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Inequality (mathematics)

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Inequality Mathematics

Mathematical relation expressed by symbols < or

## Properties on the number line

### Converse

### Transitivity

### Addition and subtraction

### Multiplication and division

### Additive inverse

### Multiplicative inverse

### Applying a function to both sides

## Formal definitions and generalizations

### Ordered fields

## Chained notation

## Sharp inequalities

## Inequalities between means

## Cauchy-Schwarz inequality

## Power inequalities

### Examples

## Well-known inequalities

## Complex numbers and inequalities

## Vector inequalities

## Systems of inequalities

## See also

## References

## Sources

## External links

This article includes a list of general references, but it remains largely unverified because it lacks sufficient corresponding inline citations. Please help to improve this article by introducing more precise citations. (May 2017) (Learn how and when to remove this template message) |

In mathematics, an **inequality** is a relation which makes a non-equal comparison between two numbers or other mathematical expressions.^{[1]}^{[2]} It is used most often to compare two numbers on the number line by their size. There are several different notations used to represent different kinds of inequalities:

- The notation
*a*<*b*means that*a*is less than*b*. - The notation
*a*>*b*means that*a*is greater than*b*.

In either case, *a* is not equal to *b*. These relations are known as **strict inequalities**,^{[2]} meaning that *a* is strictly less than or strictly greater than *b*. Equivalence is excluded.

In contrast to strict inequalities, there are two types of inequality relations that are not strict:

- The notation
*a*b or*a*?*b*means that*a*is less than or equal to*b*(or, equivalently, at most*b*, or not greater than*b*). - The notation
*a*>=*b*or*a*?*b*means that*a*is greater than or equal to*b*(or, equivalently, at least*b*, or not less than*b*).

The relation "not greater than" can also be represented by *a* ? *b*, the symbol for "greater than" bisected by a slash, "not". The same is true for "not less than" and *a* ? *b*.

The notation *a* ? *b* means that *a* is not equal to *b*, and is sometimes considered a form of strict inequality.^{[3]} It does not say that one is greater than the other; it does not even require *a* and *b* to be member of an ordered set.

In engineering sciences, less formal use of the notation is to state that one quantity is "much greater" than another, normally by several orders of magnitude. This implies that the lesser value can be neglected with little effect on the accuracy of an approximation (such as the case of ultrarelativistic limit in physics).

- The notation
*a*b means that*a*is much less than*b*. (in measure theory, however, this notation is used for absolute continuity, an unrelated concept.^{[4]}) - The notation
*a*>>*b*means that*a*is much greater than*b*.^{[5]}

In all of the cases above, any two symbols mirroring each other are symmetrical; *a* < *b* and *b* > *a* are equivalent, etc.

Inequalities are governed by the following properties. All of these properties also hold if all of the non-strict inequalities (=) are replaced by their corresponding strict inequalities (< and >) and -- in the case of applying a function -- monotonic functions are limited to *strictly* monotonic functions.

The relations = are each other's converse, meaning that for any real numbers *a* and *b*:

*a*b and*b*>=*a*are equivalent.

The transitive property of inequality states that for any real numbers *a*, *b*, *c*:^{[6]}

- If
*a*b and*b*c, then*a*c.

If *either* of the premises is a strict inequality, then the conclusion is a strict inequality:

- If
*a*b and*b*<*c*, then*a*<*c*. - If
*a*<*b*and*b*c, then*a*<*c*.

A common constant *c* may be added to or subtracted from both sides of an inequality.^{[3]} So, for any real numbers *a*, *b*, *c*:

- If
*a*b, then*a*+*c*b +*c*and*a*-*c*b -*c*.

In other words, the inequality relation is preserved under addition (or subtraction) and the real numbers are an ordered group under addition.

The properties that deal with multiplication and division state that for any real numbers, *a*, *b* and non-zero *c*:

- If
*a*b and*c*> 0, then*ac*bc and*a*/*c*b/*c*. - If
*a*b and*c*< 0, then*ac*>=*bc*and*a*/*c*>=*b*/*c*.

