The Menger sponge -iteration 4- [*L'éponge de Menger -itération 4-*].

See a related picture:

See the first objects of this family (including this one):

(SubdivisionRules=TTT TFT TTT TFT FFF TFT TTT TFT TTT)

**Definition of the "standard" Menger sponge** (related to the **Cantor triadic set**): A cube is cut into 3x3x3=27 identical smaller cubes.
Then the 7 central subcubes
(6 for each face and 1 at the center of the cube) are removed.
At last this process is iterated recursively with the 27-7=20 remaining subcubes.
The fractal dimension of the Menger sponge is equal to:
log(20)
--------- = 2.726833027860842...
log(3)

The "standard" Menger sponge can be defined by means of **subdivision rules**.
Here is the way how each of the 27 cubes of the "standard" Menger sponge at a given level is subdivided:
"standard" Menger sponge
_____________________
/ \
TTT TFT TTT
TFT FFF TFT
TTT TFT TTT
\_/
Sierpinski carpet

or again:
TTT TFT TTT TFT FFF TFT TTT TFT TTT

where 'T' ('True') and 'F' ('False') means respectively "subdivide" and "do not subdivide".
The rules are repeated at each level, but they can be changed periodically and for example:
TTT TFT TTT TFT FFF TFT TTT TFT TTT FFF FTF FFF FTF TTT FTF FFF FTF FFF
\___________________________________/ \___________________________________/
"standard" Menger sponge complement

alternates the "standard" Menger sponge and its complement.
Obviously many other rules do exist as shown below...

Beside 'F' and 'T' some other possibilities exist: 'R' that means "subdivide" or "do not subdivide" Randomly
with a given threshold between 0 and 1 (0.5 being the default value)
and 'S' that means "Stop subdividing".
Obviously 'F', 'T', 'R' and 'S' can be mixed at will...

Moreover an amazing **cross-section** can be made using the plane:
2X - 2Y + 2Z - 1 = 0

the origin of the coordinates being at the center of the main cube
and the axis being parallel to its sides.

This process can be generalized in many different ways and for example:
3 3 3
2X - 2Y + 2Z - 1 = 0

(the curved one) or again:
1 2 1 2 1 2 2
(X - ---) + (Y - ---) + (Z - ---) = R
2 2 2

(the spherical one).

**The "standard" Menger sponge**:

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**Some non linear transformations of the "standard" Menger sponge**:

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**The complement of the "standard" Menger sponge**:

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**Some extended Menger sponges**:
Two ways of extending the "standard" Menger sponge. On the one hand one can change the
used volume (from a cube to a sphere for example). On the other hand one can change the
rules of subdividing each cube as well as their numbers...

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**The fractal "standard" Menger sponge**:

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**Definition of the Sierpinski carpet** (related to the **Cantor triadic set**): A square is cut into 3x3=9 identical smaller squares.
Then the central subsquare -grey- is removed.
At last this process is iterated recursively with the 9-1=8 remaining subsquares.
The fractal dimension of the Sierpinski carpet is equal to:
log(8)
-------- = 1.892789260714372
log(3)

See the first objects of this family (including this one):

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