This shape reminds me of the feathers on an exotic bird:

(click or open in new window for full size)

(animated version)

The shape is created by reversing the digits of a number, so you could say it involves revvers and fevvers. I discovered it when I was looking at the Halton sequence. It’s a sequence of fractions created according to a simple but interesting rule. The rule works like this: take *n* in base *b*, reverse it, and divide reverse(*n*) by the first power of *b* that is greater than*n*.

For example, suppose *n* = 6 and *b* = 2. In base 2, 6 = 110 and reverse(110) = 011 = 11 = 3. The first power of 2 that is greater than 6 is 2^3 or 8. Therefore, halton(6) in base 2 equals 3/8. Here is the same procedure applied to *n* = 1..20:

1: halton(1) = 1/10_{[2]} → 1/2

2: halton(10) = 01/100_{[2]} → 1/4

3: halton(11) = 11/100_{[2]} → 3/4

4: halton(100) = 001/1000_{[2]} → 1/8

5: halton(101) = 101/1000_{[2]} → 5/8

6: halton(110) = 011/1000 → 3/8

7: halton(111) = 111/1000 → 7/8

8: halton(1000) = 0001/10000 → 1/16

9: halton(1001) = 1001/10000 → 9/16

10: halton(1010) = 0101/10000 → 5/16

11: halton(1011) = 1101/10000 → 13/16

12: halton(1100) = 0011/10000 → 3/16

13: halton(1101) = 1011/10000 → 11/16

14: halton(1110) = 0111/10000 → 7/16

15: halton(1111) = 1111/10000 → 15/16

16: halton(10000) = 00001/100000 → 1/32

17: halton(10001) = 10001/100000 → 17/32

18: halton(10010) = 01001/100000 → 9/32

19: halton(10011) = 11001/100000 → 25/32

20: halton(10100) = 00101/100000 → 5/32…

Note that the sequence always produces reduced fractions, i.e. fractions in their lowest possible terms. Once 1/2 has appeared, there is no 2/4, 4/8, 8/16…; once 3/4 has appeared, there is no 6/8, 12/16, 24/32…; and so on. If the fractions are represented as points in the interval [0,1], they look like this:

point = 1/2

point = 1/4

point = 3/4

point = 1/8

point = 5/8

point = 3/8

point = 7/8

(animated line for base = 2, *n* = 1..63)

It’s apparent that Halton points in base 2 will evenly fill the interval [0,1]. Now compare a Halton sequence in base 3:

1: halton(1) = 1/10_{[3]} → 1/3

2: halton(2) = 2/10_{[3]} → 2/3

3: halton(10) = 01/100_{[3]} → 1/9

4: halton(11) = 11/100_{[3]} → 4/9

5: halton(12) = 21/100_{[3]} → 7/9

6: halton(20) = 02/100 → 2/9

7: halton(21) = 12/100 → 5/9

8: halton(22) = 22/100 → 8/9

9: halton(100) = 001/1000 → 1/27

10: halton(101) = 101/1000 → 10/27

11: halton(102) = 201/1000 → 19/27

12: halton(110) = 011/1000 → 4/27

13: halton(111) = 111/1000 → 13/27

14: halton(112) = 211/1000 → 22/27

15: halton(120) = 021/1000 → 7/27

16: halton(121) = 121/1000 → 16/27

17: halton(122) = 221/1000 → 25/27

18: halton(200) = 002/1000 → 2/27

19: halton(201) = 102/1000 → 11/27

20: halton(202) = 202/1000 → 20/27

21: halton(210) = 012/1000 → 5/27

22: halton(211) = 112/1000 → 14/27

23: halton(212) = 212/1000 → 23/27

24: halton(220) = 022/1000 → 8/27

25: halton(221) = 122/1000 → 17/27

26: halton(222) = 222/1000 → 26/27

27: halton(1000) = 0001/10000 → 1/81

28: halton(1001) = 1001/10000 → 28/81

29: halton(1002) = 2001/10000 → 55/81

30: halton(1010) = 0101/10000 → 10/81

And here is an animated gif representing the Halton sequence in base 3 as points in the interval [0,1]:

Halton points in base 3 also evenly fill the interval [0,1]. What happens if you apply the Halton sequence to a two-dimensional square rather a one-dimensional line? Suppose the bottom left-hand corner of the square has the co-ordinates (0,0) and the top right-hand corner has the co-ordinates (1,1). Find points (x,y) inside the square, with *x* supplied by the Halton sequence in base 2 and *y* supplied by the Halton sequence in base 3. The square will gradually fill like this:

x = 1/2, y = 1/3

x = 1/4, y = 2/3

x = 3/4, y = 1/9

x = 1/8, y = 4/9

x = 5/8, y = 7/9

x = 3/8, y = 2/9

x = 7/8, y = 5/9

x = 1/16, y = 8/9

x = 9/16, y = 1/27…

animated square

The square fills with Halton points for (x ← base=2) and (y ← base=3) because 2 and 3 have no factors in common. Compare (x ← *b*=2) used with (y ← *b*=4) ← (y ← *b*=6) ← (y ← *b*=8) and (y ← *b*=10):

x ← *b*=2, y ← *b*=4

x ← *b*=2, y ← *b*=6

x ← *b*=2, y ← *b*=8

x ← *b*=2, y ← *b*=10

There are fractals in these patterns, as you can see from (x ← *b*=3, y ← *b*=9):

x ← *b*=3, y ← *b*=9

If the square is rotated, so that each vertex in turn becomes the origin (i.e., the bottom left-hand corner), this pattern appears for (x ← *b*=3, y ← *b*=9):

(colours adjusted as pixels are used again)

animated version

You can also use the Halton sequence to control the radius and angle of a point inside a circle. If radius, *r*, is controlled by base 2 and the angle, θ, is controlled by base 3, the interior of the circle fills evenly, like this:

*r* ← *b*=2, θ ← *b*=3

When the bases share factors, various patterns appear in the interior of the circle:

*r* ← *b*=2, θ ← *b*=6

*r* ← *b*=2, θ ← *b*=10

*r* ← *b*=3, θ ← *b*=6

*r* ← *b*=3, θ ← *b*=12

*r* ← *b*=6, θ ← *b*=8

*r* ← *b*=6, θ ← *b*=10

*r* ← *b*=6, θ ← *b*=12

*r* ← *b*=6, θ ← *b*=15

*r* ← *b*=6, θ ← various bases

In these shapes a point jumps clockwise from vertex to vertex. The length of the jump is found by multiplying the distance to the vertex by m = (reverse(n) / n). It should be apparent that sometimes m 1 (reverse(23)/23 = 32/23), and sometimes m = 1 (when n is a palindrome in base *b*):

reverse(n) / n, vertices = 4, base = 3

reverse(n) / n, *v* = 5, *b* = 2 (animated)

reverse(n) / n, *v* = 5, *b* = 2 (animated)

reverse(n) / n, *v* = 5, *b* = 3

reverse(n) / n, *v* = 5, *b* = various

In these shapes, m = n / reverse(n):

n / reverse(n), *v* = 4, *b* = 2

n / reverse(n), *v* = 4, *b* = 3 (animated)

n / reverse(n), *v* = 4, *b* = 3

n / reverse(n), *v* = 5, *b* = 4

n / reverse(n), *v* = 5, *b* = 4 (animated)