# Inequality (mathematics)

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 (resp., strictly greater than)*b*.

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*).

- ("not greater than" can also be represented by
*a**b*, the symbol for "greater than" bisected by a vertical line, "not". The same is true for "not less then" and*a**b.*)

If the values in question are elements of an ordered set, such as the integers or the real numbers, they can be compared in size. On the other hand, 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, or even that they can be compared in size.

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.

## Properties on the number line

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

### Converse

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

*a*≤*b*and*b*≥*a*are equivalent.

### Transitivity

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*.

### Addition and subtraction

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.

### Multiplication and division

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*.

### Additive inverse

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

- If
*a*≤*b*, then −*a*≥ −*b*.

### Multiplicative inverse

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 1/*a*≥ 1/*b*.

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 1/*a*≥ 1/*b*> 0. - If
*a*≤*b*< 0, then 0 > 1/*a*≥ 1/*b*. - If
*a*< 0 <*b*, then 1/*a*< 0 < 1/*b*.

### Applying a function to both sides

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

## Formal Definitions and Generalizations

A (non-strict) **partial order** is a binary relation ≤ over a 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).

### Ordered fields

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

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

Both (**Q**, +, ×, ≤) and (**R**, +, ×, ≤) are ordered fields, but ≤ cannot be defined in order to make (**C**, +, ×, ≤) an 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]

## Chained notation

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

*a*<

*b*and

*b*<

*c*", from which, by the transitivity property above, it also follows that

*a*<

*c*. By the above laws, one can add or subtract the same number to all three terms, or multiply or divide all three terms by same nonzero number and reverse all inequalities if that number is negative. Hence, for example,

*a*<

*b*+

*e*<

*c*is equivalent to

*a*−

*e*<

*b*<

*c*−

*e*.

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

*a*

_{i}≤

*a*

_{i+1}for

*i*= 1, 2, ...,

*n*− 1. By transitivity, this condition is equivalent to

*a*

_{i}≤

*a*

_{j}for any 1 ≤

*i*≤

*j*≤

*n*.

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 ≤ 3*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 instance, *a* < *b* = *c* ≤ *d* means that *a* < *b*, *b* = *c*, and *c* ≤ *d*. This notation exists in a few programming languages such as Python.

## Sharp inequalities

An inequality is said to be *sharp*, if it cannot be *relaxed* and still be valid in general. Formally, a universally quantified inequality *φ* is called sharp if, for every valid universally quantified inequality *ψ*, if *ψ* ⇒ *φ* holds, then *ψ* ⇔ *φ* also holds. For instance, the inequality ∀*a* ∈ ℝ. *a*^{2} ≥ 0 is sharp, whereas the inequality ∀*a* ∈ ℝ. *a*^{2} ≥ −1 is not sharp.

## Inequalities between means

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

(harmonic mean), (geometric mean), (arithmetic mean), (quadratic mean).

## Cauchy–Schwarz inequality

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

where 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

## Power inequalities

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.

### Examples

- For any real
*x*,

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

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

- If
*x*> 0, then

- If
*x*≥ 1, then

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

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

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

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

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

- This inequality was solved by I.Ilani in JSTOR,AMM,Vol.97,No.1,1990.

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

- This inequality was solved by S.Manyama in AJMAA,Vol.7,Issue 2,No.1,2010 and by V.Cirtoaje in JNSA, Vol.4, Issue 2, 130–137, 2011.

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

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

## Well-known inequalities

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

## Complex numbers and inequalities

The set of complex numbers with its operations of addition and multiplication is a field, but it is impossible to define any relation ≤ so that becomes an ordered field. To make an ordered field, it would have to satisfy the following two properties:

- if
*a*≤*b*, then*a*+*c*≤*b*+*c*; - if 0 ≤
*a*and 0 ≤*b*, then 0 ≤*a b*.

Because ≤ is a total order, for any number *a*, either 0 ≤ *a* or *a* ≤ 0 (in which case the first property above implies that 0 ≤ −*a*). In either case 0 ≤ *a*^{2}; this means that and ; so and , which means ; contradiction.

However, an operation ≤ can be defined so as to satisfy only the first property (namely, "if *a* ≤ *b*, then *a* + *c* ≤ *b* + *c*"). Sometimes the lexicographical order definition is used:

- , if or and

It can easily be proven that for this definition *a* ≤ *b* implies *a* + *c* ≤ *b* + *c*.

## Vector inequalities

Inequality relationships similar to those defined above can also be defined for column vectors. If we let the vectors (meaning that and , where and are real numbers for ), we can define the following relationships:

- , if for .
- , if for .
- , if for and .
- , if for .

Similarly, we can define relationships for , , and . 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 and , 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.

## General existence theorems

For a general system of polynomial inequalities, one can find a condition for a solution to exist. Firstly, any system of polynomial inequalities can be reduced to a system of quadratic inequalities by increasing the number of variables and equations (for example, by setting a square of a variable equal to a new variable). A single quadratic polynomial inequality in *n* − 1 variables can be written as

where *X* is a vector of the variables , and *A* is a matrix. This has a solution, for example, when there is at least one positive element on the main diagonal of *A*.

Systems of inequalities can be written in terms of matrices *A*, *B*, *C*, etc., and the conditions for existence of solutions can be written as complicated expressions in terms of these matrices. The solution for two polynomial inequalities in two variables tells us whether two conic section regions overlap or are inside each other. The general solution is not known, but such a solution could be theoretically used to solve such unsolved problems as the kissing number problem. However, the conditions would be so complicated as to require a great deal of computing time or clever algorithms.

## See also

- Binary relation
- Bracket (mathematics), for the use of similar ‹ and › signs as brackets
- Fourier–Motzkin elimination
- 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

## References

- "The Definitive Glossary of Higher Mathematical Jargon — Inequality".
*Math Vault*. 2019-08-01. Retrieved 2019-12-03. - "Inequality Definition (Illustrated Mathematics Dictionary)".
*www.mathsisfun.com*. Retrieved 2019-12-03. - "Inequality".
*www.learnalberta.ca*. Retrieved 2019-12-03. - "Absolutely continuous measures - Encyclopedia of Mathematics".
*www.encyclopediaofmath.org*. Retrieved 2019-12-03. - Weisstein, Eric W. "Much Greater".
*mathworld.wolfram.com*. Retrieved 2019-12-03. - 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 2019-12-03. - 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 2019-12-03. - Feldman, Joel (2014). "Fields" (PDF).
*math.ubc.ca*. Retrieved 2019-12-03. - Stewart, Ian (2007).
*Why Beauty Is Truth: The History of Symmetry*. Hachette UK. p. 106. ISBN 0-4650-0875-5.

## Sources

- 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, 1972–1985). "Mathematical Problem Solving".
*The Old Problem Seminar*. Kungliga Tekniska högskolan. Check date values in:`|date=`

(help) - "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.

## External links

Wikimedia Commons has media related to .Inequalities (mathematics) |

- Hazewinkel, Michiel, ed. (2001) [1994], "Inequality",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - Graph of Inequalities by Ed Pegg, Jr., Wolfram Demonstrations Project.
- AoPS Wiki entry about Inequalities