In other words, the inequality relation is preserved under multiplication and division with positive constant, but is reversed when a negative constant is involved. More generally, this applies for an ordered field. For more information, see *§ Ordered fields*.

The property for the additive inverse states that for any real numbers *a* and *b*:

- If
*a*b, then -*a*>= -*b*.

If both numbers are positive, then the inequality relation between the multiplicative inverses is opposite of that between the original numbers. More specifically, for any non-zero real numbers *a* and *b* that are both positive (or both negative):

- If
*a*b, then >= .

All of the cases for the signs of *a* and *b* can also be written in chained notation, as follows:

- If 0 <
*a*b, then >= > 0. - If
*a*b < 0, then 0 > >= . - If
*a*< 0 <*b*, then < 0 < .

Any monotonically increasing function, by its definition,^{[7]} may be applied to both sides of an inequality without breaking the inequality relation (provided that both expressions are in the domain of that function). However, applying a monotonically decreasing function to both sides of an inequality means the inequality relation would be reversed. The rules for the additive inverse, and the multiplicative inverse for positive numbers, are both examples of applying a monotonically decreasing function.

If the inequality is strict (*a* < *b*, *a* > *b*) *and* the function is strictly monotonic, then the inequality remains strict. If only one of these conditions is strict, then the resultant inequality is non-strict. In fact, the rules for additive and multiplicative inverses are both examples of applying a *strictly* monotonically decreasing function.

A few examples of this rule are:

- Raising both sides of an inequality to a power
*n*> 0 (equiv., -*n*< 0), when*a*and*b*are positive real numbers:

- 0 a b 0 a
^{n}b^{n}. - 0 a b
*a*^{-n}>=*b*^{-n}>= 0.

- Taking the natural logarithm on both sides of an inequality, when
*a*and*b*are positive real numbers:

- 0 <
*a*b ln(*a*) b). - 0 <
*a*<*b*ln(*a*) < ln(*b*). - (this is true because the natural logarithm is a strictly increasing function.)

A (non-strict) **partial order** is a binary relation set *P* which is reflexive, antisymmetric, and transitive.^{[8]} That is, for all *a*, *b*, and *c* in *P*, it must satisfy the three following clauses:

*a*a (reflexivity)- if
*a*b and*b*a, then*a*=*b*(antisymmetry) - if
*a*b and*b*c, then*a*c (transitivity)

A set with a partial order is called a **partially ordered set**.^{[9]} Those are the very basic axioms that every kind of order has to satisfy. Other axioms that exist for other definitions of orders on a set *P* include:

- For every
*a*and*b*in*P*,*a*b or*b*a (total order). - For all
*a*and*b*in*P*for which*a*<*b*, there is a*c*in*P*such that*a*<*c*<*b*(dense order). - Every non-empty subset of
*P*with an upper bound has a*least*upper bound (supremum) in*P*(least-upper-bound property).

If (*F*, +, ×) is a field and total order on *F*, then (*F*, +, ×, ordered field if and only if:

*a*b implies*a*+*c*b +*c*;- 0 a and 0 b implies 0 a ×
*b*.

Both (**Q**, +, ×, R, +, ×, ordered fields, but C, +, ×, ordered field,^{[10]} because -1 is the square of *i* and would therefore be positive.

Besides from being an ordered field, **R** also has the Least-upper-bound property. In fact, **R** can be defined as the only ordered field with that quality.^{[11]}

The notation ** a < b < c** stands for "

This notation can be generalized to any number of terms: for instance, ** a_{1}a_{2}a_{n}** means that

When solving inequalities using chained notation, it is possible and sometimes necessary to evaluate the terms independently. For instance, to solve the inequality 4*x* < 2*x* + 1 x + 2, it is not possible to isolate *x* in any one part of the inequality through addition or subtraction. Instead, the inequalities must be solved independently, yielding *x* < 1/2 and *x* >= -1 respectively, which can be combined into the final solution -1 x < 1/2.

