Absolute value

In mathematics, the absolute value or modulus of a real number , denoted , is the non-negative
value of without regard to its sign. Namely, if is a positive number, and if
is negative (in which case negating makes positive), and . For example, the absolute
value of 3 is 3, and the absolute value of −3 is also 3. The absolute value of a number may be
thought of as its distance from zero.

The graph of the absolute value

function for real numbers

The absolute value of a number may

be thought of as its distance from
zero.

Generalisations of the absolute value for real numbers occur in a wide variety of mathematical
settings. For example, an absolute value is also defined for the complex numbers, the quaternions,
ordered rings, fields and vector spaces. The absolute value is closely related to the notions of
magnitude, distance, and norm in various mathematical and physical contexts.

Terminology and notation

In 1806, Jean-Robert Argand introduced the term module, meaning unit of measure in French,
specifically for the complex absolute value,[1][2] and it was borrowed into English in 1866 as the Latin
equivalent modulus.[1] The term absolute value has been used in this sense from at least 1806 in
French[3] and 1857 in English.[4] The notation | x|, with a vertical bar on each side, was introduced
by Karl Weierstrass in 1841.[5] Other names for absolute value include numerical value[1] and
magnitude.[1] In programming languages and computational software packages, the absolute value
of is generally represented by abs(x) , or a similar expression.
The vertical bar notation also appears in a number of other mathematical contexts: for example,
when applied to a set, it denotes its cardinality; when applied to a matrix, it denotes its determinant.
Vertical bars denote the absolute value only for algebraic objects for which the notion of an
absolute value is defined, notably an element of a normed division algebra, for example a real
number, a complex number, or a quaternion. A closely related but distinct notation is the use of
vertical bars for either the Euclidean norm[6] or sup norm[7] of a vector in , although double
vertical bars with subscripts ( and , respectively) are a more common and less
ambiguous notation.

Definition and properties

Real numbers

For any real number , the absolute value or modulus of is denoted by , with a vertical bar on
each side of the quantity, and is defined as[8]

The absolute value of is thus always either a positive number or zero, but never negative. When
itself is negative ( ), then its absolute value is necessarily positive ( ).

From an analytic geometry point of view, the absolute value of a real number is that number's
distance from zero along the real number line, and more generally the absolute value of the
difference of two real numbers (their absolute difference) is the distance between them.[9] The
notion of an abstract distance function in mathematics can be seen to be a generalisation of the
absolute value of the difference (see "Distance" below).

Since the square root symbol represents the unique positive square root, when applied to a positive
number, it follows that

This is equivalent to the definition above, and may be used as an alternative definition of the
absolute value of real numbers.[10]

The absolute value has the following four fundamental properties ( , are real numbers), that are
used for generalization of this notion to other domains:
 Non-negativity
Positive-definiteness
Multiplicativity
Subadditivity, specifically the triangle inequality

Non-negativity, positive definiteness, and multiplicativity are readily apparent from the definition. To
see that subadditivity holds, first note that where , with its sign chosen
to make the result positive. Now, since and , it follows that, whichever
of is the value of , one has for all real . Consequently,
, as desired.

Some additional useful properties are given below. These are either immediate consequences of the
definition or implied by the four fundamental properties above.

Idempotence (the absolute value of the absolute value is

the absolute value)
Evenness (reflection symmetry of the graph)
Identity of indiscernibles (equivalent to positive-
definiteness)
Triangle inequality (equivalent to subadditivity)

(if ) Preservation of division (equivalent to multiplicativity)

Reverse triangle inequality (equivalent to subadditivity)

Two other useful properties concerning inequalities are:

or

These relations may be used to solve inequalities involving absolute values. For example:

The absolute value, as "distance from zero", is used to define the absolute difference between
arbitrary real numbers, the standard metric on the real numbers.
Complex numbers

The absolute value of a

complex number is the distance
of from the origin. It is also seen in
the picture that and its
complex conjugate have the same
absolute value.

