Exponentiation

Exponentiation is a mathematical operation, written as bn, involving two numbers, the base b and the exponent or power n, and pronounced as "b raised to the power of n".[1][2] When n is a positive integer, exponentiation corresponds to repeated multiplication of the base: that is, bn is the product of multiplying n bases:[2]

Graphs of y = bx for various bases b:   base 10,   base e,   base 2,   base 1/2. Each curve passes through the point (0, 1) because any nonzero number raised to the power of 0 is 1. At x = 1, the value of y equals the base because any number raised to the power of 1 is the number itself.

The exponent is usually shown as a superscript to the right of the base. In that case, bn is called "b raised to the nth power", "b raised to the power of n",[1] "the nth power of b", "b to the nth power",[3] or most briefly as "b to the nth".

One has b1 = b, and, for any positive integers m and n, one has bnbm = bn+m. To extend this property to non-positive integer exponents, b0 is defined to be 1, and bn (with n a positive integer and b not zero) is defined as 1/bn. In particular, b−1 is equal to 1/b, the reciprocal of b.

The definition of exponentiation can be extended to allow any real or complex exponent. Exponentiation by integer exponents can also be defined for a wide variety of algebraic structures, including matrices.

Exponentiation is used extensively in many fields, including economics, biology, chemistry, physics, and computer science, with applications such as compound interest, population growth, chemical reaction kinetics, wave behavior, and public-key cryptography.

History of the notationEdit

The term power (Latin: potentia, potestas, dignitas) is a mistranslation[4][5] of the ancient Greek δύναμις (dúnamis, here: "amplification"[4]) used by the Greek mathematician Euclid for the square of a line,[6] following Hippocrates of Chios.[7] In The Sand Reckoner, Archimedes discovered and proved the law of exponents, 10a ⋅ 10b = 10a+b, necessary to manipulate powers of 10.[citation needed] In the 9th century, the Persian mathematician Muhammad ibn Mūsā al-Khwārizmī used the terms مَال (māl, "possessions", "property") for a square—the Muslims, "like most mathematicians of those and earlier times, thought of a squared number as a depiction of an area, especially of land, hence property"[8]—and كَعْبَة (kaʿbah, "cube") for a cube, which later Islamic mathematicians represented in mathematical notation as the letters mīm (m) and kāf (k), respectively, by the 15th century, as seen in the work of Abū al-Hasan ibn Alī al-Qalasādī.[9]

In the late 16th century, Jost Bürgi used Roman numerals for exponents.[10]

Nicolas Chuquet used a form of exponential notation in the 15th century, which was later used by Henricus Grammateus and Michael Stifel in the 16th century. The word exponent was coined in 1544 by Michael Stifel.[11][12] Samuel Jeake introduced the term indices in 1696.[6] In the 16th century, Robert Recorde used the terms square, cube, zenzizenzic (fourth power), sursolid (fifth), zenzicube (sixth), second sursolid (seventh), and zenzizenzizenzic (eighth).[8] Biquadrate has been used to refer to the fourth power as well.

Early in the 17th century, the first form of our modern exponential notation was introduced by René Descartes in his text titled La Géométrie; there, the notation is introduced in Book I.[13]

Some mathematicians (such as Isaac Newton) used exponents only for powers greater than two, preferring to represent squares as repeated multiplication. Thus they would write polynomials, for example, as ax + bxx + cx3 + d.

Another historical synonym,[clarification needed] involution, is now rare[14] and should not be confused with its more common meaning.

In 1748, Leonhard Euler introduced variable exponents, and, implicitly, non-integer exponents by writing:

"consider exponentials or powers in which the exponent itself is a variable. It is clear that quantities of this kind are not algebraic functions, since in those the exponents must be constant."[15]

TerminologyEdit

The expression b2 = bb is called "the square of b" or "b squared", because the area of a square with side-length b is b2.

Similarly, the expression b3 = bbb is called "the cube of b" or "b cubed", because the volume of a cube with side-length b is b3.

When it is a positive integer, the exponent indicates how many copies of the base are multiplied together. For example, 35 = 3 ⋅ 3 ⋅ 3 ⋅ 3 ⋅ 3 = 243. The base 3 appears 5 times in the multiplication, because the exponent is 5. Here, 243 is the 5th power of 3, or 3 raised to the 5th power.

The word "raised" is usually omitted, and sometimes "power" as well, so 35 can be simply read "3 to the 5th", or "3 to the 5". Therefore, the exponentiation bn can be expressed as "b to the power of n", "b to the nth power", "b to the nth", or most briefly as "b to the n".

A formula with nested exponentiation, such as 357 (which means 3(57) and not (35)7), is called a tower of powers, or simply a tower.

Integer exponentsEdit

The exponentiation operation with integer exponents may be defined directly from elementary arithmetic operations.

Positive exponentsEdit

The definition of the exponentiation as an iterated multiplication can be formalized by using induction,[16] and this definition can be used as soon one has an associative multiplication:

The base case is

 

and the recurrence is

 

The associativity of multiplication implies that for any positive integers m and n,

 

and

 

Zero exponentEdit

By definition, any nonzero number raised to the 0 power is 1:[17][2]

 

This definition is the only possible that allows extending the formula

 

to zero exponents. It may used in every algebraic structure with a multiplication that has an identity.

