Inverse function

As an example, consider the real-valued function of a real variable given by f(x) = 5x − 7. Thinking of this as a step-by-step procedure (namely, take a number x, multiply it by 5, then subtract 7 from the result), to reverse this and get x back from some output value, say y, we would undo each step in reverse order. In this case, it means to add 7 to y, and then divide the result by 5. In functional notation, this inverse function would be given by,

Not all functions have inverse functions.[nb 1] Those that do are called invertible. For a function f: XY to have an inverse, it must have the property that for every y in Y, there is exactly one x in X such that f(x) = y. This property ensures that a function g: YX exists with the necessary relationship with f.

Let f be a function whose domain is the set X, and whose codomain is the set Y. Then f is invertible if there exists a function g with domain Y and codomain X, with the property:

If f is invertible, then the function g is unique,[6] which means that there is exactly one function g satisfying this property. Moreover, it also follows that the ranges of g and f equal their respective codomains. The function g is called the inverse of f, and is usually denoted as f −1, a notation introduced by John Frederick William Herschel in 1813.[7][8][9][10][11][nb 2]

Stated otherwise, a function, considered as a binary relation, has an inverse if and only if the converse relation is a function on the codomain Y, in which case the converse relation is the inverse function.[12]

Not all functions have an inverse. For a function to have an inverse, each element yY must correspond to no more than one xX; a function f with this property is called one-to-one or an injection. If f −1 is to be a function on Y, then each element yY must correspond to some xX. Functions with this property are called surjections. This property is satisfied by definition if Y is the image of f, but may not hold in a more general context. To be invertible, a function must be both an injection and a surjection. Such functions are called bijections. The inverse of an injection f: XY that is not a bijection (that is, not a surjection), is only a partial function on Y, which means that for some yY, f −1(y) is undefined. If a function f is invertible, then both it and its inverse function f−1 are bijections.

Another convention is used in the definition of functions, referred to as the "set-theoretic" or "graph" definition using ordered pairs, which makes the codomain and image of the function the same.[13] Under this convention, all functions are surjective,[nb 3] so bijectivity and injectivity are the same. Authors using this convention may use the phrasing that a function is invertible if and only if it is an injection.[14] The two conventions need not cause confusion, as long as it is remembered that in this alternate convention, the codomain of a function is always taken to be the image of the function.

The function f: R → [0,∞) given by f(x) = x2 is not injective, since each possible result y (except 0) corresponds to two different starting points in X – one positive and one negative, and so this function is not invertible. With this type of function, it is impossible to deduce a (unique) input from its output. Such a function is called non-injective or, in some applications, information-losing.[citation needed]

If the domain of the function is restricted to the nonnegative reals, that is, the function is redefined to be f: [0, ∞) → [0, ∞) with the same rule as before, then the function is bijective and so, invertible.[15] The inverse function here is called the (positive) square root function.

Using the composition of functions, we can rewrite this statement as follows:

where idX is the identity function on the set X; that is, the function that leaves its argument unchanged. In category theory, this statement is used as the definition of an inverse morphism.

Considering function composition helps to understand the notation f −1. Repeatedly composing a function with itself is called iteration. If f is applied n times, starting with the value x, then this is written as fn(x); so f 2(x) = f (f (x)), etc. Since f −1(f (x)) = x, composing f −1 and fn yields fn−1, "undoing" the effect of one application of f.

While the notation f −1(x) might be misunderstood,[5] (f(x))−1 certainly denotes the multiplicative inverse of f(x) and has nothing to do with the inverse function of f.[11]

In keeping with the general notation, some English authors use expressions like sin−1(x) to denote the inverse of the sine function applied to x (actually a partial inverse; see below).[16][11] Other authors feel that this may be confused with the notation for the multiplicative inverse of sin (x), which can be denoted as (sin (x))−1.[11] To avoid any confusion, an inverse trigonometric function is often indicated by the prefix "arc" (for Latin arcus).[17][18] For instance, the inverse of the sine function is typically called the arcsine function, written as arcsin(x).[17][18] Similarly, the inverse of a hyperbolic function is indicated by the prefix "ar" (for Latin ārea).[18] For instance, the inverse of the hyperbolic sine function is typically written as arsinh(x).[18] Other inverse special functions are sometimes prefixed with the prefix "inv", if the ambiguity of the f −1 notation should be avoided.[1][18]

Since a function is a special type of binary relation, many of the properties of an inverse function correspond to properties of converse relations.

If an inverse function exists for a given function f, then it is unique.[19] This follows since the inverse function must be the converse relation, which is completely determined by f.

There is a symmetry between a function and its inverse. Specifically, if f is an invertible function with domain X and codomain Y, then its inverse f −1 has domain Y and image X, and the inverse of f −1 is the original function f. In symbols, for functions f:XY and f−1:YX,[19]

This statement is a consequence of the implication that for f to be invertible it must be bijective. The involutory nature of the inverse can be concisely expressed by[20]

Notice that the order of g and f have been reversed; to undo f followed by g, we must first undo g, and then undo f.