Since the complex numbers are not ordered, the definition given at the top for the real absolute
value cannot be directly applied to complex numbers. However, the geometric interpretation of the
absolute value of a real number as its distance from 0 can be generalised. The absolute value of a
complex number is defined by the Euclidean distance of its corresponding point in the complex
plane from the origin. This can be computed using the Pythagorean theorem: for any complex
number

where and are real numbers, the absolute value or modulus of is denoted and is defined
by[11]

the Pythagorean addition of and , where and denote the real and
imaginary parts of , respectively. When the imaginary part is zero, this coincides with the
definition of the absolute value of the real number .

When a complex number is expressed in its polar form as its absolute value is

Since the product of any complex number and its complex conjugate , with the same
absolute value, is always the non-negative real number , the absolute value of a complex
number is the square root of which is therefore called the absolute square or squared
modulus of :

This generalizes the alternative definition for reals: .

The complex absolute value shares the four fundamental properties given above for the real
absolute value. The identity is a special case of multiplicativity that is often useful by
itself.

Absolute value function

The graph of the absolute value function for real numbers

Composition of absolute value with a cubic

function in different orders
The real absolute value function is continuous everywhere. It is differentiable everywhere except for
x = 0. It is monotonically decreasing on the interval (−∞, 0] and monotonically increasing on the
interval [0, +∞). Since a real number and its opposite have the same absolute value, it is an even
function, and is hence not invertible. The real absolute value function is a piecewise linear, convex
function.

For both real and complex numbers the absolute value function is idempotent (meaning that the
absolute value of any absolute value is itself).

Relationship to the sign function

The absolute value function of a real number returns its value irrespective of its sign, whereas the
sign (or signum) function returns a number's sign irrespective of its value. The following equations
show the relationship between these two functions:

or

and for x ≠ 0,

Relationship to the max and min functions

Let , then

and

Derivative

The real absolute value function has a derivative for every x ≠ 0, but is not differentiable at x = 0. Its
derivative for x ≠ 0 is given by the step function:[12][13]
The real absolute value function is an example of a continuous function that achieves a global
minimum where the derivative does not exist.

The subdifferential of | x| at x = 0 is the interval [−1, 1].[14]

The complex absolute value function is continuous everywhere but complex differentiable nowhere
because it violates the Cauchy–Riemann equations.[12]

The second derivative of | x| with respect to x is zero everywhere except zero, where it does not
exist. As a generalised function, the second derivative may be taken as two times the Dirac delta
function.

Antiderivative

The antiderivative (indefinite integral) of the real absolute value function is

where C is an arbitrary constant of integration. This is not a complex antiderivative because

complex antiderivatives can only exist for complex-differentiable (holomorphic) functions, which the
complex absolute value function is not.

Derivatives of compositions

The following two formulae are special cases of the chain rule:

if the absolute value is inside a function, and

if another function is inside the absolute value. In the first case, the derivative is always
discontinuous at in the first case and where in the second case.
Distance

The absolute value is closely related to the idea of distance. As noted above, the absolute value of a
real or complex number is the distance from that number to the origin, along the real number line,
for real numbers, or in the complex plane, for complex numbers, and more generally, the absolute
value of the difference of two real or complex numbers is the distance between them.

The standard Euclidean distance between two points

and

in Euclidean n-space is defined as:

This can be seen as a generalisation, since for and real, i.e. in a 1-space, according to the
alternative definition of the absolute value,

and for and complex numbers, i.e. in a 2-space,

The above shows that the "absolute value"-distance, for real and complex numbers, agrees with the
standard Euclidean distance, which they inherit as a result of considering them as one and two-
dimensional Euclidean spaces, respectively.