Intuitionally,   may be interpreted as the empty product of copies of b. So, the equality   is a special case of the general convention for the empty product.

The case of 00 is more complicated. In contexts where only integer powers are considered, the value 0 is generally assigned to   but, otherwise, the choice of whether to assign it a value and what value to assign may depend on context. For more details, see Zero to the power of zero.

Negative exponentsEdit

Exponentiation with negative exponents is defined by the following identity, which holds for any integer n and nonzero b:

 [2]

Raising 0 to a negative exponent is undefined, but in some circumstances, it may be interpreted as infinity ( ).

This definition of exponentiation with negative exponents is the only one that allows extending the identity   to negative exponents (consider the case  ).

The same definition applies to invertible elements in a multiplicative monoid, that is, an algebraic structure, with an associative multiplication and a multiplicative identity denoted 1 (for example, the square matrices of a given dimension). In particular, in such a structure, the inverse of an invertible element x is standardly denoted  

Identities and propertiesEdit

The following identities, often called exponent rules, hold for all integer exponents, provided that the base is non-zero:[2]

 

Unlike addition and multiplication, exponentiation is not commutative. For example, 23 = 8 ≠ 32 = 9. Also unlike addition and multiplication, exponentiation is not associative. For example, (23)2 = 82 = 64, whereas 2(32) = 29 = 512. Without parentheses, the conventional order of operations for serial exponentiation in superscript notation is top-down (or right-associative), not bottom-up[18][19][20][21] (or left-associative). That is,

 

which, in general, is different from

 

Powers of a sumEdit

The powers of a sum can normally be computed from the powers of the summands by the binomial formula

 

However, this formula is true only if the summands commute (i.e. that ab = ba), which is implied if they belong to a structure that is commutative. Otherwise, if a and b are, say, square matrices of the same size, this formula cannot be used. It follows that in computer algebra, many algorithms involving integer exponents must be changed when the exponentiation bases do not commute. Some general purpose computer algebra systems use a different notation (sometimes ^^ instead of ^) for exponentiation with non-commuting bases, which is then called non-commutative exponentiation.

Combinatorial interpretationEdit

For nonnegative integers n and m, the value of nm is the number of functions from a set of m elements to a set of n elements (see cardinal exponentiation). Such functions can be represented as m-tuples from an n-element set (or as m-letter words from an n-letter alphabet). Some examples for particular values of m and n are given in the following table:

nm The nm possible m-tuples of elements from the set {1, ..., n}
05 = 0 none
14 = 1 (1, 1, 1, 1)
23 = 8 (1, 1, 1), (1, 1, 2), (1, 2, 1), (1, 2, 2), (2, 1, 1), (2, 1, 2), (2, 2, 1), (2, 2, 2)
32 = 9 (1, 1), (1, 2), (1, 3), (2, 1), (2, 2), (2, 3), (3, 1), (3, 2), (3, 3)
41 = 4 (1), (2), (3), (4)
50 = 1 ()

Particular basesEdit

Powers of tenEdit

In the base ten (decimal) number system, integer powers of 10 are written as the digit 1 followed or preceded by a number of zeroes determined by the sign and magnitude of the exponent. For example, 103 = 1000 and 10−4 = 0.0001.

Exponentiation with base 10 is used in scientific notation to denote large or small numbers. For instance, 299792458 m/s (the speed of light in vacuum, in metres per second) can be written as 2.99792458×108 m/s and then approximated as 2.998×108 m/s.

SI prefixes based on powers of 10 are also used to describe small or large quantities. For example, the prefix kilo means 103 = 1000, so a kilometre is 1000 m.

Powers of twoEdit

The first negative powers of 2 are commonly used, and have special names, e.g.: half and quarter.

Powers of 2 appear in set theory, since a set with n members has a power set, the set of all of its subsets, which has 2n members.

Integer powers of 2 are important in computer science. The positive integer powers 2n give the number of possible values for an n-bit integer binary number; for example, a byte may take 28 = 256 different values. The binary number system expresses any number as a sum of powers of 2, and denotes it as a sequence of 0 and 1, separated by a binary point, where 1 indicates a power of 2 that appears in the sum; the exponent is determined by the place of this 1: the nonnegative exponents are the rank of the 1 on the left of the point (starting from 0), and the negative exponents are determined by the rank on the right of the point.

Powers of oneEdit

The powers of one are all one: 1n = 1.

Powers of zeroEdit

If the exponent n is positive (n > 0), the nth power of zero is zero: 0n = 0.

If the exponent n is negative (n < 0), the nth power of zero 0n is undefined, because it must equal   with n > 0, and this would be   according to above.

The expression 00 is either defined as 1, or it is left undefined (see Zero to the power of zero).

Powers of negative oneEdit

If n is an even integer, then (−1)n = 1.

If n is an odd integer, then (−1)n = −1.