Occasionally, chained notation is used with inequalities in different directions, in which case the meaning is the logical conjunction of the inequalities between adjacent terms. For example, the defining condiction of a zigzag poset is written as *a*_{1} < *a*_{2} > *a*_{3} < *a*_{4} > *a*_{5} < *a*_{6} > ... . Mixed chained notation is used more often with compatible relations, like <, =, a < *b* = *c* d means that *a* < *b*, *b* = *c*, and *c* d. This notation exists in a few programming languages such as Python. In contrast, in programming languages that provide an ordering on the type of comparison results, such as C, even homogeneous chains may have a completely different meaning.^{[12]}

An inequality is said to be *sharp*, if it cannot be *relaxed* and still be valid in general. Formally, a universally quantified inequality *?* is called sharp if, for every valid universally quantified inequality *?*, if holds, then also holds. For instance, the inequality is sharp, whereas the inequality is not sharp.^{[]}

There are many inequalities between means. For example, for any positive numbers *a*_{1}, *a*_{2}, ..., *a*_{n} we have where

$H={\frac {n}{{\frac {1}{a_{1}}}+{\frac {1}{a_{2}}}+\cdots +{\frac {1}{a_{n}}}}}$ (harmonic mean), $G={\sqrt[{n}]{a_{1}\cdot a_{2}\cdots a_{n}}}$ (geometric mean), $A={\frac {a_{1}+a_{2}+\cdots +a_{n}}{n}}$ (arithmetic mean), $Q={\sqrt {\frac {a_{1}^{2}+a_{2}^{2}+\cdots +a_{n}^{2}}{n}}}$ (quadratic mean).

The Cauchy-Schwarz inequality states that for all vectors *u* and *v* of an inner product space it is true that

- $|\langle \mathbf {u} ,\mathbf {v} \rangle |^{2}\leq \langle \mathbf {u} ,\mathbf {u} \rangle \cdot \langle \mathbf {v} ,\mathbf {v} \rangle ,$

where $\langle \cdot ,\cdot \rangle$ is the inner product. Examples of inner products include the real and complex dot product; In Euclidean space *R*^{n} with the standard inner product, the Cauchy-Schwarz inequality is

- $\left(\sum _{i=1}^{n}u_{i}v_{i}\right)^{2}\leq \left(\sum _{i=1}^{n}u_{i}^{2}\right)\left(\sum _{i=1}^{n}v_{i}^{2}\right).$

A "**power inequality**" is an inequality containing terms of the form *a*^{b}, where *a* and *b* are real positive numbers or variable expressions. They often appear in mathematical olympiads exercises.

- For any real
*x*,

- $e^{x}\geq 1+x.$

- If
*x*> 0 and*p*> 0, then

- ${\frac {x^{p}-1}{p}}\geq \ln(x)\geq {\frac {1-{\frac {1}{x^{p}}}}{p}}.$

- In the limit of
*p*-> 0, the upper and lower bounds converge to ln(*x*).

- If
*x*> 0, then

- $x^{x}\geq \left({\frac {1}{e}}\right)^{\frac {1}{e}}.$

- If
*x*> 0, then

- $x^{x^{x}}\geq x.$

- If
*x*,*y*,*z*> 0, then

- $\left(x+y\right)^{z}+\left(x+z\right)^{y}+\left(y+z\right)^{x}>2.$

- For any real distinct numbers
*a*and*b*,

- ${\frac {e^{b}-e^{a}}{b-a}}>e^{(a+b)/2}.$

- If
*x*,*y*> 0 and 0 <*p*< 1, then

- $x^{p}+y^{p}>\left(x+y\right)^{p}.$

- If
*x*,*y*,*z*> 0, then

- $x^{x}y^{y}z^{z}\geq \left(xyz\right)^{(x+y+z)/3}.$

- If
*a*,*b*> 0, then^{[13]}

- $a^{a}+b^{b}\geq a^{b}+b^{a}.$

- If
*a*,*b*> 0, then^{[14]}

- $a^{ea}+b^{eb}\geq a^{eb}+b^{ea}.$

- If
*a*,*b*,*c*> 0, then

- $a^{2a}+b^{2b}+c^{2c}\geq a^{2b}+b^{2c}+c^{2a}.$

- If
*a*,*b*> 0, then

- $a^{b}+b^{a}>1.$

Mathematicians often use inequalities to bound quantities for which exact formulas cannot be computed easily. Some inequalities are used so often that they have names:

