The rhombic dodecahedron can be used to tessellate 3-dimensional space. It can be stacked to fill a space much like hexagons fill a plane.

This tessellation can be seen as the Voronoi tessellation of theface-centred cubic lattice. Some minerals such as garnet form a rhombic dodecahedral crystal habit. Honeybees use the geometry of rhombic dodecahedra to form honeycomb from a tessellation of cells each of which is a hexagonal prism capped with half a rhombic dodecahedron. The rhombic dodecahedron also appears in the unit cells of diamond and diamondoids. In these cases, four vertices are absent, but the chemical bonds lie on the remaining edges

The rhombic dodecahedron forms the hull of the vertex-first projection of a tesseract to 3 dimensions. There are exactly two ways of decomposing a rhombic dodecahedron into 4 congruent parallelepipeds, giving 8 possible parallelepipeds. The 8 cells of the tesseract under this projection map precisely to these 8 parallelepipeds.

The rhombic dodecahedron forms the maximal cross-section of a 24-cell, and also forms the hull of its vertex-first parallel projection into 3 dimensions. The rhombic dodecahedron can be decomposed into 6 congruent (but non-regular) square dipyramids meeting at a single vertex in the center; these form the images of 6 pairs of the 24-cell’s octahedral cells. The remaining 12 octahedral cells project onto the faces of the rhombic dodecahedron. The non-regularity of these images are due to projective distortion; the facets of the 24-cell are regular octahedra in 4-space.

This decomposition gives an interesting method for constructing the rhombic dodecahedron: cut a cube into 6 congruent square pyramids, and attach them to the faces of a second cube. The triangular faces of each pair of adjacent pyramids lie on the same plane, and so merge into rhombuses. The 24-cell may also be constructed in an analogous way using two tesseracts.

The tesseract represents 4 th dimension and is the projection or shadow of the rhombic dodecahedron

A 3D projection of an 8-cell performing adouble rotation about two orthogonal planes

1111 familiar ?

The rhombic dodecahedron forms the convex hull of the tesseracts vertex-first parallel-projection. The number of vertices in the layers of this projection is 1 4 6 4 1 – the fourth row in Pascal’s triangle.

Pascal triangle

Each number in the triangle is the sum of the two directly above it.

Patterns and properties

Pascal’s triangle has many properties and contains many patterns of numbers.

[edit]Rows

• When adding all the digits in a single row, each successive row has twice the value of the row preceding it. For example, row 1 has a value of 1, row 2 has a value of 2, row 3 has a value of 4, and so forth.
• The value of a row, if each entry is considered a decimal place (and numbers larger than 9 carried over accordingly) is a power of 11 ( 11ⁿ, for row n). Thus, in row two, ’1,2,1′ becomes 11², while ’1,5,10,10,5,1′ in row six becomes (after carrying) 161,051, which is 11⁵. This property is explained by setting x = ’10′ in the binomial expansion of (x + 1)^row=n, and adjusting values to the decimal system. But x can be chosen to allow rows to represent values in any base – such as base 3; 1 2 1₃['1,2,1'] = 4² (16), 2 1 0 1₃ ['1,3,3,1'] = 4³ (64) – or base 9; 1 2 1₉ = 10² (100), 1 3 3 1₉ = 10³ (1000) and 1 6 2 1 5 1₉ ['1,5,10,10,5,1'] = 10⁵ (100,000). In particular (see next property), for x = 1 place value remains constant(1^place=1). Thus entries can simply be added in interpreting the value of a row.
• The sum of the elements of row m is equal to 2^m−1. For example, the sum of the elements of row 5 is 1 + 4 + 6 + 4 + 1 = 16, which is equal to 2⁴ = 16. This follows from the binomial theorem proved above, applied to (1 + 1)^m−1.
• If rows are numbered starting with n = 0, the sum of the elements in the row is simply 2ⁿ, so row 0 adds to 2⁰ = 1, row 1 adds to 2¹ = 2, etc.
• Some of the numbers in Pascal’s triangle correlate to numbers in Lozanić’s triangle.
• The sum of the squares of the elements of row m equals the middle element of row (2m − 1). For example, 1² + 4² + 6² + 4² + 1² = 70. In general form:

• Another interesting pattern is that on any row m, wherem is odd, the middle term minus the term two spots to the left equals a Catalan number, specifically the (m + 1)/2 Catalan number. For example: on row 5, 6 − 1 = 5, which is the 3^rd Catalan number, and (5 + 1)/2 = 3.
• Another interesting property of Pascal’s triangle is that in rows where the second number (immediately following ’1′) is prime, all the terms in that row except the 1s are multiples of that prime.

A particularly popular polyhedron is the pyramid. If we restrict ourselves to regular polygons for faces, there are three possible pyramids: the triangle-based tetrahedron, the square pyramid, and the pentagonal pyramid. Being bounded by regular polygons, these last two fall within the class of Johnson solids. One interesting property of pyramids is that like the tetrahedron, their duals are also pyramids. (Incidentally, the Egyptian pyramids have square bases but the triangular side faces are not quite equilateral; they are very close to half a golden rhombus.)

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