Rational function

In mathematics, a rational function is any function which can be written as the ratio of two polynomial functions.

Contents

Definitions

Rational function of degree 2 :
y = \frac{x^2-3x-2}{x^2-4}

In the case of one variable, x\,, a function is called a rational function if and only if it can be written in the form

f(x) = \frac{P(x)}{Q(x)}

where P\, and Q\, are polynomial functions in x\, and Q\, is not the zero polynomial. The domain of f\, is the set of all points x\, for which the denominator Q(x)\, is not zero, where one assumes that the fraction is written in its lower degree terms, that is, \textstyle P and \textstyle Q have no common factor of positive degree.

Every polynomial function is a rational function with Q(x) = 1. A function that cannot be written in this form (for example, f(x) = sin(x)) is not a rational function (but the adjective "irrational" is not generally used for functions, but only for real numbers).

An expression of the form \frac{P(x)}{Q(x)} is called a rational expression. The x need not be a variable. In abstract algebra the x is called an indeterminate.

A rational equation is an equation in which two rational expressions are set equal to each other. These expressions obey the same rules as fractions. The equations can be solved by cross-multiplying. Division by zero is undefined, so that a solution causing formal division by zero is rejected.

Examples

Rational function of degree 3 :
y = \frac{x^3-2x}{2(x^2-5)}

The rational function f(x) = \frac{x^3-2x}{2(x^2-5)} is not defined at x^2=5 \leftrightarrow x=\pm \sqrt{5}.

The rational function f(x) = \frac{x^2 + 2}{x^2 + 1} is defined for all real numbers, but not for all complex numbers, since if x were the square root of -1 (i.e. the imaginary unit) or its negative, then formal evaluation would lead to division by zero: f(i) = \frac{i^2 + 2}{i^2 + 1} = \frac{-1 + 2}{-1 + 1} = \frac{1}{0}, which is undefined.

The rational function f(x) = \frac{x^3-2x}{2(x^2-5)}, as x approaches infinity, is asymptotic to \frac{x}{2}.

A constant function such as f(x) = π is a rational function since constants are polynomials. Although f(x) is irrational for all x, note that what is rational is the function, not necessarily the values of the function.

Taylor series

The coefficients of a Taylor series of any rational function satisfy a linear recurrence relation, which can be found by setting the rational function equal to its Taylor series and collecting like terms.

For example,

\frac{1}{x^2 - x + 2} = \sum_{k=0}^{\infty} a_k x^k.

Multiplying through by the denominator and distributing,

1 = (x^2 - x + 2) \sum_{k=0}^{\infty} a_k x^k
1 = \sum_{k=0}^{\infty} a_k x^{k+2} - \sum_{k=0}^{\infty} a_k x^{k+1} + 2\sum_{k=0}^{\infty} a_k x^k.

After adjusting the indices of the sums to get the same powers of x, we get

1 = \sum_{k=2}^{\infty} a_{k-2} x^k - \sum_{k=1}^{\infty} a_{k-1} x^k + 2\sum_{k=0}^{\infty} a_k x^k.

Combining like terms gives

1 = 2a_0 + (2a_1 - a_0)x + \sum_{k=2}^{\infty} (a_{k-2} - a_{k-1} + 2a_k) x^k.

Since this holds true for all x in the radius of convergence of the original Taylor series, we can compute as follows. Since the constant term on the left must equal the constant term on the right it follows that

a_0 = \frac{1}{2}.

Then, since there are no powers of x on the left, all of the coefficients on the right must be zero, from which it follows that

a_1 = \frac{1}{4}
a_{k} = \frac{1}{2} (a_{k-1} - a_{k-2})\quad for\ k \ge 2.

Conversely, any sequence that satisfies a linear recurrence determines a rational function when used as the coefficients of a Taylor series. This is useful in solving such recurrences, since by using partial fraction decomposition we can write any rational function as a sum of factors of the form 1 / (ax + b) and expand these as geometric series, giving an explicit formula for the Taylor coefficients; this is the method of generating functions.

Complex analysis

In complex analysis, a rational function

f(z) = \frac{P(z)}{Q(z)}

is the ratio of two polynomials with complex coefficients, where Q is not the zero polynomial and P and Q have no common factor (this avoids f taking the indeterminate value 0/0). The domain and range of f are usually taken to be the Riemann sphere, which avoids any need for special treatment at the poles of the function (where Q(z) is 0).

The degree of a rational function is the maximum of the degrees of its constituent polynomials P and Q. If the degree of f is d then the equation

f(z) = w \,

has d distinct solutions in z except for certain values of w, called critical values, where two or more solutions coincide. f can therefore be thought of as a d-fold covering of the w-sphere by the z-sphere.

Rational functions with degree 1 are called Möbius transformations and are automorphisms of the Riemann sphere. Rational functions are representative examples of meromorphic functions.

Abstract algebra

In abstract algebra the concept of a polynomial is extended to include formal expressions in which the coefficients of the polynomial can be taken from any field. In this setting given a field F and some indeterminate X, a rational expression is any element of the field of fractions of the polynomial ring F[X]. Any rational expression can be written as the quotient of two polynomials P/Q with Q ≠ 0, although this representation isn't unique. P/Q is equivalent to R/S, for polynomials P, Q, R, and S, when PS = QR. However since F[X] is a unique factorization domain, there is a unique representation for any rational expression P/Q with P and Q polynomials of lowest degree and Q chosen to be monic. This is similar to how a fraction of integers can always be written uniquely in lowest terms by canceling out common factors.

The field of rational expressions is denoted F(X). This field is said to be generated (as a field) over F by (a transcendental element) X, because F(X) does not contain any proper subfield containing both F and the element X.

Like polynomials, rational expressions can also be generalized to n indeterminates X1,..., Xn, by taking the field of fractions of F[X1,..., Xn], which is denoted by F(X1,..., Xn).

An extended version of the abstract idea of rational function is used in algebraic geometry. There the function field of an algebraic variety V is formed as the field of fractions of the coordinate ring of V (more accurately said, of a Zariski-dense affine open set in V). Its elements f are considered as regular functions in the sense of algebraic geometry on non-empty open sets U, and also may be seen as morphisms to the projective line.

Applications

These objects are first encountered in school algebra. In more advanced mathematics they play an important role in ring theory, especially in the construction of field extensions. They also provide an example of a nonarchimedean field (see Archimedean property).

Rational functions are used in numerical analysis for interpolation and approximation of functions, for example the Padé approximations introduced by Henri Padé. Approximations in terms of rational functions are well suited for computer algebra systems and other numerical software. Like polynomials, they can be evaluated straightforwardly, and at the same time they express more diverse behavior than polynomials.

Rational functions are used to approximate or model more complex equations in science and engineering including (i) fields and forces in physics, (ii) spectroscopy in analytical chemistry, (iii) enzyme kinetics in biochemistry, (iv) electronic circuitry, (v) aerodynamics, (vi) medicine concentrations in vivo, (vii) wave functions for atoms and molecules, (viii) optics and photography to improve image resolution, and (ix) acoustics and sound.

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