antistrephon Paradox
Banach-Tarski Paradox
Berry Paradox
Boundary Paradox
Finitude Paradox
First Boring Number Paradox
Grelling's Paradox
Grue-Bleen Paradox
Liar Paradox
Quine's Paradox
Prisoner's Dilemma
Russell Paradox
Santa Sentence Paradox
Sid's Paradox
Sorites Paradox
Unexpected Hanging Paradox
Zeno's Paradox of the Arrow
Zenos Paradox of the Moving Rows
Zeno's Paradox of the Tortoise and Achilles
parallelogram
A quadrilateral with opposite sides (and opposite angles) equal.
parallel postulate
The 5th postulate of Euclidean geometry.
partially ordered set
A set with a partial order defined on it.
partition
General: A partition of a set X is a collection of subsets of X such that every element of X is in exactly one of the subsets. Such a partition is given by (and gives rise to) an equivalence relation on X. For example, division modulo 3 partitions the set of natural numbers into three subsets, each containing all those numbers leaving remainders of 0, 1, or 2 respectively when divided by 3.
Algebra: A partition of a matrix is a division of the matrix into conformable submatrices.
Analysis: A partition of a space is a collection of pairwise disjoint regions of the space whose union is the entire space. For example, a partition of an interval [a, b] of the real line is given by a finite set of points {xi} such that a = x1 < x2 < . . . < xn = b which divide the interval into disjoint subintervals.
Number Theory: Given a positive integer n, a partition of n is a set of positive integers whose sum is n. For example, 4 = 3 + 1 = 2 + 2 = 2 + 1 + 1 = 1 + 1 + 1 + 1 are the four possible partitions of the number 4.
permutation
A permutation on a finite set is a reordering of its elements. On a set of n elements there are n! (n-factorial) permutations, and together they form a group, the symmetric group on n elements.
The number of permutations of a subselection of r elements from a set of n elements is usually denoted nPr, and is given by
A cyclic permutation, also called a cycle, of an ordered collection of n objects is obtained by replacing each kth object with the (k - 1)th object, and replacing the first object with the nth object. In other words, each object advances one place in the order, and the last object “wraps around” to the first position. The number of objects n is called the degree (sometimes length) of the cycle. A cycle of length 2 is called a transposition. Every permutation can be factored as a product of transpositions, and is called an even (or odd) permutation if it factors into an even (resp. odd) number of transpositions.
The subgroup of the symmetric group on n elements consisting of all the even permutations is called the alternating group on n elements.
Cf. combination.
perpendicular
Two lines are perpendicular if they intersect in a right angle. In particular, two lines in the Cartesian plane are perpendicular if their respective slopes m1 and m2 satisfy m 1 × m2 = –1.
Two curves intersect perpendicularly if their tangent lines at the point of intersection are perpendicular.
Perpendicularity of planes and surfaces is defined analogously. Cf. normal.
perpendicular distance
Geometry: The shortest distance between a point and a line, or between a point and a plane. Sometimes called directed distance.
Pi (p)
The real number that is equal to the ratio of the circumference of any circle to its diameter, approximately equal to 3.1415926. It is known to be a transcendental number.
plane figure
A figure, i.e., a graphical (visual) representation of points, lines, curves, or regions in the geometric plane. A plane figure, such as a polygon, that bounds a finite region is called closed.
Platonic solid
ARTICLE
A solid having congruent, regular polygonal faces. There are five Platonic solids: the icosahedron, tetrahedron, octahedron, dodecahedron, and cube.
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