Because of this, powers of −1 are useful for expressing alternating sequences. For a similar discussion of powers of the complex number i, see § Powers of complex numbers.

Large exponentsEdit

The limit of a sequence of powers of a number greater than one diverges; in other words, the sequence grows without bound:

bn → ∞ as n → ∞ when b > 1

This can be read as "b to the power of n tends to +∞ as n tends to infinity when b is greater than one".

Powers of a number with absolute value less than one tend to zero:

bn → 0 as n → ∞ when |b| < 1

Any power of one is always one:

bn = 1 for all n if b = 1

Powers of –1 alternate between 1 and –1 as n alternates between even and odd, and thus do not tend to any limit as n grows.

If b < –1, bn, alternates between larger and larger positive and negative numbers as n alternates between even and odd, and thus does not tend to any limit as n grows.

If the exponentiated number varies while tending to 1 as the exponent tends to infinity, then the limit is not necessarily one of those above. A particularly important case is

(1 + 1/n)ne as n → ∞

See § The exponential function below.

Other limits, in particular those of expressions that take on an indeterminate form, are described in § Limits of powers below.

Power functionsEdit

 
Power functions for  
 
Power functions for  

Real functions of the form  , where  , are sometimes called power functions.[citation needed] When   is an integer and  , two primary families exist: for   even, and for   odd. In general for  , when   is even   will tend towards positive infinity with increasing  , and also towards positive infinity with decreasing  . All graphs from the family of even power functions have the general shape of  , flattening more in the middle as   increases.[22] Functions with this kind of symmetry ( ) are called even functions.

When   is odd,  's asymptotic behavior reverses from positive   to negative  . For  ,   will also tend towards positive infinity with increasing  , but towards negative infinity with decreasing  . All graphs from the family of odd power functions have the general shape of  , flattening more in the middle as   increases and losing all flatness there in the straight line for  . Functions with this kind of symmetry ( ) are called odd functions.

For  , the opposite asymptotic behavior is true in each case.[22]

List of whole-number powersEdit

n n2 n3 n4 n5 n6 n7 n8 n9 n10
2 4 8 16 32 64 128 256 512 1024
3 9 27 81 243 729 2187 6561 19683 59049
4 16 64 256 1024 4096 16384 65536 262144 1048576
5 25 125 625 3125 15625 78125 390625 1953125 9765625
6 36 216 1296 7776 46656 279936 1679616 10077696 60466176
7 49 343 2401 16807 117649 823543 5764801 40353607 282475249
8 64 512 4096 32768 262144 2097152 16777216 134217728 1073741824
9 81 729 6561 59049 531441 4782969 43046721 387420489 3486784401
10 100 1000 10000 100000 1000000 10000000 100000000 1000000000 10000000000

Rational exponentsEdit

 
From top to bottom: x1/8, x1/4, x1/2, x1, x2, x4, x8.

If x is a nonnegative real number, and n is a positive integer,   or   denotes the unique positive real nth root of x, that is, the unique positive real number y such that  

If x is a positive real number, and   is a rational number, with p and q ≠ 0 integers, then   is defined as

 

The equality on the right may be derived by setting   and writing  

If r is a positive rational number,   by definition.

All these definitions are required for extending the identity   to rational exponents.

On the other hand, there are problems with the extension of these definitions to bases that are not positive real numbers. For example, a negative real number has a real nth root, which is negative if n is odd, and no real root if n is even. In the latter case, whichever complex nth root one chooses for   the identity   cannot be satisfied. For example,

 

See § Real exponents and § Powers of complex numbers for details on the way these problems may be handled.

Real exponentsEdit

For positive real numbers, exponentiation to real powers can be defined in two equivalent ways, either by extending the rational powers to reals by continuity (§ Limits of rational exponents, below), or in terms of the logarithm of the base and the exponential function (§ Powers via logarithms, below). The result is always a positive real number, and the identities and properties shown above for integer exponents remain true with these definitions for real exponents. The second definition is more commonly used, since it generalizes straightforwardly to complex exponents.

On the other hand, exponentiation to a real power of a negative real number is much more difficult to define consistently, as it may be non-real and have several values (see § Real exponents with negative bases). One may choose one of these values, called the principal value, but there is no choice of the principal value for which the identity

 

is true; see § Failure of power and logarithm identities. Therefore, exponentiation with a basis that is not a positive real number is generally viewed as a multivalued function.

Limits of rational exponentsEdit

 
The limit of e1/n is e0 = 1 when n tends to the infinity.

Since any irrational number can be expressed as the limit of a sequence of rational numbers, exponentiation of a positive real number b with an arbitrary real exponent x can be defined by continuity with the rule[23]

 

where the limit is taken over rational values of r only. This limit exists for every positive b and every real x.

For example, if x = π, the non-terminating decimal representation π = 3.14159... and the monotonicity of the rational powers can be used to obtain intervals bounded by rational powers that are as small as desired, and must contain  

 

So, the upper bounds and the lower bounds of the intervals form two sequences that have the same limit, denoted  

This defines   for every positive b and real x as a continuous function of b and x. See also Well-defined expression.

