Fig. 1a: A wide view of the office. Photo by Aris Noutsos.
Fig. 1b: A narrower view towards some of the most complex models of 4-dimensional polytopes. Photo by Aris Noutsos.
Fig. 1c: This is me in my polytope garden, holding a projection of the Grand Antiprism in 3 dimensions! Photo by Aris Noutsos.
|Geometric models in my office:|
|- 3-D paper models: Regular and Quasi-regular Polyhedra|
|- 3-D Zometool models|
|- The fourth dimension: Regular and Quasi-regular Polychora|
Convex Regular and Quasi-regular Polychora
Non-convex Regular Polychora
|- Zometool models of Coxeter-Dynkin diagrams, Triality and regular polychoron compounds|
|- Beyond the fourth dimension|
|Related pages in this site:|
|- The many stellations of the Rhombic triacontahedron|
|- My old collection of Uniform polyhedra|
Fig. 2: The convex polyhedra. Bottom row, from right to left: 1 - The Tetrahedron (the only model that has tetrahedral symmetry), 2 - its rectification, the Octahedron, 3 - the dual of the Octahedron, the Cube, 4 - the rectification of the Cube and Octahedron - the Cuboctahedron and 5 - the dual of the Cuboctahedron, the Rhombic dodecahedron (these last four have octahedral symmetry). Top row, right to left: 1 - the Icosahedron and 2 - its dual, the Dodecahedron, 3 - the rectification of the Icosahedron and the Dodecahedron - the Icosidodecahedron and 4 - the dual of the Icosidodecahedron, the Rhombic triacontahedron (these four have icosahedral symmetry). The five polyhedra on the right (Tetrahedron, Octahedron, Cube, Icosahedron and Dodecahedron) are the five platonic polyhedra, the only regular convex polyhedra. The Cuboctahedron and Icosidodecahedron (those with two colors) are the only convex quasi-regular polyhedra, and the two rhombic polyhedra in blue are their duals.
Frame of picture taken by Jason Wu.
Fig. 3: The platonic polyhedra, this time as dice. Picture by Aris Noutsos.
Fig. 4: The four Kepler-Poinsot polyhedra: the Small stellated dodecahedron (top left) and its dual, the Great dodecahedron (bottom left), the Great stellated dodecahedron (top right) and its dual, the Great icosahedron (bottom right).
Frame of picture taken by Jason Wu.
The relations between the Dodecahedron, Icosahedron and the Kepler-Poinsot polyhedra are displayed in Fig. 5 below. The vertical scale indicates the density. Duality is given by the two-sided golden arrows. For each face of a polyhedron, there is a perpendicular line from the centre of the dual polyhedron to its vertex; for each edge of a polyhedron, the dual polyhedron has a perpendicular edge. The duality operation transforms the face of a polyhedron into the vertex figure of its dual.
There are two ``stellation'' operations, that extend the faces of a particular polyhedron. The first, edge stellation, extends the edges of the faces in the same direction until they meet again, this is illustrated by the blue arrows. This can only start from polyhedra with pentagonal faces. The second operation, Greatening, replaces a face with a larger one of the same kind in the same plane; this is displayed by the green arrows. Both operations conserve the number of faces and their planes; therefore all polyhedra derived from the Dodecahedron (Great dodecahedron, Small stellated dodecahedron and Great dodecahedron) have 12 faces, as their names indicate. They represent the full set of stellations of the Dodecahedron; the model in Fig. 11b is useful to see how they can be derived geometrically by the process of extending the planes of the faces. The Great icosahedron, being a greatening of the Icosahedron, has 20 triangular faces. It is the only of the 59 stellations the Icosahedron that is regular.
In Fig. 5 we also present the faceting, or ``conservation'' operations, which as we describe below are the duals of the stellation operations. The solid red lines indicate faceting with conservation of edge arrangement: Indeed, the Great dodecahedron has the same edge arrangement as the Icosahedron, and the Great icosahedron has the same edge arrangement of the Small stellated dodecahedron, as can be seen in Fig. 4. The dashed lines in Fig. 5 indicate faceting with conservation of the vertex figure. Both the solid red lines and the dashed lines imply conservation of the vertex arrangement, which all faceting operations must obey by definition.
It follows from these conservation laws that the four polyhedra in the red circuit must share the same vertex arrangement - in this case of the Icosahedron. This means that they are facetings of the Icosahedron (see point number 3 above when introducing the non-convex polyhedra). The two polyhedra in the pink dotted line have the vertex arrangement and vertex figure of the Dodecahedron.
Why these conservation relations exist is explained below.
Fig. 5: Relations between the Dodecahedron, Icosahedron and the Kepler-Poinsot polyhedra.
(Technical bit: please skip if you're devoid of curiosity).