For example, let f(x) = 3x and let g(x) = x + 5. Then the composition g ∘ f is the function that first multiplies by three and then adds five,

To reverse this process, we must first subtract five, and then divide by three,

More generally, a function f : XX is equal to its own inverse, if and only if the composition f ∘ f is equal to idX. Such a function is called an involution.

Single-variable calculus is primarily concerned with functions that map real numbers to real numbers. Such functions are often defined through formulas, such as:

A surjective function f from the real numbers to the real numbers possesses an inverse, as long as it is one-to-one. That is, the graph of y = f(x) has, for each possible y value, only one corresponding x value, and thus passes the horizontal line test.

The following table shows several standard functions and their inverses:

One approach to finding a formula for f −1, if it exists, is to solve the equation y = f(x) for x.[22] For example, if f is the function

Sometimes, the inverse of a function cannot be expressed by a formula with a finite number of terms. For example, if f is the function

then f is a bijection, and therefore possesses an inverse function f −1. The formula for this inverse has an infinite number of terms:

This is identical to the equation y = f(x) that defines the graph of f, except that the roles of x and y have been reversed. Thus the graph of f −1 can be obtained from the graph of f by switching the positions of the x and y axes. This is equivalent to reflecting the graph across the line y = x.[23][5]

A continuous function f is invertible on its range (image) if and only if it is either strictly increasing or decreasing (with no local maxima or minima).[citation needed] For example, the function

is invertible, since the derivative f′(x) = 3x2 + 1 is always positive.

If the function f is differentiable on an interval I and f′(x) ≠ 0 for each xI, then the inverse f −1 is differentiable on f(I).[24] If y = f(x), the derivative of the inverse is given by the inverse function theorem,

This result follows from the chain rule (see the article on inverse functions and differentiation).

The inverse function theorem can be generalized to functions of several variables. Specifically, a differentiable multivariable function f : RnRn is invertible in a neighborhood of a point p as long as the Jacobian matrix of f at p is invertible. In this case, the Jacobian of f −1 at f(p) is the matrix inverse of the Jacobian of f at p.

Even if a function f is not one-to-one, it may be possible to define a partial inverse of f by restricting the domain. For example, the function

is not one-to-one, since x2 = (−x)2. However, the function becomes one-to-one if we restrict to the domain x ≥ 0, in which case

(If we instead restrict to the domain x ≤ 0, then the inverse is the negative of the square root of y.) Alternatively, there is no need to restrict the domain if we are content with the inverse being a multivalued function:

Sometimes, this multivalued inverse is called the full inverse of f, and the portions (such as x and −x) are called branches. The most important branch of a multivalued function (e.g. the positive square root) is called the principal branch, and its value at y is called the principal value of f −1(y).

For a continuous function on the real line, one branch is required between each pair of local extrema. For example, the inverse of a cubic function with a local maximum and a local minimum has three branches (see the adjacent picture).

These considerations are particularly important for defining the inverses of trigonometric functions. For example, the sine function is not one-to-one, since

for every real x (and more generally sin(x + 2πn) = sin(x) for every integer n). However, the sine is one-to-one on the interval [− π/2,  π/2], and the corresponding partial inverse is called the arcsine. This is considered the principal branch of the inverse sine, so the principal value of the inverse sine is always between −π/2 and π/2. The following table describes the principal branch of each inverse trigonometric function:[25]

Left and right inverses are not necessarily the same. If g is a left inverse for f, then g may or may not be a right inverse for f; and if g is a right inverse for f, then g is not necessarily a left inverse for f. For example, let f: R[0, ∞) denote the squaring map, such that f(x) = x2 for all x in R, and let g: [0, ∞)R denote the square root map, such that g(x) = x for all x ≥ 0. Then f(g(x)) = x for all x in [0, ∞); that is, g is a right inverse to f. However, g is not a left inverse to f, since, e.g., g(f(−1)) = 1 ≠ −1.

If f: XY, a left inverse for f (or retraction of f ) is a function g: YX such that composing f with g from the left gives the identity function[citation needed]:

Thus, g must equal the inverse of f on the image of f, but may take any values for elements of Y not in the image.

A function f is injective if and only if it has a left inverse or is the empty function.[citation needed]

A right inverse for f (or section of f ) is a function h: YX such that[citation needed]

A function f has a right inverse if and only if it is surjective (though constructing such an inverse in general requires the axiom of choice).

An inverse that is both a left and right inverse (a two-sided inverse), if it exists, must be unique. In fact, if a function has a left inverse and a right inverse, they are both the same two-sided inverse, so it can be called the inverse.

If f: XY is any function (not necessarily invertible), the preimage (or inverse image) of an element yY, is the set of all elements of X that map to y:[citation needed]

The preimage of y can be thought of as the image of y under the (multivalued) full inverse of the function f.

For example, take a function f: RR, where f: xx2. This function is not invertible for reasons discussed in § Example: Squaring and square root functions. Yet preimages may be defined for subsets of the codomain: