# Exponential function – Wikipedia

Mathematical function, denoted exp(x) or e^x

The **exponential function** is a mathematical function denoted by

or

${displaystyle e^{x}}$ (where the argument x is written as an exponent). Unless otherwise specified, the term generally refers to the positive-valued function of a real variable, although it can be extended to the complex numbers or generalized to other mathematical objects like matrices or Lie algebras. The exponential function originated from the notion of exponentiation (repeated multiplication), but modern definitions (there are several equivalent characterizations) allow it to be rigorously extended to all real arguments, including irrational numbers. Its ubiquitous occurrence in pure and applied mathematics led mathematician Walter Rudin to opine that the exponential function is “the most important function in mathematics”.^{[1]}

The exponential function satisfies the exponentiation identity

which, along with the definition

${displaystyle e=exp(1)}$, shows that

${displaystyle e^{n}=underbrace {etimes cdots times e} _{n{text{ factors}}}}$for positive integers n, and relates the exponential function to the elementary notion of exponentiation. The base of the exponential function, its value at 1,

${displaystyle e=exp(1)}$, is a ubiquitous mathematical constant called Euler’s number.

While other continuous nonzero functions

${displaystyle f:mathbb {R} to mathbb {R} }$ that satisfy the exponentiation identity are also known as *exponential functions*, the exponential function exp is the unique real-valued function of a real variable whose derivative is itself and whose value at 0 is 1; that is,

for all real x, and

${displaystyle exp(0)=1.}$ Thus, exp is sometimes called the **natural exponential function** to distinguish it from these other exponential functions, which are the functions of the form

where the base b is a positive real number. The relation

${displaystyle b^{x}=e^{xln b}}$for positive b and real or complex x establishes a strong relationship between these functions, which explains this ambiguous terminology.

The real exponential function can also be defined as a power series. This power series definition is readily extended to complex arguments to allow the complex exponential function

${displaystyle exp :mathbb {C} to mathbb {C} }$to be defined. The complex exponential function takes on all complex values except for 0 and is closely related to the complex trigonometric functions, as shown by Euler’s formula.

Motivated by more abstract properties and characterizations of the exponential function, the exponential can be generalized to and defined for entirely different kinds of mathematical objects (for example, a square matrix or a Lie algebra).

In applied settings, exponential functions model a relationship in which a constant change in the independent variable gives the same proportional change (that is, percentage increase or decrease) in the dependent variable. This occurs widely in the natural and social sciences, as in a self-reproducing population, a fund accruing compound interest, or a growing body of manufacturing expertise. Thus, the exponential function also appears in a variety of contexts within physics, computer science, chemistry, engineering, mathematical biology, and economics.

The real exponential function is a bijection from

${displaystyle mathbb {R} }$to

${displaystyle (0;infty )}$.^{[2]} Its inverse function is the natural logarithm, denoted

^{[nb 1]}

^{[nb 2]} or

because of this, some old texts^{[3]} refer to the exponential function as the **antilogarithm**.

The graph of

${displaystyle y=e^{x}}$ is upward-sloping, and increases faster as x increases.^{[4]} The graph always lies above the x-axis, but becomes arbitrarily close to it for large negative x; thus, the x-axis is a horizontal asymptote. The equation

means that the slope of the tangent to the graph at each point is equal to its y-coordinate at that point.

## Relation to more general exponential functions[edit]

The exponential function

${displaystyle f(x)=e^{x}}$ is sometimes called the *natural exponential function* for distinguishing it from the other exponential functions. The study of any exponential function can easily be reduced to that of the natural exponential function, since per definition, for positive b,

As functions of a real variable, exponential functions are uniquely characterized by the fact that the derivative of such a function is directly proportional to the value of the function. The constant of proportionality of this relationship is the natural logarithm of the base b:

For *b* > 1, the function

is increasing (as depicted for *b* = *e* and *b* = 2), because

*b* < 1, the function is decreasing (as depicted for *b* = 1/2); and for *b* = 1 the function is constant.