- Azuma's inequality
- Bernoulli's inequality
- Bell's inequality
- Boole's inequality
- Cauchy-Schwarz inequality
- Chebyshev's inequality
- Chernoff's inequality
- Cramér-Rao inequality
- Hoeffding's inequality
- Hölder's inequality
- Inequality of arithmetic and geometric means
- Jensen's inequality
- Kolmogorov's inequality
- Markov's inequality
- Minkowski inequality
- Nesbitt's inequality
- Pedoe's inequality
- Poincaré inequality
- Samuelson's inequality
- Triangle inequality

The set of complex numbers C with its operations of addition and multiplication is a field, but it is impossible to define any relation (C, +, ×, becomes an ordered field. To make

- if , then ;
- if and , then .

Because total order, for any number *a*, either or

However, an operation *a* b, then "). Sometimes the lexicographical order definition is used:

- , if
- , or
- and

It can easily be proven that for this definition implies .

Inequality relationships similar to those defined above can also be defined for column vectors. If we let the vectors $x,y\in \mathbb {R} ^{n}$ (meaning that $x=(x_{1},x_{2},\ldots ,x_{n})^{\mathsf {T}}$ and $y=(y_{1},y_{2},\ldots ,y_{n})^{\mathsf {T}}$, where $x_{i}$ and $y_{i}$ are real numbers for $i=1,\ldots ,n$), we can define the following relationships:

- $x=y$, if $x_{i}=y_{i}$ for $i=1,\ldots ,n$.
- $x<y$, if $x_{i}<y_{i}$ for $i=1,\ldots ,n$.
- $x\leq y$, if $x_{i}\leq y_{i}$ for $i=1,\ldots ,n$ and $x\neq y$.
- $x\leqq y$, if $x_{i}\leq y_{i}$ for $i=1,\ldots ,n$.

Similarly, we can define relationships for $x>y$, $x\geq y$, and $x\geqq y$. This notation is consistent with that used by Matthias Ehrgott in *Multicriteria Optimization* (see References).

The trichotomy property (as stated above) is not valid for vector relationships. For example, when $x=(2,5)^{\mathsf {T}}$ and $y=(3,4)^{\mathsf {T}}$, there exists no valid inequality relationship between these two vectors. Also, a multiplicative inverse would need to be defined on a vector before this property could be considered. However, for the rest of the aforementioned properties, a parallel property for vector inequalities exists.

Systems of linear inequalities can be simplified by Fourier-Motzkin elimination.^{[15]}

The cylindrical algebraic decomposition is an algorithm that allows testing whether a system of polynomial equations and inequalities has solutions, and, if solutions exist, describing them. The complexity of this algorithm is doubly exponential in the number of variables. It is an active research domain to design algorithms that are more efficient in specific cases.

- Binary relation
- Bracket (mathematics), for the use of similar signs as brackets
- Inclusion (set theory)
- Inequation
- Interval (mathematics)
- List of inequalities
- List of triangle inequalities
- Partially ordered set
- Relational operators, used in programming languages to denote inequality