The exponential functionEdit

The exponential function is often defined as   where   is Euler's number. For avoiding circular reasoning, this definition cannot be used here. So, a definition of the exponential function, denoted   and of Euler's number are given, which are independent from exponentiation. Then a proof is sketched that, if one uses the definition of exponentiation given in preceding sections, one has

 

There are many equivalent ways to define the exponential function, one of them being to define it as the inverse function of the natural logarithm. Precisely, the natural logarithm is the antiderivative of   that takes the value 0 for x = 1:

 

This defines the logarithm as an increasing function from the positive reals to the real numbers. It inverse function, the exponential function, is thus an increasing function from the real numbers to the positive reals, which is commonly denoted exp. One has

 

and the exponential identity

 

for every x and y.

Euler's number can be defined as  . It follows from the preceding equations that   when x is an integer (this results from the repeated-multiplication definition of the exponentiation). If x is real,   results from the definitions given in preceding sections, by using the exponential identity if x is rational, and the continuity of the exponential function otherwise.

The exponential function satisfies the equation

 

As this series converges for every complex value of x this equation allows defining the exponential function, and thus   for any complex argument z. This extended exponential function still satifies the exponential identity, and is commonly used for defining exponentiation for complex base and exponent.

Powers via logarithmsEdit

The definition of ex as the exponential function allows defining bx for every positive real numbers b, in terms of exponential and logarithm function. Specifically, the fact that the natural logarithm ln(x) is the inverse of the exponential function ex means that one has

 

for every b > 0. For preserving the identity   one must have

 

So,   can be used as an alternative definition of bx for any positive real b. This agrees with the definition given above using rational exponents and continuity, with the advantage to extend straightforwardly to any complex exponent.

Complex exponents with a positive real baseEdit

If b is a positive real number, exponentiation with base b and complex exponent is defined by mean of the exponential function with complex argument (see the end of § The exponential function, above) as

 

where   denotes the natural logarithm of b.

This satisfies the identity

 

In general,   is not defined, since bz is not a real number. If a meaning is given to the exponentiation of a complex number (see § Powers of complex numbers, below), one has, in general,

 

unless z is real or w is integer.

Euler's formula,

 

allows expressing the polar form of   in terms of the real and imaginary parts of z, namely

 

where the absolute value of the trigonometric factor is one. This results from

 

Non-integer powers of complex numbersEdit

In the preceding sections, exponentiation with non-integer exponents has been defined for positive real bases only. For other bases, difficulties appear already with the apparently simple case of nth roots, that is, of exponents   where n is a positive integer. Although the general theory of exponentiation with non-integer exponents applies to nth roots, this case deserves to be considered first, since it does not need to use complex logarithms, and is therefore easier to understand.

nth roots of a complex numberEdit

Every nonzero complex number z may be witten in polar form as

 

where   is the absolute value of z, and   is its argument. The argument is defined up to an integer multiple of 2π; this means that, if   is the argument of a complex number, then   is also an argument of the same complex number.

The polar form of the product of two complex numbers is obtained by multiplying the absolute values and adding the arguments. It follows that the polar form of an nth root of a complex number can be obtained by taking the nth root of the absolute value and dividing its argument by n:

 

If   is added to   the complex number in not changed, but this adds   to the argument of the nth root, and provides a new nth root. This can be done n times, and provides the n nth roots of the complex number.

It is usual to choose one of the n nth root as the principal root. The common choice is to choose the nth root for which   that is, the nth root that has the largest real part, and, if they are two, the one with positive imaginary part. This makes the principal nth root a continuous function in the whole complex plane, except for negative real values of the radicand. This function equals the usual nth root for positive real radicands. For negative real radicands, and odd exponents, the principal nth root is not real, although the usual nth root is real. Analytic continuation shows that the principal nth root is the unique complex differentiable function that extends the usual nth root to the complex plane without the nonpositive real numbers.

If the complex number is moved around zero by increasing its argument, after an increment of   the complex number comes back to its initial position, and its nth roots are permuted circularly (they are multiplied by  ). This shows that it is not possible to define a nth root function that is not continuous in the whole complex plane.

Roots of unityEdit

 
The three third roots of 1

The nth roots of unity are the n complex numbers such that wn = 1, where n is a positive integer. They arise in various areas of mathematics, such as in discrete Fourier transform or algebraic solutions of algebraic equations (Lagrange resolvent).

The n nth roots of unity are the n first powers of  , that is   The nth roots of unity that have this generating property are called primitive nth roots of unity; they have the form   with k coprime with n. The unique primitive square root of unity is   the primitive fourth roots of unity are   and  

The nth roots of unity allow expressing all nth roots of a complex number z as the n products of a given nth roots of z with a nth root of unity.

Geometrically, the nth roots of unity lie on the unit circle of the complex plane at the vertices of a regular n-gon with one vertex on the real number 1.

