Atom Domain
Robert P. Munafo, 2023 Mar 19.
A period domain, also called "atom domain", is a region (a set of points) sharing a property that is easily computed and makes for a good representation function. This method was first described by Peitgen and Richter in The Beauty of Fractals (1986).
Rendering Mandelbrot images with period domains in different colors makes it easy to find certain island Mumolecules and embedded Julia sets.
Given a single specific point, the "atomdomain period" of that point is an integer that is equal to the period of a nearby muatom. This might be called a "nearby dominant period", "period of the dominant muatom", or a "tuning" period.
Contents
Size Relationships for Islands
Origin
The first atomdomain image appears in The Beauty of Fractals, figure 34 (on page 61), with this description on the following page:
Figures 33 and 34 are included from some ongoing research on M. Given c in M we define
(4.24) a(c) = inf { p_{c}^{k}(0) : k = 1, 2, ...}
and
index (c) = k provided a(c) = p_{c}^{k}(0) .
Figure 33 shows levels of a(c) in alternating colors while Fig. 34 shows the distribiution of index (c) on M. Remarkably each satellite is distinguished by a component of some fixed index and index (c) introduces a Fibonacci partitition in M.
"p_{c}^{k}(0)" had previously been defined as the standard iteration algorithm for the Mandelbrot set.
Definition
Starting with the standard iteration calculation:
Z_{0} = 0
Z_{N+1} = Z_{N}^{2} + C
and defining R_{N}=Z_{N} (where "R" stands for "radius", i.e. the distance from the origin to Z_{N}); the period of the dominant muatom or "atomdomain period" is:
AtomDomainPeriod = N(minimum(R_{i}))
This is the value of N when R_{N}=Z_{N} reaches a minimum (not counting the initial value R_{0}=Z_{0} which is always zero). In other words, the Nth iterate is closer to the origin than any other iterate(s).
Above we see (on the left) an ordinary view of the Mandelbrot set and (on the right) the same view showing period domains. All of the points in the green area have a dominant period of 2. This means that when iterating these points, the value of Z_{2} has a smaller magnitude than all of the other iterates (except for the initial Z_{0}=0, which does not count).
Similarly, all the points in the three light blue regions have a dominant period of 3, so for them, the Z_{3} iterate is smaller than all other iterates.
In practice, we can only compute Z_{N}^{2}+C a finite number of times. A calculation of the period domain ends up being just the same as the normal Mandelbrot calculation, except that you take note of which iteration produces the smallest value of Z. In pesudocode, it comes out something like this:
function period_domain param(c) : complex param(max_iterations) : integer param(escape_radius) : real should be 2.0 or larger result: integer begin function declare z : complex declare iterations : integer declare still_iterating : boolean declare domain_period : integer declare minimum_magnitude : real let still_iterating = true let iterations = 1 let z = c let minimum_magnitude = magnitude(z) let domain_period = 1 while (still_iterating) do let z = z^{2} + c let iterations = iterations + 1 if (magnitude(z) < minimum_magnitude) then let minimum_magnitude = magnitude(z) let domain_period = iterations end if if (magnitude(z) > escape_radius) then let still_iterating = false else if (iterations >= max_iterations) then let still_iterating = false end if end while let result = domain_period end functionThis function yields an excellent way of locating muatoms of a given period. The image produced contains regions of solid color that surround the muatoms, because the minimum R_{N} value occurs when the number of iterations is equal to the period of the muatom.
In the images below and in the secondorder embedded Julia set article, each color corresponds to a dominant period value: period 1 is white, period 2 is blue, 3 is pink, 4 is orange, 5 is yellow, etc.
If desired, a "maximum domain_period" parameter can be used in the algorithm, to detect certain low periods without interference from the many higher periods that always occur near muatoms. Here are four images of the mumolecule R2F(14/15B1)S in Elephant valley an island whose period is 17:
period domains 17 and 34 
period domains 17, 34 and 51 
period domains 17 through 68 
period domains 17 through 85 
Island mumolecules and their surrounding embedded Julia sets can be located by this method. To make this easier, some other quality such as brightness or saturation could be altered based on the relative magnitude of the minimum radius R_{N}.
Properties
For cardioid muatoms, the size of the perioddomain is typically much larger than the cardioid (qualitatively, for nontuned islands, the "diameter" of the cardioid's perioddomain "blob" is approximately the square root of the "diameter" of the cardioid. Since the "diameter" of the cardioid is a very small number like 0.000001, its square root is less small, like 0.001, and therefore much bigger by comparison).