Why do these conservation relations exist? The answer is because they can be described as duals to the stellation operations. In particular:
1) Conservation of edges (red) = find dual + edge stellate + find dual
2) Conservation of vertex figure (dotted) = find dual + greaten + find dual
As an example of 1), let's move from the Icosahedron to the Great dodecahedron along the solid red line. This is the same as finding the dual of the Icosahedron (Dodecahedron), edge stellating (Small stellated dodecahedron) and finding the dual of the latter (Great dodecahedron). The first operation transformed the edges of the Icosahedron into the perpendicular edges, those of the Dodecahedron. The second operation merely extended those edges, but kept their directions. The last operation find edges perpendicular to those of the Small stellated dodecahedron - those of the original Icosahedron!
As an example of 2), let's move from the Dodecahedron to the Great stellated dodecahedron along the dotted pink line. This is the same as finding the dual of the Dodecahedron (Icosahedron), greatening (Great Icosahedron) and finding the dual of the latter (Great stellated dodecahedron). The first duality converts the triangular vertex figure of the Dodecahedron into the triangular face of the Icosahedron. Then greatening to a Great Icosahedron keeps the same type of face (as greatening always does). In the final dual transformation, the triangular faces of the Great Icosahedron are converted back to triangular vertex figures, as in the original Dodecahedron. Thus this operation preserves the vertex figure.
All faceting operations preserve the vertex arrangements. This is because their dual (the stellation operations) preserve the planes of the faces.
(end of technical bit)
Fig. 6: Two quasi-regular star polyhedra and their duals. Top right: the rectification of the Great dodecahedron and the Small stellated Icosahedron - the Dodecadodecahedron, bottom right: its dual, the Medial rhombic triacontahedron; Top left: the rectification of the Great Icosahedron and the Great stellated dodecahedron - the Great icosidodecahedron, bottom left: its dual, the Great rhombic triacontahedron.
The operation of rectification corresponds to finding the middle of the edges of a polyhedron and transforming it in a new vertex. Doing this for any two dual polyhedra results in the same polyhedron, since their edges are perpendicular and intersect at their midpoints. Doing this for the Kepler-Poinsot polyhedra, we always find the vertex arrangement of the Icosidodecahedron. Thus the Dodecadodecahedron and the Great Icosidodecahedron (in the top row) must be facetings of the Icosidodecahedron. Therefore their duals are stellations of the dual of the Icosidodecahedron, the Rhombic triacontahedron.
Frame of picture taken by Jason Wu.
Fig. 7: The ditrigonal quasi-regular star polyhedra and their duals. The Small ditrigonal icosidodecahedron (bottom right) and its dual, the Small triambic icosahedron (top right), the Ditrigonal dodecadodecahedron (bottom centre) and its dual, the Medial triambic icosahedron (top left), the Great ditrigonal icosidodecahedron (bottom left) and its dual, the Great triambic icosahedron (top left) - this is represented by the same model as the Medial triambic icosahedron as they only differ in their internal structure.
The three ditrigonal quasi-regular polyhedra share the same edge arrangement among themselves, and have the same vertex arrangement as the Dodecahedron, i.e., they are among its many facetings. This is also true for the compound of five cubes. Therefore their duals (in blue) (and the dual of the compound of five cubes, the compound of five octahedra) are stellations of the Icosahedron.
Fig. 8: Not all polyhedron models are made of paper. Here is a stained glass dodecadodecahedron. Made by Naomi Janches.
Fig. 9: Myself holding a beautiful geometrical model that illustrates many interesting relations between the Platonic polyhedra and their symmetries. This is sold by Zometool as Kepler's Kosmos, and was designed originally by John H. Conway. Photo by Aris Noutsos.
Fig. 10: Myself holding a model of an Octahedron with its only possible stellation, known as the Stella Octangula. This can also be seen as an intersection of two Tetrahedra. The outer vertices are those of a cube. These relations can be seen in the model above. Photo by Aris Noutsos.
Fig. 11a: This is the stellation pattern of the Dodecahedron; its face is the inner pentagon. The inner pentagram is the face of the Small stellated dodecahedron. The outer pentagon is the face of the Great Dodecahedron, and the outer pentagram is the face of the Great stellated dodecahedron.
Fig. 11b: Extending the 12 faces of the Dodecahedron as in Fig. 11a, we obtain the four stellations of the Dodecahedron listed in Fig. 11 a.
Fig. 12a: An incomplete stellation diagram of the Icosahedron, representing some of the figures that can be built with a small Zometool model. The inner Triangle if the face of the Icosahedron, the Green triangle is a face of the compound of five Tetrahedra and the outer triangle is a face of the Great icosahedron. An intermediate figure where each face is composed of three disjointed (blue) triangles, touching the face of the Icosahedron at its vertex, is the self-dual Excavated dodecahedron (which is, therefore, another faceting of the Dodecahedron). These are only four of the 59 stellations of the Icosahedron.