As the number   is the primitive nth root of unity with the smallest positive argument, it is called the principal primitive nth root of unity, sometimes shortened as principal nth root of unity, although this terminology can be confused with the principal value of   which is 1.[24][25][26]

Complex exponentiationEdit

Defining exponentiation with complex bases leads to difficulties that are similar to those described in the preceding section, except that there are, in general, there are infinitely many possible values for  . So, either a principal value is defined, which is not continuous for the values of z that are real and nonpositive, or   is defined as a multivalued function.

In all cases, the complex logarithm is used to define complex exponentiation as

 

where   is the variant of the complex logarithm that is used, which is, a function or a multivalued function such that

 

for every z in its domain of definition.

Principal valueEdit

The principal value of the complex logarithm is the unique function, commonly denoted   such that, for every nonzero complex number z,

 

and the imaginary part of z satisfies

 

The principal value of the complex logarithm is not defined for   it is discontinuous at negative real values of z, and it is holomorphic (that is, complex differentiable) elsewhere. If z is real and positive, the principal value of the complex logarithm is the natural logarithm:  

The principal value of   is defined as   where   is the principal value of the logarithm.

The function   is holomorphic except in the neibourhood of the points where z is real and nonpositive.

If z is real and positive, the principal value of   equals its usual value defined above. If   where n is an integer, this principal value is the same as the one defined above.

Multivalued functionEdit

In some contexts, there is a problem with the discontinuity of the principal values of   and   at the negative real values of z. In this case, it is useful to consider these functions as multivalued functions.

If   denotes one of the values of the multivalued logarithm (typically its principal value), the other values are   where k is any integer. Similarly, if   is one value of the exponentiation, then the other values are given by

 

where k is any integer.

Different values of k give different values of   unless w is a rational number, that is, there is an integer d such that dw is an integer. This results from the periodicity of the exponential function, more specifically, that   if and only if   is an integer multiple of  

If   is a rational number with m and n coprime integers with   then   has exactly n values. In the case   these values are the same as those described in § nth roots of a complex number. If w is an integer, there is only one value that agrees with that of § Integer exponents.

The multivalued exponentiation is holomorphic for   in the sense that its graph consists of several sheets that define each a holomorphic function in the neighborhood of every point. If z varies continuously along a circle around 0, then, after a turn, the value of   has changed of sheet.

ComputationEdit

The canonical form   of   can be computed from the canonical form of z and w. Although this can be described by a single formula, it is clearer to split the computation in several steps.

  • Polar form of z. If   is the canonical form of z (a and b being real), then its polar form is
     
    where   and   (see atan2 for the definition of this function).
  • Logarithm of z. The principal value of this logarithm is   where   denotes the natural logarithm. The other values of the logarithm are obtained by adding   for any integer k.
  • Canonical form of   If   with c and d real, the values of   are
     
    the principal value corresponding to  
  • Final result. Using the identities   and   one gets
     
    with   for the principal value.
ExamplesEdit
  •  
    The polar form of i is   and the values of   are thus
     
    It follows that
     
    So, all values of   are real, the principal one being
     
  •  
    Similarly, the polar form of −2 is   So, the above described method gives the values
     
    In this case, all the values have the same argument   and different absolute values.

In both examples, all values of   have the same argument. More generally, this is true if and only if the real part of w is an integer.

Failure of power and logarithm identitiesEdit

Some identities for powers and logarithms for positive real numbers will fail for complex numbers, no matter how complex powers and complex logarithms are defined as single-valued functions. For example:

  • The identity log(bx) = x ⋅ log b holds whenever b is a positive real number and x is a real number. But for the principal branch of the complex logarithm one has
     

    Regardless of which branch of the logarithm is used, a similar failure of the identity will exist. The best that can be said (if only using this result) is that:

     

    This identity does not hold even when considering log as a multivalued function. The possible values of log(wz) contain those of z ⋅ log w as a proper subset. Using Log(w) for the principal value of log(w) and m, n as any integers the possible values of both sides are:

     
  • The identities (bc)x = bxcx and (b/c)x = bx/cx are valid when b and c are positive real numbers and x is a real number. But, for the principal values, one has
     

    and

     

    On the other hand, when x is an integer, the identities are valid for all nonzero complex numbers.

    If exponentiation is considered as a multivalued function then the possible values of (−1 ⋅ −1)1/2 are {1, −1}. The identity holds, but saying {1} = {(−1 ⋅ −1)1/2} is wrong.
  • The identity (ex)y = exy holds for real numbers x and y, but assuming its truth for complex numbers leads to the following paradox, discovered in 1827 by Clausen:[27] For any integer n, we have:
    1.  
    2.   (taking the  -th power of both sides)
    3.   (using   and expanding the exponent)
    4.   (using  )
    5.   (dividing by e)
    but this is false when the integer n is nonzero. The error is the following: by definition,   is a notation for   a true function, and   is a notation for   which is a multi-valued function. Thus the notation is ambiguous when x = e. Here, before expanding the exponent, the second line should be
     
    Therefore, when expanding the exponent, one has implicitly supposed that   for complex values of z, which is wrong, as the complex logarithm is multivalued. In other words, the wrong identity (ex)y = exy must be replaced by the identity
     
    which is a true identity between multivalued functions.