The period domain of any .1/2a descendant of a cardioid (i.e. the largest circular muatom on any island) is a roughly circular region with diameter 4 times as great as the muatom itself. In the case of highly distorted mumolecules, the .1/2a's period domain is an ellipse whose major axis is somewhat larger, and minor axis somewhat smaller, than 4 times the .1/2a muatom's diameter.
Regardless of any distortion, the boundary of the .1/2a's period domain, and the boundaries of the period domains of all mumolecules in the island's R2t series pass through the tip of the island (the most extreme point of the .F(1/2B1) filament). Here is an example:
Here we see the highlydistorted island from the R2.C(0) article. Note the mustardcolored ellipse belonging to the .1/2a muatom, the red ellipse belonging to the .F(1/2B1)Sa island's cardioid, the blue ellipse belonging to the .F(1/2B1)FS[2]Sa island's cardioid, and so on, the boundaries of which all pass through the tip.
Other descendants (noncardioid "circular" muatoms) whose name ends in .1/2a (such as R2.1/3/1.2a) also have an elliptical perioddomain. The ellipse passes through the nucleus of that muatom's parent (in this example, passing through the nucleus of R2.1/3a) and also passing through the tip of the parent's muunit (in this example, the branch point R2F(1/3B*)). Again, the ellipse has a major axis somewhat more than 4 times the diameter of the muatom and minor axis somewhat smaller.
All other descendants have a perioddomain with a teardrop shape, of a width about 3 to 4 times the muatom's diameter and a length about 6 or 7 times the muatom's diameter. For descendants ending in .1/3a and .1/4a the teardrop's tip touches the parent's nucleus; in the .1/4a case it tapers down to a zero angle (like a cusp). For higherorder descendants the tips do not reach the parent's nucleus. The rounded end of the teardrop extends some distance past the muatom (in calculated images this is usually obscured by other perioddomains of higher periods).
Size Relationships for Islands
For islands that do not have any tuning apart from their own, there is no single direct relationship between the size of the island and of its perioddomain. However, there are classes of islands that have a direct size relationship to their atom domains.
Here are the first few islands in the R2t series. As discussed in that article, their sizes diminish by 16 each time and their locations get 4 times closer to the tip. Since the perioddomains are all mutually tangent at the tip, it follows that the perioddomain is 4 times smaller. Thus we can see that for these islands, the size of the period domain is approximately the square root of the size of the island.

For the islands in this table, the relationship is approximately:
size_{island} ≅ 0.1 (size_{perioddomain})^{2}
or equivalently:
size_{perioddomain} ≅ √(10 size_{island})
For other nontuned islands the relationship is a bit different.
Islands that are part of a muunit smaller than the entire Mandelbrot set R2 are tuned, and always have a composite (nonprime) period. Here is a series approaching the Feigenbaum point from the west; As above we start with R2F(1/2B1)S but each time the period doubles.

In this case the relationship is simple: the perioddomain is about 20 times the size of the mumolecule's cardioid. Each step involves an additional tuning factor of .1/2, and the scaling factor at each step is the Feigenbaum constant ≅ 4.67.
Examples
Here again is the full view of R2. The colors are: white for period 1, blue for period 2, pink for 3, orange for 4, yellow for 5 and so on. Note in particular that small color "bubbles" around the various islands such as period4 R2F(1/3B1)S.
This view shows the immediate vicinity of R2F(1/2B1)S. Periods that are multiples of 3 (the island's period) are most prominent, and the locations of embedded Julia sets around it are made clear.
Here the period domains clearly show the locations and periods of the nucleus and paramecia of a 2fold embedded Julia set.
Several more images with atomdomain coloring are included in the secondorder embedded Julia set article.
revisions: 19961024 oldest on record; 20120421 add images; 20120422 expand description, add tables of measurements; 20120423 add Feigenbaum sequence; 20120424 extend algorithm description; 20120529 add distorted island example and table of contents 20121202 no simple size relationship for cardioids 20141105 add Peitgen & Richter quote; 20230319 add periods to the tables of examples
From the Mandelbrot Set Glossary and Encyclopedia, by Robert Munafo, (c) 19872023. Muency index
This page was written in the "embarrassingly readable" markup language RHTF, and was last updated on 2023 Mar 21. s.27