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Normal modal logic K
Normal modal logic K is the smallest normal modal logic, containing classical propositional logic, the Distribution Axiom, and the Necessitation Rule, named after Saul Kripke.
Normal operator
A normal operator is a linear operator on an inner product space that commutes with its adjoint, T*T = TT, the class for which the spectral theorem holds.
Normal space
A normal space is a topological space in which any two disjoint closed sets can be separated by disjoint open neighborhoods; it underlies Urysohn's lemma and the Tietze extension theorem.
Normal subgroup
A normal subgroup, also called an invariant subgroup, is a subgroup H of a group G with aH = Ha for every element, equivalently fixed under conjugation by all of G.
Normal surface
A normal surface is a properly embedded surface in a triangulated 3-manifold meeting each tetrahedron in standard disks, a theory introduced by Kneser and developed by Haken in the 1960s.
Normality test
A normality test is a statistical procedure that assesses whether a sample of data plausibly comes from a normal distribution, including the Shapiro-Wilk and Anderson-Darling tests.
Normalization
Normalization, in statistics and machine learning, adjusts data or model activations to a common scale or range, using methods such as min-max scaling and z-score standardization.
Normalization (statistics)
Normalization in statistics, also called normalizing variables, covers methods such as Z-score standardization and min-max scaling that prevent features with large values from dominating learning.
Normalized Laplacian
The normalized Laplacian is a matrix associated with a graph, formed by scaling the combinatorial Laplacian by vertex degrees; Fan Chung developed it in her book Spectral Graph Theory.
Norman Routledge
Norman Arthur Routledge (1928–2013) was an English mathematician and schoolteacher who taught at Eton College, was a fellow of King's College, Cambridge, and preserved Alan Turing's letters.
Normed algebra
A normed algebra is an associative algebra equipped with a submultiplicative norm, meaning |ab| ≤ |a|·|b|; when the norm is complete it is called a Banach algebra.
Normed vector space
A normed vector space, or normed linear space, is a vector space over the real or complex numbers equipped with a norm assigning each vector a nonnegative length.
North American Industry Classification System
The North American Industry Classification System (NAICS) is the standard used by the statistical agencies of the United States, Canada, and Mexico to classify business establishments for economic statistics.
Notation for differentiation
Notation for differentiation is the set of symbols used to write a derivative, including Leibniz's dy/dx, introduced in 1675, Lagrange's f'(x), and Newton's dot notation for time derivatives.
Novikov conjecture
The Novikov conjecture is a conjecture in geometric topology, formulated by Sergei Novikov around 1970, stating that the higher signatures of closed oriented manifolds are invariant under oriented homotopy equivalence.
NP (complexity)
NP, or nondeterministic polynomial time, is the complexity class of decision problems whose proposed solutions can be verified in polynomial time; whether NP is larger than P is a famous unsolved problem.
Nth root
An nth root of a number a is a number b satisfying b^n = a; over the reals, odd roots of negatives are real while even roots are not.
Nuclear dimension
Nuclear dimension is a numerical invariant of C-algebras, introduced by Wilhelm Winter and Joachim Zacharias in 2009 as a noncommutative version of topological covering dimension.
Nuclear operator
A nuclear operator is a continuous linear operator between topological vector spaces written as an absolutely convergent sum of rank-one operators, a theory created by Alexandre Grothendieck in the 1950s.
Nuclear space
A nuclear space, introduced by Grothendieck in 1955, is a locally convex space in which every continuous linear map into a Banach space is a nuclear operator.
Nugget effect
The nugget effect is the nonzero value a variogram approaches as sample separation tends to zero, caused by measurement error and microscale variation; a term coined in mining geostatistics.
Null hypothesis
The null hypothesis (H0) is the default proposition, typically stating that an effect is absent, which a statistical test attempts to reject in favor of an alternative.
Null set
A null set, also called a measure-zero set, is a measurable set of measure zero, like the rationals or the uncountable Cantor set, that integration treats as negligible.
Null test
A null test is a statistical validation procedure in physics and astronomy that checks whether data constructed to contain no signal, such as differences of data splits, look like noise.
Number
A number is a mathematical object used to count, measure, and label, built up in nested systems from the natural numbers to the integers, rationals, reals, and complex numbers.
Number field
A number field is a subfield of the complex numbers that is a finite extension of the rational numbers, and the central object of algebraic number theory.
Number needed to treat
The number needed to treat (NNT) is a medical measure of treatment effect: the average number of patients who must receive a treatment for one additional patient to benefit.
Number of the beast
The number of the beast, also called the beast number (Greek: Χξς), is 666, given in Revelation 13:18 as the number of a beastly figure who forces everyone to receive a mark.
Number theory
Number theory, also called the theory of numbers or higher arithmetic, is the branch of mathematics studying the integers, primes, divisibility, and equations solved in whole numbers.
Number-theoretic transform
The number-theoretic transform (NTT) is a discrete Fourier transform computed modulo a prime instead of over the complex numbers, giving exact convolution for polynomial multiplication and cryptography.