The properties of the absolute value of the difference of two real or complex numbers: non-
negativity, identity of indiscernibles, symmetry and the triangle inequality given above, can be seen
to motivate the more general notion of a distance function as follows:

A real valued function d on a set X × X is called a metric (or a distance function) on X, if it satisfies
the following four axioms:[15]
 Non-negativity
Identity of indiscernibles
Symmetry
Triangle inequality

Generalizations

Ordered rings

The definition of absolute value given for real numbers above can be extended to any ordered ring.
That is, if a is an element of an ordered ring R, then the absolute value of a, denoted by | a|, is
defined to be:[16]

where −a is the additive inverse of a, 0 is the additive identity, and < and ≥ have the usual meaning
with respect to the ordering in the ring.

Fields

The four fundamental properties of the absolute value for real numbers can be used to generalise
the notion of absolute value to an arbitrary field, as follows.

A real-valued function v on a field F is called an absolute value (also a modulus, magnitude, value, or
valuation)[17] if it satisfies the following four axioms:

Non-negativity

Positive-definiteness

Multiplicativity

Subadditivity or the triangle inequality

Where 0 denotes the additive identity of F. It follows from positive-definiteness and multiplicativity
that v(1) = 1, where 1 denotes the multiplicative identity of F. The real and complex absolute values
defined above are examples of absolute values for an arbitrary field.

If v is an absolute value on F, then the function d on F × F, defined by d(a, b) = v(a − b), is a metric
and the following are equivalent:

d satisfies the ultrametric inequality for all x, y, z in F.

is bounded in R.

for every .

for all .

for all .

An absolute value which satisfies any (hence all) of the above conditions is said to be non-
Archimedean, otherwise it is said to be Archimedean.[18]

Vector spaces

Again the fundamental properties of the absolute value for real numbers can be used, with a slight
modification, to generalise the notion to an arbitrary vector space.

A real-valued function on a vector space V over a field F, represented as ‖ · ‖, is called an absolute

value, but more usually a norm, if it satisfies the following axioms:

For all a in F, and v, u in V,

Non-negativity

Positive-definiteness

Positive homogeneity or positive scalability

Subadditivity or the triangle inequality

The norm of a vector is also called its length or magnitude.

In the case of Euclidean space , the function defined by

is a norm called the Euclidean norm. When the real numbers are considered as the one-
dimensional vector space , the absolute value is a norm, and is the p-norm (see Lp space) for
any p. In fact the absolute value is the "only" norm on , in the sense that, for every norm ‖ · ‖ on
, ‖ x ‖ = ‖ 1 ‖ ⋅ | x |.

The complex absolute value is a special case of the norm in an inner product space, which is
identical to the Euclidean norm when the complex plane is identified as the Euclidean plane .

Composition algebras

Every composition algebra A has an involution x → x* called its conjugation. The product in A of an
element x and its conjugate x* is written N(x) = x x* and called the norm of x.

The real numbers , complex numbers , and quaternions are all composition algebras with
norms given by definite quadratic forms. The absolute value in these division algebras is given by
the square root of the composition algebra norm.

In general the norm of a composition algebra may be a quadratic form that is not definite and has
null vectors. However, as in the case of division algebras, when an element x has a non-zero norm,
then x has a multiplicative inverse given by x*/N(x).

See also

Least absolute values

Notes

1. Oxford English Dictionary, Draft Revision, June 2008

2. Nahin, O'Connor and Robertson (http://www-history.mcs.st-andrews.ac.uk/Mathematicians/Ar

gand.html) , and functions.Wolfram.com. (http://functions.wolfram.com/ComplexComponent
s/Abs/35/) ; for the French sense, see Littré, 1877

3. Lazare Nicolas M. Carnot, Mémoire sur la relation qui existe entre les distances respectives de
cinq point quelconques pris dans l'espace, p. 105 at Google Books (https://books.google.com/b
ooks?id=YyIOAAAAQAAJ&pg=PA105)
 4. James Mill Peirce, A Text-book of Analytic Geometry at Internet Archive (https://archive.org/det
ails/atextbookanalyt00peirgoog/page/n60) . The oldest citation in the 2nd edition of the
Oxford English Dictionary is from 1907. The term absolute value is also used in contrast to
relative value.

