In mathematics, a Misiurewicz point is a parameter value in the Mandelbrot set (the parameter space of complex quadratic maps) and also in real quadratic maps of the interval for which the critical point is strictly pre-periodic (i.e., it becomes periodic after finitely many iterations but is not periodic itself). By analogy, the term Misiurewicz point is also used for parameters in a multibrot set where the unique critical point is strictly pre-periodic. This term makes less sense for maps in greater generality that have more than one free critical point because some critical points might be periodic and others not. These points are named after the Polish-American mathematician Michał Misiurewicz, who was the first to study them.
A parameter is a Misiurewicz point if it satisfies the equations:
- is a critical point of ,
- and are positive integers,
- denotes the -th iterate of .
The term "Misiurewicz point" is used ambiguously: Misiurewicz originally investigated maps in which all critical points were non-recurrent; that is, in which there exists a neighborhood for every critical point that is not visited by the orbit of this critical point. This meaning is firmly established in the context of the dynamics of iterated interval maps. Only in very special cases does a quadratic polynomial have a strictly periodic and unique critical point. In this restricted sense, the term is used in complex dynamics; a more appropriate one would be Misiurewicz–Thurston points (after William Thurston, who investigated post-critically finite rational maps).
A complex quadratic polynomial has only one critical point. By a suitable conjugation any quadratic polynomial can be transformed into a map of the form which has a single critical point at . The Misiurewicz points of this family of maps are roots of the equations:
Subject to the condition that the critical point is not periodic, where:
- k is the pre-period
- n is the period
- denotes the n-fold composition of with itself i.e. the nth iteration of .
For example, the Misiurewicz points with k= 2 and n= 1, denoted by M2,1, are roots of:
The root c= 0 is not a Misiurewicz point because the critical point is a fixed point when c= 0, and so is periodic rather than pre-periodic. This leaves a single Misiurewicz point M2,1 at c = −2.
Properties of Misiurewicz points of complex quadratic mapping
If is a Misiurewicz point, then in the corresponding Julia set all periodic cycles are repelling (in particular the cycle that the critical orbit falls onto).
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Misiurewicz points can be classified according to several criteria. One such criterion is the number of external rays that land on such a point.
- Branch points, which can divide the Mandelbrot set into two or more sub-regions, have three or more external arguments (or angles).
- Non-branch points have exactly two external arguments (points of arcs within the Mandelbrot set). These points are less conspicuous and not easily found in pictures.
- Branch tips, or end points, will have one external argument.
Another criterion is how such a point looks within a plot of part of the set. Misiurewicz points can be found at the centers of spirals as well as at points where two or more branches meet.
Most Misiurewicz parameters in the Mandelbrot set look like "centers of spirals". The explanation for this is that at a Misiurewicz parameter, the critical value jumps onto a repelling periodic cycle after finitely many iterations. At each point during the cycle, the Julia set is asymptotically self-similar by a complex multiplication by the derivative of this cycle. If the derivative is non-real, this implies that the Julia set near the periodic cycle has a spiral structure. Thus, a similar spiral structure occurs in the Julia set near the critical value and by Tan Lei's theorem, also in the Mandelbrot set near any Misiurewicz parameter for which the repelling orbit has a non-real multiplier. Depending on the value of this multiplier, the spiral shape can seem either more or less pronounced. The number of arms at the spiral equals the number of branches at the Misiurewicz parameter, and this equals the number of branches at the critical value in the Julia set. Even the principal Misiurewicz point in the 1/3-limb, at the end of the parameter rays at angles 9/56, 11/56, and 15/56, turns out to be asymptotically a spiral with infinitely many turns, even though this is hard to see without magnification.
External arguments of Misiurewicz points, measured in turns are:
- Rational numbers
- Proper fractions with an even denominator
where: a and b are positive integers and b is odd, subscript number shows base of numeral system.
Examples of Misiurewicz points of complex quadratic mapping
- Is a tip of the filament
- Landing point of the external ray for the angle =1/6
- Its critical orbits is 
- Is the endpoint of main antenna of Mandelbrot set
- Is landing point of only one external ray (parameter ray) of angle 1/2
- Its critical orbit:
- Is a principal Misiurewicz point of the 1/3 limb
- It has 3 external rays: 9/56, 11/56 and 15/56.
These are points which are not-branch and not-end points.
Point is near a Misiurewicz point . It is
- A center of a two-arms spiral
- A landing point of 2 external rays with angles: and Where denominator is
- Preperiodic point with pre-period and period
Point is near a Misiurewicz point ,
- Which is landing point for pair of rays: ,
- Has pre-period and period
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- Michał Misiurewicz home page, Indiana University-Purdue University Indianapolis
- Wellington de Melo, Sebastian van Strien, "One-dimensional dynamics". Monograph, Springer Verlag (1991)
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- Lei.pdf Tan Lei, "Similarity between the Mandelbrot set and Julia Sets", Communications in Mathematical Physics 134 (1990), pp. 587-617.
- Fractal Geometry Yale University Michael Frame, Benoit Mandelbrot (1924-2010), and Nial Neger November 6, 2022
- The boundary of the Mandelbrot set Archived 2003-03-28 at the Wayback Machine by Michael Frame, Benoit Mandelbrot, and Nial Neger
- Binary Decimal Numbers and Decimal Numbers Other Than Base Ten by Thomas Kim-wai Yeung and Eric Kin-keung Poon
- Tip of the filaments by Robert P. Munafo
- Preperiodic (Misiurewicz) points in the Mandelbrot se by Evgeny Demidov
- tip of main antennae by Robert P. Munafo
- Michał Misiurewicz (1981), "Absolutely continuous measures for certain maps of an interval" (in French). Publications Mathématiques de l'IHÉS, 53 (1981), p. 17-51