Irrationality and transcendenceEdit

If b is a positive real algebraic number, and x is a rational number, then bx is an algebraic number. This results from the theory of algebraic extensions. This remains true if b is any algebraic number, in which case, all values of bx (as a multivalued function) are algebraic. If x is irrational (that is, not rational), and both b and x are algebraic, Gelfond–Schneider theorem asserts that all values of bx are transcendental (that is, not algebraic), except if b equals 0 or 1.

In other words, if x is irrational and   then at least one of b, x and bx is transcendental.

Integer powers in algebraEdit

The definition of exponentiation with positive integer exponents as repeated multiplication may apply to any associative operation denoted as a multiplication.[nb 1] The definition of   requires further the existence of a multiplicative identity.[28]

An algebraic structure consisting of a set together with an associative operation denoted multiplicatively, and a multiplicative identity denoted by 1 is a monoid. In such a monoid, exponentiation of an element x is defined inductively by

  •  
  •   for every nonnegative integer n.

If n is a negative integer,   is defined only if x has a multiplicative inverse.[29] In this case, the inverse of x is denoted   and   is defined as  

Exponentiation with integer exponents obeys the following laws, for x and y in the algebraic structure, and m and n integers:

 

These definitions are widely used in many areas of mathematics, notably for groups, rings, fields, square matrices (which form a ring). They apply also to functions from a set to itself, which form a monoid under function composition. This includes, as specific instances, geometric transformations, and endomorphisms of any mathematical structure.

When there are several operations that may be repeated, it is common to indicate the repeated operation by placing its symbol in the superscript, before the exponent. For example, if f is a real function whose valued can be multiplied,   denotes the exponentiation with respect of multiplication, and   may denote exponentiation with respect of function composition. That is,

 

and

 

Commonly,   is denoted   while   is denoted  

In a groupEdit

A multiplicative group is a set with as associative operation denoted as multiplication, that has an identity element, and such that every element has an inverse.

So, if G is a group,   is defined for every   and every integer n.

The set of all powers of an element of a group form a subgroup. A group (or subgroup) that consists of all powers of a specific element x is the cyclic group generated by x. If all the powers of x are distinct, the group is isomorphic to the additive group   of the integers. Otherwise, the cyclic group is finite (it has a finite number of elements), and its number of elements is the order of x. If the order of x is n, then   and the cyclic group generated by x consists of the n first powers of x (starting indifferently from the exponent 0 or 1).

Order of elements play a fundamental role in group theory. For example, the order of an element in a finite group is always a divisor of the number of elements of the group (the order of the group). The possible orders of group elements are important in the study of the structure of a group (see Sylow theorems), and in the classification of finite simple groups.

Superscript notation is also used for conjugation; that is, gh = h−1gh, where g and h are elements of a group. This notation cannot be confused with exponentiation, since the superscript is not an integer. The motivation of this notation is that conjugation obeys some of the laws of exponentiation, namely   and  

In a ringEdit

In a ring, it may occurs that some nonzero elements satisfy   for some integer n. Such an element is said nilpotent. In a commutative ring, the nilpotent elements form an ideal, called the nilradical of the ring.

If the nilradical is reduced to the zero ideal (that is, if   implies   for every positive integer n), the commutative ring is said reduced. Reduced rings important in algebraic geometry, since the coordinate ring of an affine algebraic set is always a reduced ring.

More generally, given an ideal I in a commutative ring R, the set of the elements of R that have a power in I is an ideal, called the radical of I. The nilradical is the radical of the zero ideal. A radical ideal is an ideal that equals its own radical. In a polynomial ring   over a field k, an ideal is radical if and only if it is the set of all polynomials that are zero on an affine algebraic set (this is a consequence of Hilbert's Nullstellensatz).

Matrices and linear operatorsEdit

If A is a square matrix, then the product of A with itself n times is called the matrix power. Also   is defined to be the identity matrix,[30] and if A is invertible, then  .

Matrix powers appear often in the context of discrete dynamical systems, where the matrix A expresses a transition from a state vector x of some system to the next state Ax of the system.[31] This is the standard interpretation of a Markov chain, for example. Then   is the state of the system after two time steps, and so forth:   is the state of the system after n time steps. The matrix power   is the transition matrix between the state now and the state at a time n steps in the future. So computing matrix powers is equivalent to solving the evolution of the dynamical system. In many cases, matrix powers can be expediently computed by using eigenvalues and eigenvectors.

Apart from matrices, more general linear operators can also be exponentiated. An example is the derivative operator of calculus,  , which is a linear operator acting on functions   to give a new function  . The n-th power of the differentiation operator is the n-th derivative:

 

These examples are for discrete exponents of linear operators, but in many circumstances it is also desirable to define powers of such operators with continuous exponents. This is the starting point of the mathematical theory of semigroups.[32] Just as computing matrix powers with discrete exponents solves discrete dynamical systems, so does computing matrix powers with continuous exponents solve systems with continuous dynamics. Examples include approaches to solving the heat equation, Schrödinger equation, wave equation, and other partial differential equations including a time evolution. The special case of exponentiating the derivative operator to a non-integer power is called the fractional derivative which, together with the fractional integral, is one of the basic operations of the fractional calculus.

Finite fieldsEdit

A field is an algebraic structure in which multiplication, addition, subtraction, and division are defined and satisfy the properties that multiplication is associative and every nonzero element has a multiplicative inverse. This implies that exponentiation with integer exponents is well-defined, except for nonpositive powers of 0. Common examples are the complex numbers and their subfields, the rational numbers and the real numbers, which have been considered earlier in this article, and are all infinite.

A finite field is a field with a finite number of elements. This number of elements is either a prime number or a prime power; that is, it has the form   where p is a prime number, and k is a positive integer. For every such q, there are fields with q elements. The fields with q elements are all isomorphic, which allows, in general, working as if there were only one field with q elements, denoted  

One has

 

for every  

A primitive element in   is an element g such the set of the q − 1 first powers of g (that is,  ) equals the set of the nonzero elements of   There are   primitive elements in   where   is Euler's totient function.

In   the Freshman's dream identity

 

is true for the exponent p. As   in   It follows that the map

 

is linear over   and is a field automorphism, called the Frobenius automorphism. If   the field   has k automorphisms, which are the k first powers (under composition) of F. In other words, the Galois group of   is cyclic of order k, generated by the Frobenius automorphism.

The Diffie–Hellman key exchange is an application of exponentiation in finite fields that is widely used for secure communications. It uses the fact that exponentiation is computationally inexpensive, whereas the inverse operation, the discrete logarithm, is computationally expensive. More precisely, if g is a primitive element in   then   can be efficiently computed with exponentiation by squaring for any e, even if q is large, while there is no known algorithm allowing retrieving e from   if q is sufficiently large.

Over setsEdit

If n is a natural number, and A is an arbitrary set, then the expression An is often used to denote the set of ordered n-tuples of elements of A. This is equivalent to letting An denote the set of functions from the set {0, 1, 2, ..., n − 1} to the set A; the n-tuple (a0, a1, a2, ..., an−1) represents the function that sends i to ai.

For an infinite cardinal number κ and a set A, the notation Aκ is also used to denote the set of all functions from a set of size κ to A. This is sometimes written κA to distinguish it from cardinal exponentiation, defined below.

This generalized exponential can also be defined for operations on sets or for sets with extra structure. For example, in linear algebra, it makes sense to index direct sums of vector spaces over arbitrary index sets. That is, we can speak of

 

where each Vi is a vector space.

Then if Vi = V for each i, the resulting direct sum can be written in exponential notation as VN, or simply VN with the understanding that the direct sum is the default. We can again replace the set N with a cardinal number n to get Vn, although without choosing a specific standard set with cardinality n, this is defined only up to isomorphism. Taking V to be the field R of real numbers (thought of as a vector space over itself) and n to be some natural number, we get the vector space that is most commonly studied in linear algebra, the real vector space Rn.

If the base of the exponentiation operation is a set, the exponentiation operation is the Cartesian product unless otherwise stated. Since multiple Cartesian products produce an n-tuple, which can be represented by a function on a set of appropriate cardinality, SN becomes simply the set of all functions from N to S in this case:

 

This fits in with the exponentiation of cardinal numbers, in the sense that |SN| = |S||N|, where |X| is the cardinality of X. When "2" is defined as {0, 1}, we have |2X| = 2|X|, where 2X, usually denoted by P(X), is the power set of X; each subset Y of X corresponds uniquely to a function on X taking the value 1 for xY and 0 for xY.

In category theoryEdit

In a Cartesian closed category, the exponential operation can be used to raise an arbitrary object to the power of another object. This generalizes the Cartesian product in the category of sets. If 0 is an initial object in a Cartesian closed category, then the exponential object 00 is isomorphic to any terminal object 1.

Of cardinal and ordinal numbersEdit

In set theory, there are exponential operations for cardinal and ordinal numbers.

If κ and λ are cardinal numbers, the expression κλ represents the cardinality of the set of functions from any set of cardinality λ to any set of cardinality κ.[33] If κ and λ are finite, then this agrees with the ordinary arithmetic exponential operation. For example, the set of 3-tuples of elements from a 2-element set has cardinality 8 = 23. In cardinal arithmetic, κ0 is always 1 (even if κ is an infinite cardinal or zero).

Exponentiation of cardinal numbers is distinct from exponentiation of ordinal numbers, which is defined by a limit process involving transfinite induction.

Repeated exponentiationEdit

Just as exponentiation of natural numbers is motivated by repeated multiplication, it is possible to define an operation based on repeated exponentiation; this operation is sometimes called hyper-4 or tetration. Iterating tetration leads to another operation, and so on, a concept named hyperoperation. This sequence of operations is expressed by the Ackermann function and Knuth's up-arrow notation. Just as exponentiation grows faster than multiplication, which is faster-growing than addition, tetration is faster-growing than exponentiation. Evaluated at (3, 3), the functions addition, multiplication, exponentiation, and tetration yield 6, 9, 27, and 7625597484987 (= 327 = 333 = 33) respectively.

