An "Irrational Number" is a real number that can't be expressed as a ratio of two integers. It is therefore not a rational number, that is, not a fraction.

The most common example used is the square root of two, which can easily be proven to be an irrational number in several different ways. Other examples include $\pi$ and $e,$ although they are not just irrational, they are transcendental numbers.

AbsoluteValue
AlgebraicNumber
ArgandDiagram
CauchySequence
ComplexConjugate
ComplexNumber
ComplexPlane
ContinuedFraction
ContinuumHypothesis
CountableSet
CubeRoot
Denominator
DifferenceOfTwoSquares
DividingComplexNumbers
DividingRationalNumbers
DividingRealNumbers
DividingWholeNumbers
Divisor
DomainOfAFunction
Function
FundamentalTheoremOfAlgebra
GeorgCantor
ImaginaryNumber
ImproperFraction
Integer
Logarithm
MagnitudeOfAVector
MixedNumber
MultiplyingRationalNumbers
MultiplyingRealNumbers
NewUserIntroduction
NewtonsMethod
Numerator
PolarRepresentationOfAComplexNumber
Quaternion
ReducingFractionsToLowestTerms
RiemannHypothesis
RootTwoIsIrrational
RootsOfPolynomials
SquareRoot
SubtractingRationalNumbers
SubtractingRealNumbers
TranscendentalNumber
UncountableSet
Euclid
FermatNumber
SquareNumber
RationalNumber
RealNumber
A4Paper
DedekindCut
EIsIrrational
MathematicsTaxonomy
PiIsIrrational
TypesOfNumber

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IrrationalNumber
Integer
SquareRoot
TranscendentalNumber
AlgebraicNumber
CauchySequence
ContinuedFraction
CountableSet
DedekindCut
Denominator
Numerator
Pythagoras
ReducingFractionsToLowestTerms
RootTwoIsIrrational
(none) CoDomainOfAFunction
ComplexNumber
DomainOfAFunction
Function
ImageOfAFunction
ImaginaryNumber
Polynomial
RiemannSurface
Root
WholeNumber