Euler’s number *e* = 2.71828… is the unique base for which the constant of proportionality is 1, since

, so that the function is its own derivative:

This function, also denoted as exp *x*, is called the “natural exponential function”,^{[5]}^{[6]} or simply “the exponential function”. Since any exponential function can be written in terms of the natural exponential as

, it is computationally and conceptually convenient to reduce the study of exponential functions to this particular one. The natural exponential is hence denoted by

or

The former notation is commonly used for simpler exponents, while the latter is preferred when the exponent is a complicated expression.

For real numbers c and d, a function of the form

${displaystyle f(x)=ab^{cx+d}}$is also an exponential function, since it can be rewritten as

## Formal definition[edit]

The real exponential function

${displaystyle exp colon mathbb {R} to mathbb {R} }$ can be characterized in a variety of equivalent ways. It is commonly defined by the following power series:^{[1]}^{[7]}

Since the radius of convergence of this power series is infinite, this definition is, in fact, applicable to all complex numbers

${displaystyle zin mathbb {C} }$(see § Complex plane for the extension of

${displaystyle exp x}$to the complex plane). The constant e can then be defined as

${textstyle e=exp 1=sum _{k=0}^{infty }(1/k!).}$

The term-by-term differentiation of this power series reveals that

${textstyle {frac {d}{dx}}exp x=exp x}$for all real x, leading to another common characterization of

${displaystyle exp x}$as the unique solution of the differential equation

satisfying the initial condition

${displaystyle y(0)=1.}$Based on this characterization, the chain rule shows that its inverse function, the natural logarithm, satisfies

${textstyle {frac {d}{dy}}log _{e}y=1/y}$for

${displaystyle y>0,}$${textstyle log _{e}y=int _{1}^{y}{frac {dt}{t}},.}$

This relationship leads to a less common definition of the real exponential function

${displaystyle exp x}$as the solution

${displaystyle y}$to the equation

By way of the binomial theorem and the power series definition, the exponential function can also be defined as the following limit:^{[8]}^{[7]}

It can be shown that every continuous, nonzero solution of the functional equation

${displaystyle f(x+y)=f(x)f(y)}$is an exponential function,

${displaystyle f:mathbb {R} to mathbb {R} , xmapsto e^{kx},}$with

${displaystyle kin mathbb {R} .}$

## Overview[edit]

The exponential function arises whenever a quantity grows or decays at a rate proportional to its current value. One such situation is continuously compounded interest, and in fact it was this observation that led Jacob Bernoulli in 1683^{[9]} to the number

now known as *e*. Later, in 1697, Johann Bernoulli studied the calculus of the exponential function.^{[9]}

If a principal amount of 1 earns interest at an annual rate of *x* compounded monthly, then the interest earned each month is *x*/12 times the current value, so each month the total value is multiplied by (1 + *x*/12), and the value at the end of the year is (1 + *x*/12)^{12}. If instead interest is compounded daily, this becomes (1 + *x*/365)^{365}. Letting the number of time intervals per year grow without bound leads to the limit definition of the exponential function,

first given by Leonhard Euler.^{[8]}

This is one of a number of characterizations of the exponential function; others involve series or differential equations.

From any of these definitions it can be shown that the exponential function obeys the basic exponentiation identity,

which justifies the notation *e*^{x} for exp *x*.

The derivative (rate of change) of the exponential function is the exponential function itself. More generally, a function with a rate of change *proportional* to the function itself (rather than equal to it) is expressible in terms of the exponential function. This function property leads to exponential growth or exponential decay.

The exponential function extends to an entire function on the complex plane. Euler’s formula relates its values at purely imaginary arguments to trigonometric functions. The exponential function also has analogues for which the argument is a matrix, or even an element of a Banach algebra or a Lie algebra.

