In mathematics, a multiplicative inverse or reciprocal for a number x,
denoted by 1/x or x−1, is a number which when multiplied by x yields
the multiplicative identity, 1. The multiplicative inverse of a
fraction a/b is b/a. For the multiplicative inverse of a real number,
divide 1 by the number. For example, the reciprocal of 5 is one fifth
(1/5 or 0.2), and the reciprocal of 0.25 is 1 divided by 0.25, or 4.
The reciprocal function, the function f(x) that maps x to 1/x, is one
of the simplest examples of a function which is its own inverse (an
involution).
The term reciprocal was in common use at least as far back as the
third edition of
Encyclopædia Britannica
Contents 1 Examples and counterexamples 2 Complex numbers 3 Calculus 4 Algorithms 5 Reciprocals of irrational numbers 6 Further remarks 7 Applications 8 See also 9 Notes 10 References Examples and counterexamples[edit] In the real numbers, zero does not have a reciprocal because no real number multiplied by 0 produces 1 (the product of any number with zero is zero). With the exception of zero, reciprocals of every real number are real, reciprocals of every rational number are rational, and reciprocals of every complex number are complex. The property that every element other than zero has a multiplicative inverse is part of the definition of a field, of which these are all examples. On the other hand, no integer other than 1 and −1 has an integer reciprocal, and so the integers are not a field. In modular arithmetic, the modular multiplicative inverse of a is also defined: it is the number x such that ax ≡ 1 (mod n). This multiplicative inverse exists if and only if a and n are coprime. For example, the inverse of 3 modulo 11 is 4 because 4 · 3 ≡ 1 (mod 11). The extended Euclidean algorithm may be used to compute it. The sedenions are an algebra in which every nonzero element has a multiplicative inverse, but which nonetheless has divisors of zero, i.e. nonzero elements x, y such that xy = 0. A square matrix has an inverse if and only if its determinant has an inverse in the coefficient ring. The linear map that has the matrix A−1 with respect to some base is then the reciprocal function of the map having A as matrix in the same base. Thus, the two distinct notions of the inverse of a function are strongly related in this case, while they must be carefully distinguished in the general case (as noted above). The trigonometric functions are related by the reciprocal identity: the cotangent is the reciprocal of the tangent; the secant is the reciprocal of the cosine; the cosecant is the reciprocal of the sine. A ring in which every nonzero element has a multiplicative inverse is a division ring; likewise an algebra in which this holds is a division algebra. Complex numbers[edit] As mentioned above, the reciprocal of every nonzero complex number z = a + bi is complex. It can be found by multiplying both top and bottom of 1/z by its complex conjugate z ¯ = a − b i displaystyle bar z =abi and using the property that z z ¯ = ‖ z ‖ 2 displaystyle z bar z =z^ 2 , the absolute value of z squared, which is the real number a2 + b2: 1 z = z ¯ z z ¯ = z ¯ ‖ z ‖ 2 = a − b i a 2 + b 2 = a a 2 + b 2 − b a 2 + b 2 i . displaystyle frac 1 z = frac bar z z bar z = frac bar z z^ 2 = frac abi a^ 2 +b^ 2 = frac a a^ 2 +b^ 2  frac b a^ 2 +b^ 2 i. In particular, if z=1 (z has unit magnitude), then 1 / z = z ¯ displaystyle 1/z= bar z . Consequently, the imaginary units, ±i, have additive inverse equal to multiplicative inverse, and are the only complex numbers with this property. For example, additive and multiplicative inverses of i are −(i) = −i and 1/i = −i, respectively. For a complex number in polar form z = r(cos φ + i sin φ), the reciprocal simply takes the reciprocal of the magnitude and the negative of the angle: 1 z = 1 r ( cos ( − φ ) + i sin ( − φ ) ) . displaystyle frac 1 z = frac 1 r left(cos(varphi )+isin(varphi )right). Geometric intuition for the integral of 1/x. The three integrals from 1 to 2, from 2 to 4, and from 4 to 8 are all equal. Each region is the previous region scaled vertically down by 50%, then horizontally by 200%. Extending this, the integral from 1 to 2k is k times the integral from 1 to 2, just as ln 2k = k ln 2. Calculus[edit] In real calculus, the derivative of 1/x = x−1 is given by the power rule with the power −1: d d x x − 1 = ( − 1 ) x ( − 1 ) − 1 = − x − 2 = − 1 x 2 . displaystyle frac d dx x^ 1 =(1)x^ (1)1 =x^ 2 = frac 1 x^ 2 . The power rule for integrals (Cavalieri's quadrature formula) cannot be used to compute the integral of 1/x, because doing so would result in division by 0: ∫ 1 x d x = x 0 0 + C displaystyle int frac 1 x ,dx= frac x^ 0 0 +C Instead the integral is given by: ∫ 1 a 1 x d x = ln a , displaystyle int _ 1 ^ a frac 1 x ,dx=ln a, ∫ 1 x d x = ln x + C . displaystyle int frac 1 x ,dx=ln x+C. where ln is the natural logarithm. To show this, note that d d x e x = e x displaystyle frac d dx e^ x =e^ x , so if y = e x displaystyle y=e^ x and x = ln y displaystyle x=ln y , we have:[2] d y d x = y ⇒ d y y = d x ⇒ ∫ 1 y d y = ∫ 1 d x ⇒ ∫ 1 y d y = x + C = ln y + C . displaystyle frac dy dx =yquad Rightarrow quad frac dy y =dxquad Rightarrow quad int frac 1 y ,dy=int 1,dxquad Rightarrow quad int frac 1 y ,dy=x+C=ln y+C. Algorithms[edit] The reciprocal may be computed by hand with the use of long division. Computing the reciprocal is important in many division algorithms, since the quotient a/b can be computed by first computing 1/b and then multiplying it by a. Noting that f ( x ) = 1 / x − b displaystyle f(x)=1/xb has a zero at x = 1/b,