Limits of powersEdit

Zero to the power of zero gives a number of examples of limits that are of the indeterminate form 00. The limits in these examples exist, but have different values, showing that the two-variable function xy has no limit at the point (0, 0). One may consider at what points this function does have a limit.

More precisely, consider the function f(x, y) = xy defined on D = {(x, y) ∈ R2 : x > 0}. Then D can be viewed as a subset of R2 (that is, the set of all pairs (x, y) with x, y belonging to the extended real number line R = [−∞, +∞], endowed with the product topology), which will contain the points at which the function f has a limit.

In fact, f has a limit at all accumulation points of D, except for (0, 0), (+∞, 0), (1, +∞) and (1, −∞).[34] Accordingly, this allows one to define the powers xy by continuity whenever 0 ≤ x ≤ +∞, −∞ ≤ y ≤ +∞, except for 00, (+∞)0, 1+∞ and 1−∞, which remain indeterminate forms.

Under this definition by continuity, we obtain:

  • x+∞ = +∞ and x−∞ = 0, when 1 < x ≤ +∞.
  • x+∞ = 0 and x−∞ = +∞, when 0 ≤ x < 1.
  • 0y = 0 and (+∞)y = +∞, when 0 < y ≤ +∞.
  • 0y = +∞ and (+∞)y = 0, when −∞ ≤ y < 0.

These powers are obtained by taking limits of xy for positive values of x. This method does not permit a definition of xy when x < 0, since pairs (x, y) with x < 0 are not accumulation points of D.

On the other hand, when n is an integer, the power xn is already meaningful for all values of x, including negative ones. This may make the definition 0n = +∞ obtained above for negative n problematic when n is odd, since in this case xn → +∞ as x tends to 0 through positive values, but not negative ones.

Efficient computation with integer exponentsEdit

Computing bn using iterated multiplication requires n − 1 multiplication operations, but it can be computed more efficiently than that, as illustrated by the following example. To compute 2100, apply Horner's rule to the exponent 100 written in binary:

 .

Then compute the following terms in order, reading Horner's rule from right to left.

22 = 4
2 (22) = 23 = 8
(23)2 = 26 = 64
(26)2 = 212 = 4096
(212)2 = 224 = 16777216
2 (224) = 225 = 33554432
(225) 2 2 = 250 = 1125899906842624
(225)2 = 2100 = 1267650600228229401496703205376

This series of steps only requires 8 multiplications instead of 99.

In general, the number of multiplication operations required to compute bn can be reduced to   by using exponentiation by squaring, where   denotes the number of 1 in the binary representation of n. For some exponents (100 is not among them), the number of multiplications can be further reduced by computing and using the minimal addition-chain exponentiation. Finding the minimal sequence of multiplications (the minimal-length addition chain for the exponent) for bn is a difficult problem, for which no efficient algorithms are currently known (see Subset sum problem), but many reasonably efficient heuristic algorithms are available.[35] However, in practical computations, exponentiation by squaring is efficient enough, and much more easy to implement.

Iterated functionsEdit

Function composition is a binary operation that is defined on functions such that the codomain of the function written on the right is included in the domain of the function written on the left. It is denoted   and defined as

 

for every x in the domain of f.

If the domain of a function f equals its codomain, one may compose the function with itself an arbitrary number of time, and this defines the nth power of the function under composition, commonly called the nth iterate of the function. Thus   denotes generally the nth iterate of f; for example,   means  [36]

When a multiplication is defined on the codomain of the function, this defines a multiplication on functions, the pointwise multiplication, which induces another exponentiation. When using functional notation, the two kinds of exponentiation are generaly distinguished by placing the exponent of the functional iteration before the parentheses enclosing the arguments of the function, and placing the exponent of pointwise multiplication after the parentheses. Thus   and   When functional notation is not used, disambiguation is often done by placing the composition symbol before the exponent; for example   and   For historical reasons, the exponent of a repeated multiplication is placed before the argument for some specific functions, typically the trigonometric functions. So,   and   mean both   and not   which, in any case, is rarely considered. Historically, several variants of these notations were used by different authors.[37][38][39]

In this context, the exponent   denotes always the inverse function, if it exists. So   For the multiplicative inverse fractions are generally used as in  

In programming languagesEdit

Programming languages generally express exponentiation either as an infix operator or as a (prefix) function, as they are linear notations which do not support superscripts:

Many other programming languages lack syntactic support for exponentiation, but provide library functions:

For certain exponents there are special ways to compute xy much faster than through generic exponentiation. These cases include small positive and negative integers (prefer x · x over x2; prefer 1/x over x−1) and roots (prefer sqrt(x) over x0.5, prefer cbrt(x) over x1/3).

Not all programming languages adhere to the same association convention for exponentiation: while the Wolfram language, Google Search and others use right-association (i.e. a^b^c is evaluated as a^(b^c)), many computer programs such as Microsoft Office Excel and Matlab associate to the left (i.e. a^b^c is evaluated as (a^b)^c).

See alsoEdit

NotesEdit

  1. ^ More generally, power associativity is sufficient for the definition.

ReferencesEdit

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