## Derivatives and differential equations[edit]

The importance of the exponential function in mathematics and the sciences stems mainly from its property as the unique function which is equal to its derivative and is equal to 1 when *x* = 0. That is,

Functions of the form *ce*^{x} for constant *c* are the only functions that are equal to their derivative (by the Picard–Lindelöf theorem). Other ways of saying the same thing include:

- The slope of the graph at any point is the height of the function at that point.
- The rate of increase of the function at
*x*is equal to the value of the function at*x*. - The function solves the differential equation
*y*′ =*y*. - exp is a fixed point of derivative as a functional.

If a variable’s growth or decay rate is proportional to its size—as is the case in unlimited population growth (see Malthusian catastrophe), continuously compounded interest, or radioactive decay—then the variable can be written as a constant times an exponential function of time. Explicitly for any real constant *k*, a function *f*: **R** → **R** satisfies *f*′ = *kf* if and only if *f*(*x*) = *ce*^{kx} for some constant *c*. The constant *k* is called the **decay constant**, **disintegration constant**,^{[10]}**rate constant**,^{[11]} or **transformation constant**.^{[12]}

Furthermore, for any differentiable function *f*, we find, by the chain rule:

## Continued fractions for *e*^{x}[edit]

A continued fraction for *e*^{x} can be obtained via an identity of Euler:

The following generalized continued fraction for *e*^{z} converges more quickly:^{[13]}

or, by applying the substitution *z* = *x*/*y*:

with a special case for *z* = 2:

This formula also converges, though more slowly, for *z* > 2. For example:

## Complex plane[edit]

As in the real case, the exponential function can be defined on the complex plane in several equivalent forms. The most common definition of the complex exponential function parallels the power series definition for real arguments, where the real variable is replaced by a complex one:

Alternatively, the complex exponential function may defined by modelling the limit definition for real arguments, but with the real variable replaced by a complex one:

For the power series definition, term-wise multiplication of two copies of this power series in the Cauchy sense, permitted by Mertens’ theorem, shows that the defining multiplicative property of exponential functions continues to hold for all complex arguments:

The definition of the complex exponential function in turn leads to the appropriate definitions extending the trigonometric functions to complex arguments.

In particular, when *z* = *it* (t real), the series definition yields the expansion

In this expansion, the rearrangement of the terms into real and imaginary parts is justified by the absolute convergence of the series. The real and imaginary parts of the above expression in fact correspond to the series expansions of cos *t* and sin *t*, respectively.

This correspondence provides motivation for *defining* cosine and sine for all complex arguments in terms of

and the equivalent power series:^{[14]}

The functions exp, cos, and sin so defined have infinite radii of convergence by the ratio test and are therefore entire functions (that is, holomorphic on

${displaystyle mathbb {C} }$). The range of the exponential function is

${displaystyle mathbb {C} setminus {0}}$, while the ranges of the complex sine and cosine functions are both

${displaystyle mathbb {C} }$in its entirety, in accord with Picard’s theorem, which asserts that the range of a nonconstant entire function is either all of

${displaystyle mathbb {C} }$, or

${displaystyle mathbb {C} }$excluding one lacunary value.

These definitions for the exponential and trigonometric functions lead trivially to Euler’s formula:

We could alternatively define the complex exponential function based on this relationship. If *z* = *x* + *iy*, where x and y are both real, then we could define its exponential as

where exp, cos, and sin on the right-hand side of the definition sign are to be interpreted as functions of a real variable, previously defined by other means.^{[15]}

For

${displaystyle tin mathbb {R} }$, the relationship

${displaystyle {overline {exp(it)}}=exp(-it)}$holds, so that

${displaystyle left|exp(it)right|=1}$for real

${displaystyle t}$and

${displaystyle tmapsto exp(it)}$ maps the real line (mod 2*π*) to the unit circle in the complex plane. Moreover, going from

to

${displaystyle t=t_{0}}$, the curve defined by

${displaystyle gamma (t)=exp(it)}$traces a segment of the unit circle of length

starting from *z* = 1 in the complex plane and going counterclockwise. Based on these observations and the fact that the measure of an angle in radians is the arc length on the unit circle subtended by the angle, it is easy to see that, restricted to real arguments, the sine and cosine functions as defined above coincide with the sine and cosine functions as introduced in elementary mathematics via geometric notions.