5. Nicholas J. Higham, Handbook of writing for the mathematical sciences, SIAM. ISBN 0-89871-
420-6, p. 25

6. Spivak, Michael (1965). Calculus on Manifolds. Boulder, CO: Westview. p. 1. ISBN 0805390219.

7. Munkres, James (1991). Analysis on Manifolds. Boulder, CO: Westview. p. 4. ISBN 0201510359.

8. Mendelson, p. 2 (https://books.google.com/books?id=A8hAm38zsCMC&pg=PA2) .

9. Smith, Karl (2013). Precalculus: A Functional Approach to Graphing and Problem Solving (https://
books.google.com/books?id=ZUJbVQN37bIC&pg=PA8) . Jones & Bartlett Publishers. p. 8.
ISBN 978-0-7637-5177-7.

10. Stewart, James B. (2001). Calculus: concepts and contexts. Australia: Brooks/Cole. p. A5.
ISBN 0-534-37718-1.

11. González, Mario O. (1992). Classical Complex Analysis (https://books.google.com/books?id=nc

xL7EFr7GsC&pg=PA19) . CRC Press. p. 19. ISBN 9780824784157.

12. "Weisstein, Eric W. Absolute Value. From MathWorld – A Wolfram Web Resource" (http://mathw
orld.wolfram.com/AbsoluteValue.html) .

13. Bartle and Sherbert, p. 163

14. Peter Wriggers, Panagiotis Panatiotopoulos, eds., New Developments in Contact Problems,
1999, ISBN 3-211-83154-1, p. 31–32 (https://books.google.com/books?id=tiBtC4GmuKcC&pg=
PA31)

15. These axioms are not minimal; for instance, non-negativity can be derived from the other three:
0 = d(a, a) ≤ d(a, b) + d(b, a) = 2d(a, b).

16. Mac Lane, p. 264 (https://books.google.com/books?id=L6FENd8GHIUC&pg=PA264) .

17. Shechter, p. 260 (https://books.google.com/books?id=eqUv3Bcd56EC&pg=PA260) . This

meaning of valuation is rare. Usually, a valuation is the logarithm of the inverse of an absolute
value

18. Shechter, pp. 260–261 (https://books.google.com/books?id=eqUv3Bcd56EC&pg=PA260) .

References

Bartle; Sherbert; Introduction to real analysis (4th ed.), John Wiley & Sons, 2011 ISBN 978-0-471-
43331-6.

Nahin, Paul J.; An Imaginary Tale; Princeton University Press; (hardcover, 1998). ISBN 0-691-
02795-1.

Mac Lane, Saunders, Garrett Birkhoff, Algebra, American Mathematical Soc., 1999. ISBN 978-0-
8218-1646-2.

Mendelson, Elliott, Schaum's Outline of Beginning Calculus, McGraw-Hill Professional, 2008.

ISBN 978-0-07-148754-2.

O'Connor, J.J. and Robertson, E.F.; "Jean Robert Argand" (http://www-history.mcs.st-andrews.ac.u

k/Mathematicians/Argand.html) .

Schechter, Eric; Handbook of Analysis and Its Foundations, pp. 259–263, "Absolute Values" (http
s://books.google.com/books?id=eqUv3Bcd56EC&pg=PA259) , Academic Press (1997) ISBN 0-
12-622760-8.

External links

"Absolute value" (https://www.encyclopediaofmath.org/index.php?title=Absolute_value) .

Encyclopedia of Mathematics. EMS Press. 2001 [1994].

absolute value (https://planetmath.org/AbsoluteValue) at PlanetMath.

Weisstein, Eric W. "Absolute Value" (https://mathworld.wolfram.com/AbsoluteValue.html) .

MathWorld.