![]() | ||
In mathematics, and particularly complex dynamics, the escaping set of an entire function ƒ consists of all points that tend to infinity under the repeated application of ƒ. That is, a complex number
Contents
For example, for
tends to infinity.
History
The iteration of transcendental entire functions was first studied by Pierre Fatou in 1926 The escaping set occurs implicitly in his study of the explicit entire functions
The first study of the escaping set for a general transcendental entire function is due to Alexandre Eremenko. He conjectured that every connected component of the escaping set of a transcendental entire function is unbounded. This has become known as Eremenko's Conjecture. There are many partial results on this problem but as of 2013 the conjecture is still open.
Eremenko also asked whether every escaping point can be connected to infinity by a curve in the escaping set; it was later shown that this is not the case. Indeed, there exist entire functions whose escaping sets do not contain any curves at all.
Properties
The following properties are known to hold for the escaping set of any non-constant and non-linear entire function. (Here nonlinear means that the function is not of the form
Note that the final statement does not imply Eremenko's Conjecture. (Indeed, there exist connected spaces in which the removal of a single dispersion point leaves the remaining space totally disconnected.)
Polynomials
For a polynomial of degree at least 2, the point at infinity is an (super-)attracting fixed point, and the escaping set is precisely the basin of attraction of this fixed point ( infinity). Hence in this case,
For instance the escaping set of the complex quadratic polynomial
Transcendental entire functions
For transcendental entire functions, the escaping set is much more complicated than for polynomials: in the simplest cases like the one illustrated in the picture it consists on uncountably many curves, called hairs or rays. In other examples the structure of the escaping set can be very different (a spider's web). As mentioned above, there are examples of entire functions whose escaping set contains no curves.