Robert P. Munafo, 2012 May 29.
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. Rendering Mandelbrot images with period domains in different colors makes it easy to find certain island Mu-molecules and embedded Julia sets.
Given a single specific point, the "atom-domain period" of that point is an integer that is equal to the period of a nearby mu-atom. This might be called a "nearby dominant period", "period of the dominant mu-atom", or a "tuning" period.
Starting with the standard iteration calculation:
Z0 = 0
ZN+1 = ZN2 + C
and defining RN=|ZN|, the period of the dominant mu-atom or "atom-domain period" is:
Atom-Domain-Period = N(minimum(Ri))
This is the value of N when RN=|ZN| reaches a minimum (not counting the initial value R0=|Z0| which is always zero).
Here 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 dark blue area have a dominant period of 2. This means that when iterating these points, the value of Z2 has a smaller magnitude than all of the other iterates (except for the initial Z0=0, which does not count).
Similarly, all the points in the three pink regions have a dominant period of 3, so for them, the Z3 iterate is smaller than all other iterates.
In practice, we can only compute ZN2+C a finite number of times. A calculation of the period domain ends up being just the ame 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 = z2 + 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 function
This function yields an excellent way of locating mu-atoms of a given period. The image produced contains regions of solid color that surround the mu-atoms, because the minimum RN value occurs when the number of iterations is equal to the period of the mu-atom.
In the images below and in the second-order 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 mu-atoms. Here are four images of the mu-molecule 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 mu-molecules 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 RN.
For cardioid mu-atoms, the size of the period-domain is typically much larger than the cardioid (qualitatively, for non-tuned islands, the "diameter" of the cardioid's period-domain "blob" is approximately the square root of the "diameter" of the cardioid).
The period domain of any .1/2a descendant of a cardioid (i.e. the largest circular mu-atom on any island) is a roughly circular region with diameter 4 times as great as the mu-atom itself. In the case of highly distorted mu-molecules, 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 mu-atom's diameter.
Regardless of any distortion, the boundary of the .1/2a's period domain, and the boundaries of the period domains of all mu-molecules 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 highly-distorted island from the R2.C(0) article. Note the mustard-colored ellipse belonging to the .1/2a mu-atom, the red ellipse belonging to the .F(1/2B1)Sa island's cardioid, the blue ellipse belonging to the .F(1/2B1)FSSa island's cardioid, and so on, the boundaries of which all pass through the tip.
Other descendants (non-cardioid "circular" mu-atoms) whose name ends in .1/2a (such as R2.1/3/1.2a) also have an elliptical period-domain. The ellipse passes through the nucleus of that mu-atom's parent (in this example, passing through the nucleus of R2.1/3a) and also passing through the tip of the parent's mu-unit (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 mu-atom and minor axis somewhat smaller.
All other descendants have a period-domain with a teardrop shape, of a width about 3 to 4 times the mu-atom's diameter and a length about 6 or 7 times the mu-atom'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 higher-order descendants the tops do not reach the parent's nucleus. The rounded end of the teardrop extends some distance past the mu-atom (in calculated images this is usually obscured by other period-domains of higher periods).
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 period-domain. 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 period-domains are all mutually tangent at the tip, it follows that the period-domain 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:
sizeisland ≅ 0.1 (sizeperiod-domain)2
sizeperiod-domain ≅ √(10 sizeisland)
For other non-tuned islands the relationship is a bit different.
Islands that are part of a mu-unit smaller than the entire Mandelbrot set R2 are tuned, and always have a composite (non-prime) 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 period-domain is about 20 times the size of the mu-molecule'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.
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 period-4 R2F(1/3B1)S.
Several more examples are shown in the second-order 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
From the Mandelbrot Set Glossary and Encyclopedia, by Robert Munafo, (c) 1987-2013. Mu-ency index