Newton's method
x 0 displaystyle x_ 0 and iterating using the rule: x n + 1 = x n − f ( x n ) f ′ ( x n ) = x n − 1 / x n − b − 1 / x n 2 = 2 x n − b x n 2 = x n ( 2 − b x n ) . displaystyle x_ n+1 =x_ n  frac f(x_ n ) f'(x_ n ) =x_ n  frac 1/x_ n b 1/x_ n ^ 2 =2x_ n bx_ n ^ 2 =x_ n (2bx_ n ). This continues until the desired precision is reached. For example, suppose we wish to compute 1/17 ≈ 0.0588 with 3 digits of precision. Taking x0 = 0.1, the following sequence is produced: x1 = 0.1(2 − 17 × 0.1) = 0.03 x2 = 0.03(2 − 17 × 0.03) = 0.0447 x3 = 0.0447(2 − 17 × 0.0447) ≈ 0.0554 x4 = 0.0554(2 − 17 × 0.0554) ≈ 0.0586 x5 = 0.0586(2 − 17 × 0.0586) ≈ 0.0588 A typical initial guess can be found by rounding b to a nearby power of 2, then using bit shifts to compute its reciprocal. In constructive mathematics, for a real number x to have a reciprocal, it is not sufficient that x ≠ 0. There must instead be given a rational number r such that 0 < r < x. In terms of the approximation algorithm described above, this is needed to prove that the change in y will eventually become arbitrarily small. Graph of f(x) = xx showing the minimum at (1/e, e−1/e). This iteration can also be generalised to a wider sort of inverses, e.g. matrix inverses. Reciprocals of irrational numbers[edit] Every number excluding zero has a reciprocal, and reciprocals of certain irrational numbers can have important special properties. Examples include the reciprocal of e (≈ 0.367879) and the golden ratio's reciprocal (≈ 0.618034). The first reciprocal is special because no other positive number can produce a lower number when put to the power of itself; f ( 1 / e ) displaystyle f(1/e) is the global minimum of f ( x ) = x x displaystyle f(x)=x^ x . The second number is the only positive number that is equal to its reciprocal plus one: φ = 1 / φ + 1 displaystyle varphi =1/varphi +1 . Its additive inverse is the only negative number that is equal to its reciprocal minus one: − φ = − 1 / φ − 1 displaystyle varphi =1/varphi 1 . The function f ( n ) = n + ( n 2 + 1 ) , n ∈ N , n > 0 displaystyle f(n)=n+ sqrt (n^ 2 +1) ,nin mathbb N ,n>0 gives an infinite number of irrational numbers that differ with their reciprocal by an integer. For example, f ( 2 ) displaystyle f(2) is the irrational 2 + 5 displaystyle 2+ sqrt 5 . Its reciprocal 1 / ( 2 + 5 ) displaystyle 1/(2+ sqrt 5 ) is − 2 + 5 displaystyle 2+ sqrt 5 , exactly 4 displaystyle 4 less. Such irrational numbers share a curious property: they have the same fractional part as their reciprocal. Further remarks[edit] If the multiplication is associative, an element x with a multiplicative inverse cannot be a zero divisor (x is a zero divisor if some nonzero y, xy = 0). To see this, it is sufficient to multiply the equation xy = 0 by the inverse of x (on the left), and then simplify using associativity. In the absence of associativity, the sedenions provide a counterexample. The converse does not hold: an element which is not a zero divisor is not guaranteed to have a multiplicative inverse. Within Z, all integers except −1, 0, 1 provide examples; they are not zero divisors nor do they have inverses in Z. If the ring or algebra is finite, however, then all elements a which are not zero divisors do have a (left and right) inverse. For, first observe that the map f(x) = ax must be injective: f(x) = f(y) implies x = y: a x = a y ⇒ a x − a y = 0 ⇒ a ( x − y ) = 0 ⇒ x − y = 0 ⇒ x = y . displaystyle begin aligned ax&=ay&quad Rightarrow &quad axay=0\&&quad Rightarrow &quad a(xy)=0\&&quad Rightarrow &quad xy=0\&&quad Rightarrow &quad x=y.end aligned Distinct elements map to distinct elements, so the image consists of the same finite number of elements, and the map is necessarily surjective. Specifically, ƒ (namely multiplication by a) must map some element x to 1, ax = 1, so that x is an inverse for a. Applications[edit] The expansion of the reciprocal 1/q in any base can also act [3] as a source of pseudorandom numbers, if q is a "suitable" safe prime, a prime of the form 2p + 1 where p is also a prime. A sequence of pseudorandom numbers of length q − 1 will be produced by the expansion. See also[edit] Division (mathematics) Fraction (mathematics) Group (mathematics) Ring (mathematics) Division algebra Exponential decay Unit fractions – reciprocals of integers Hyperbola Repeating decimal List of sums of reciprocals Sixsphere coordinates Notes[edit] ^ " In equall Parallelipipedons the bases are reciprokall to their
altitudes". OED "Reciprocal" §3a. Sir
Henry Billingsley
References[edit] Maximally Periodic Reciprocals, Matthews R.A.J. Bulletin of the
Institute of
Mathematics