**^**"The Definitive Glossary of Higher Mathematical Jargon -- Inequality".*Math Vault*. 2019-08-01. Retrieved .- ^
^{a}^{b}"Inequality Definition (Illustrated Mathematics Dictionary)".*www.mathsisfun.com*. Retrieved . - ^
^{a}^{b}"Inequality".*www.learnalberta.ca*. Retrieved . **^**"Absolutely continuous measures - Encyclopedia of Mathematics".*www.encyclopediaofmath.org*. Retrieved .**^**Weisstein, Eric W. "Much Greater".*mathworld.wolfram.com*. Retrieved .**^**Drachman, Bryon C.; Cloud, Michael J. (2006).*Inequalities: With Applications to Engineering*. Springer Science & Business Media. pp. 2-3. ISBN 0-3872-2626-5.**^**"ProvingInequalities".*www.cs.yale.edu*. Retrieved .**^**Simovici, Dan A. & Djeraba, Chabane (2008). "Partially Ordered Sets".*Mathematical Tools for Data Mining: Set Theory, Partial Orders, Combinatorics*. Springer. ISBN 9781848002012.**^**Weisstein, Eric W. "Partially Ordered Set".*mathworld.wolfram.com*. Retrieved .**^**Feldman, Joel (2014). "Fields" (PDF).*math.ubc.ca*. Retrieved .**^**Stewart, Ian (2007).*Why Beauty Is Truth: The History of Symmetry*. Hachette UK. p. 106. ISBN 0-4650-0875-5.**^**Brian W. Kernighan and Dennis M. Ritchie (Apr 1988).*The C Programming Language*. Prentice Hall Software Series (2nd ed.). Englewood Cliffs/NJ: Prentice Hall. ISBN 0131103628. Here: Sect.A.7.9*Relational Operators*, p.167: Quote: "a<b<c is parsed as (a<b)<c"**^**Laub, M.; Ilani, Ishai (1990). "E3116".*The American Mathematical Monthly*.**97**(1): 65-67. doi:10.2307/2324012. JSTOR 2324012.**^**Manyama, S. (2010). "Solution of One Conjecture on Inequalities with Power-Exponential Functions" (PDF).*Australian Journal of Mathematical Analysis and Applications*.**7**(2): 1.**^**Gärtner, Bernd; Matou?ek, Ji?í (2006).*Understanding and Using Linear Programming*. Berlin: Springer. ISBN 3-540-30697-8.

- Hardy, G., Littlewood J. E., Pólya, G. (1999).
*Inequalities*. Cambridge Mathematical Library, Cambridge University Press. ISBN 0-521-05206-8.CS1 maint: multiple names: authors list (link) - Beckenbach, E. F., Bellman, R. (1975).
*An Introduction to Inequalities*. Random House Inc. ISBN 0-394-01559-2.CS1 maint: multiple names: authors list (link) - Drachman, Byron C., Cloud, Michael J. (1998).
*Inequalities: With Applications to Engineering*. Springer-Verlag. ISBN 0-387-98404-6.CS1 maint: multiple names: authors list (link) - Grinshpan, A. Z. (2005), "General inequalities, consequences, and applications",
*Advances in Applied Mathematics*,**34**(1): 71-100, doi:10.1016/j.aam.2004.05.001 - Murray S. Klamkin. "'Quickie' inequalities" (PDF).
*Math Strategies*. - Arthur Lohwater (1982). "Introduction to Inequalities". Online e-book in PDF format.
- Harold Shapiro (2005). "Mathematical Problem Solving".
*The Old Problem Seminar*. Kungliga Tekniska högskolan. - "3rd USAMO". Archived from the original on 2008-02-03.
- Pachpatte, B. G. (2005).
*Mathematical Inequalities*. North-Holland Mathematical Library.**67**(first ed.). Amsterdam, The Netherlands: Elsevier. ISBN 0-444-51795-2. ISSN 0924-6509. MR 2147066. Zbl 1091.26008. - Ehrgott, Matthias (2005).
*Multicriteria Optimization*. Springer-Berlin. ISBN 3-540-21398-8. - Steele, J. Michael (2004).
*The Cauchy-Schwarz Master Class: An Introduction to the Art of Mathematical Inequalities*. Cambridge University Press. ISBN 978-0-521-54677-5.

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Inequality_(mathematics)

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