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Every rational function can be naturally extended to a function whose domain and range are the whole Riemann sphere (complex projective line). A complex rational function with degree one is a Möbius transformation. Rational functions are representative examples of meromorphic functions. [3]
Dirichlet function: is an indicator function that matches 1 to rational numbers and 0 to irrationals. It is nowhere continuous. Thomae's function: is a function that is continuous at all irrational numbers and discontinuous at all rational numbers. It is also a modification of Dirichlet function and sometimes called Riemann function.
Rational functions are quotients of two polynomial functions, and their domain is the real numbers with a finite number of them removed to avoid division by zero. The simplest rational function is the function , whose graph is a hyperbola, and whose domain is the whole real line except for 0.
For example, a quadratic for the numerator and a cubic for the denominator is identified as a quadratic/cubic rational function. The rational function model is a generalization of the polynomial model: rational function models contain polynomial models as a subset (i.e., the case when the denominator is a constant).
In algebraic geometry, the function field of an algebraic variety V consists of objects that are interpreted as rational functions on V.In classical algebraic geometry they are ratios of polynomials; in complex geometry these are meromorphic functions and their higher-dimensional analogues; in modern algebraic geometry they are elements of some quotient ring's field of fractions.
By starting with the field of rational functions, two special types of transcendental extensions (the logarithm and the exponential) can be added to the field building a tower containing elementary functions. A differential field F is a field F 0 (rational functions over the rationals Q for example) together with a derivation map u → ∂u.
By definition, a rational function is just a rational map whose range is the projective line. Composition of functions then allows us to " pull back " rational functions along a rational map, so that a single rational map f : V → W {\displaystyle f\colon V\to W} induces a homomorphism of fields K ( W ) → K ( V ) {\displaystyle K(W)\to K(V)} .
Here is an example of a rational function which possesses a Herman ring. [1]() = ()where = … such that the rotation number of ƒ on the unit circle is () /.. The picture shown on the right is the Julia set of ƒ: the curves in the white annulus are the orbits of some points under the iterations of ƒ while the dashed line denotes the unit circle.