The complex exponential function is periodic with period 2*πi* and

holds for all

${displaystyle zin mathbb {C} ,kin mathbb {Z} }$.

When its domain is extended from the real line to the complex plane, the exponential function retains the following properties:

Extending the natural logarithm to complex arguments yields the complex logarithm log *z*, which is a multivalued function.

We can then define a more general exponentiation:

for all complex numbers *z* and *w*. This is also a multivalued function, even when *z* is real. This distinction is problematic, as the multivalued functions log *z* and *z*^{w} are easily confused with their single-valued equivalents when substituting a real number for *z*. The rule about multiplying exponents for the case of positive real numbers must be modified in a multivalued context:

(*e ^{z}*)

^{w}

_{}≠

*e*, but rather (

^{zw}*e*)

^{z}^{w}

_{}=

*e*

^{(z + 2niπ)w}multivalued over integers

*n*

See failure of power and logarithm identities for more about problems with combining powers.

The exponential function maps any line in the complex plane to a logarithmic spiral in the complex plane with the center at the origin. Two special cases exist: when the original line is parallel to the real axis, the resulting spiral never closes in on itself; when the original line is parallel to the imaginary axis, the resulting spiral is a circle of some radius.

Considering the complex exponential function as a function involving four real variables:

the graph of the exponential function is a two-dimensional surface curving through four dimensions.

Starting with a color-coded portion of the

${displaystyle xy}$domain, the following are depictions of the graph as variously projected into two or three dimensions.

The second image shows how the domain complex plane is mapped into the range complex plane:

- zero is mapped to 1
- the real
- the imaginary
- values with negative real parts are mapped inside the unit circle
- values with positive real parts are mapped outside of the unit circle
- values with a constant real part are mapped to circles centered at zero
- values with a constant imaginary part are mapped to rays extending from zero

The third and fourth images show how the graph in the second image extends into one of the other two dimensions not shown in the second image.

The third image shows the graph extended along the real

${displaystyle x}$axis. It shows the graph is a surface of revolution about the

${displaystyle x}$axis of the graph of the real exponential function, producing a horn or funnel shape.

The fourth image shows the graph extended along the imaginary

${displaystyle y}$axis. It shows that the graph’s surface for positive and negative

${displaystyle y}$values doesn’t really meet along the negative real

${displaystyle v}$axis, but instead forms a spiral surface about the

${displaystyle y}$axis. Because its

${displaystyle y}$ values have been extended to ±2*π*, this image also better depicts the 2π periodicity in the imaginary

value.

### Computation of *a*^{b} where both *a* and *b* are complex[edit]

Complex exponentiation *a*^{b} can be defined by converting *a* to polar coordinates and using the identity (*e*^{ln a})^{b}_{} = *a*^{b}:

However, when *b* is not an integer, this function is multivalued, because *θ* is not unique (see failure of power and logarithm identities).

## Matrices and Banach algebras[edit]

The power series definition of the exponential function makes sense for square matrices (for which the function is called the matrix exponential) and more generally in any unital Banach algebra *B*. In this setting, *e*^{0} = 1, and *e*^{x} is invertible with inverse *e*^{−x} for any *x* in *B*. If *xy* = *yx*, then *e*^{x + y} = *e*^{x}*e*^{y}, but this identity can fail for noncommuting *x* and *y*.

Some alternative definitions lead to the same function. For instance, *e*^{x} can be defined as

Or *e*^{x} can be defined as *f*_{x}(1), where *f*_{x} : **R** → *B* is the solution to the differential equation *df _{x}*/

*dt*(

*t*) =

*x f*

_{x}(

*t*), with initial condition

*f*

_{x}(0) = 1; it follows that

*f*

_{x}(

*t*) =

*e*

^{tx}for every t in

**R**.

## Lie algebras[edit]

Given a Lie group *G* and its associated Lie algebra

, the exponential map is a map

${displaystyle {mathfrak {g}}}$ ↦ *G* satisfying similar properties. In fact, since **R** is the Lie algebra of the Lie group of all positive real numbers under multiplication, the ordinary exponential function for real arguments is a special case of the Lie algebra situation. Similarly, since the Lie group GL(*n*,**R**) of invertible *n* × *n* matrices has as Lie algebra M(*n*,**R**), the space of all *n* × *n* matrices, the exponential function for square matrices is a special case of the Lie algebra exponential map.

The identity exp(*x* + *y*) = exp *x* exp *y* can fail for Lie algebra elements *x* and *y* that do not commute; the Baker–Campbell–Hausdorff formula supplies the necessary correction terms.

## Transcendency[edit]

The function *e*^{z} is not in **C**(*z*) (that is, is not the quotient of two polynomials with complex coefficients).

For *n* distinct complex numbers {*a*_{1}, …, *a*_{n}}, the set {*e*^{a1z}, …, *e*^{anz}} is linearly independent over **C**(*z*).

The function *e*^{z} is transcendental over **C**(*z*).

## Computation[edit]

When computing (an approximation of) the exponential function near the argument 0, the result will be close to 1, and computing the value of the difference

${displaystyle e^{x}-1}$with floating-point arithmetic may lead to the loss of (possibly all) significant figures, producing a large calculation error, possibly even a meaningless result.

Following a proposal by William Kahan, it may thus be useful to have a dedicated routine, often called `expm1`

, for computing *e ^{x}* − 1 directly, bypassing computation of

*e*

^{x}. For example, if the exponential is computed by using its Taylor series

one may use the Taylor series of

${displaystyle e^{x}-1}$:

This was first implemented in 1979 in the Hewlett-Packard HP-41C calculator, and provided by several calculators,^{[16]}^{[17]}operating systems (for example Berkeley UNIX 4.3BSD^{[18]}), computer algebra systems, and programming languages (for example C99).^{[19]}

In addition to base *e*, the IEEE 754-2008 standard defines similar exponential functions near 0 for base 2 and 10:

and

${displaystyle 10^{x}-1}$.

A similar approach has been used for the logarithm (see lnp1).^{[nb 3]}

An identity in terms of the hyperbolic tangent,

gives a high-precision value for small values of *x* on systems that do not implement expm1(*x*).

## See also[edit]

**^**The notation ln*x*is the ISO standard and is prevalent in the natural sciences and secondary education (US). However, some mathematicians (for example, Paul Halmos) have criticized this notation and prefer to use log*x*for the natural logarithm of x.**^**In pure mathematics, the notation log*x*generally refers to the natural logarithm of x or a logarithm in general if the base is immaterial.**^**A similar approach to reduce round-off errors of calculations for certain input values of trigonometric functions consists of using the less common trigonometric functions versine, vercosine, coversine, covercosine, haversine, havercosine, hacoversine, hacovercosine, exsecant and excosecant.

## References[edit]

- ^
^{a}^{b}Rudin, Walter (1987).*Real and complex analysis*(3rd ed.). New York: McGraw-Hill. p. 1. ISBN 978-0-07-054234-1. **^**Meier, John; Smith, Derek (2017-08-07).*Exploring Mathematics*. Cambridge University Press. p. 167. ISBN 978-1-107-12898-9.**^**Converse, Henry Augustus; Durell, Fletcher (1911).*Plane and Spherical Trigonometry*. Durell’s mathematical series. C. E. Merrill Company. p. 12.Inverse Use of a Table of Logarithms; that is, given a logarithm, to find the number corresponding to it, (called its antilogarithm) …

[1]**^**“Exponential Function Reference”.*www.mathsisfun.com*. Retrieved 2020-08-28.**^**Goldstein, Larry Joel; Lay, David C.; Schneider, David I.; Asmar, Nakhle H. (2006).*Brief calculus and its applications*(11th ed.). Prentice–Hall. ISBN 978-0-13-191965-5. (467 pages)**^**Courant; Robbins (1996). Stewart (ed.).*What is Mathematics? An Elementary Approach to Ideas and Methods*(2nd revised ed.). Oxford University Press. p. 448. ISBN 978-0-13-191965-5.*This natural exponential function is identical with its derivative.*This is really the source of all the properties of the exponential function, and the basic reason for its importance in applications…- ^
^{a}^{b}Weisstein, Eric W. “Exponential Function”.*mathworld.wolfram.com*. Retrieved 2020-08-28. - ^
^{a}^{b}Maor, Eli.*e: the Story of a Number*. p. 156. - ^
^{a}^{b}O’Connor, John J.; Robertson, Edmund F. (September 2001). “The number e”.*School of Mathematics and Statistics*. University of St Andrews, Scotland. Retrieved 2011-06-13. **^**Serway (1989, p. 384)**^**Simmons (1972, p. 15)**^**McGraw-Hill (2007)**^**Lorentzen, L.; Waadeland, H. (2008). “A.2.2 The exponential function.”.*Continued Fractions*. Atlantis Studies in Mathematics. Vol. 1. p. 268. doi:10.2991/978-94-91216-37-4. ISBN 978-94-91216-37-4.**^**Rudin, Walter (1976).*Principles of Mathematical Analysis*. New York: McGraw-Hill. p. 182. ISBN 978-0-07-054235-8.**^**Apostol, Tom M. (1974).*Mathematical Analysis*(2nd ed.). Reading, Mass.: Addison Wesley. pp. 19. ISBN 978-0-201-00288-1.**^***HP 48G Series – Advanced User’s Reference Manual (AUR)*(4 ed.). Hewlett-Packard. December 1994 [1993]. HP 00048-90136, 0-88698-01574-2. Retrieved 2015-09-06.**^***HP 50g / 49g+ / 48gII graphing calculator advanced user’s reference manual (AUR)*(2 ed.). Hewlett-Packard. 2009-07-14 [2005]. HP F2228-90010. Retrieved 2015-10-10. [2]**^**Beebe, Nelson H. F. (2017-08-22). “Chapter 10.2. Exponential near zero”.*The Mathematical-Function Computation Handbook – Programming Using the MathCW Portable Software Library*(1 ed.). Salt Lake City, UT, USA: Springer International Publishing AG. pp. 273–282. doi:10.1007/978-3-319-64110-2. ISBN 978-3-319-64109-6. LCCN 2017947446. S2CID 30244721.Berkeley UNIX 4.3BSD introduced the expm1() function in 1987.

**^**Beebe, Nelson H. F. (2002-07-09). “Computation of expm1 = exp(x)−1” (PDF). 1.00. Salt Lake City, Utah, USA: Department of Mathematics, Center for Scientific Computing, University of Utah. Retrieved 2015-11-02.

*McGraw-Hill Encyclopedia of Science & Technology*(10th ed.). New York: McGraw-Hill. 2007. ISBN 978-0-07-144143-8.- Serway, Raymond A.; Moses, Clement J.; Moyer, Curt A. (1989),
*Modern Physics*, Fort Worth: Harcourt Brace Jovanovich, ISBN 0-03-004844-3 - Simmons, George F. (1972),
*Differential Equations with Applications and Historical Notes*, New York: McGraw-Hill, LCCN 75173716

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