Fig. 12b: Extending the 20 faces of the Icosahedron as shown above, we have the four stellations of the Icosahedron mentioned above.
Fig. 13a: The stellation diagram of the Rhombic dodecahedron built with Zometool. The inner diamond if the face of the Rhombic dodecahedron.
Fig. 13b: The four stellations of the Rhombic dodecahedron. Each of the 12 faces of the Rhombic dodecahedron has been extended along their planes as shown above.
Fig. 14a: The inner region of the stellation diagram of the Rhombic triacontahedron built with Zometool. The outer Rhombus is the face of the Great rhombic triacontahedron. All other struts represent intersections of its 30 rhombic faces.
Fig. 14b: Excluding most struts and connectors, we see the faces of some of the more important stellations of the Rhombic triacontahedron. The inner red Rhombus is one of the 30 faces of the Rhombic triacontahedron. The yellow Rhombus is one of the 30 faces of the Medial rhombic triacontahedron. The blue square is one of the 30 faces of the compound of 5 cubes. As in Fig. 14a, the outer Rhombus is one of the 30 faces of the Great rhombic triacontahedron.
Fig. 14c: Extending the 30 faces of the Rhombic triacontahedron along their planes we obtain its stellations. If we include only the four stellations mentioned in Fig. 14b, then we obtain this model. The four stellations can be found very easily since they are represented by different colours. This is the model currently in my office since it is consistent with the ``4 stellations'' theme set by the previous models.
Fig. 14d: If, for each facial plane we include the complete set of struts and balls as in Fig. 14a, then we obtain this model, which represents the full set of stellations of the Rhombic triacontahedron up to the Great rhombic triacontahedron (the outer shape). As in Fig. 14a, every line represents the intersection of two faces of the Great rhombic triacontahedron.
For more on the latter figures, see the page dedicated to the many stellations of the Rhombic triacontahedron.
Fig. 15a: Front row: Vertex-first projections of three convex regular polychora: On the left is the simplest regular polychoron, the 5-Cell, which is bound by 5 Tetrahedral cells and has 5 vertices, with Tetrahedral vertex figures. This is the 4-dimensional analogue of the Tetrahedron. In higher dimensions, the analogues are known as Simplexes, all of them self-dual. This is one of the few projections in this page where the "near" and "far" sides of the projections do not coincide.
At the centre is the vertex-first projection of the 16-cell, which is bound by 16 Tetrahedral cells and has 8 vertices with Octahedral vertex figures. This the 4-D analogue of the Octahedron. The analogues in any dimension are known as Orthoplexes.
On the right is the projection of its dual, the 8-Cell, more commonly known as the Tesseract. This is bound by 8 Cubes and has 16 vertices with Tetrahedral vertex figures. This is the 4-D Hypercube.
As one might expect, Tesseracts can fill 4-D space, but more interestingly, the 16-cell can do that as well: The vertex arrangement of this 16-cell honeycomb is known as the F4 lattice. If a 3-sphere is centered in each point of this lattice, the resulting arrangement is the densest possible regular sphere packing in 4 dimensions, with kissing number 24.
In the row further back are the rectifications of these polychora (also, rectifications of the particular projections in the first row).
On the left is the cell-first projection of the Rectified 5-cell. For each of the 5 Tetrahedra of the 5-cell, this has 5 rectifications of the Tetrahedron, or Octahedra. Each vertex of the 5-cell was replaced by its vertex figure, a Tetrahedron. This is one of the 3 non-regular uniform polychora that has only regular cells, these are known as a Semi-regular polychora. In particular, this is a member of a multi-dimensional family of Semi-regular polytopes, where the facets are Simplexes and Orthoplexes, known as the k21 polytope family, where k is the dimension − 4. Thus the Rectified 5-cell is the 021 polytope. More about this family below. This is another of the projections in this page where the "near" and "far" sides of the projections do not coincide.
In the middle is a cell-first projection of the Rectified 16-cell. More about this object below.
On the right is tetrahedral cell-first projection of the Rectified tesseract. Each of the 8 Cubes of the Tesseract was replaced with rectifications of the Cube, Cuboctahedra. Each of the 16 vertices of the Tesseract was replaced by the Tesseract's vertex figure, Tetrahedra. This is another case of a model where the "near" and "far" sides of the projections do not coincide: this is the reason why the model displays two crossed Tetrahedra at the centre (the "Stella Octangula" design).
The two models on the left have pentachoric symmetry, the other four share the hexadecachoric symmetry, but one of them (the Rectified 16-cell, below) has a higher degree of symmetry.
Picture taken by Jason Wu
Fig. 15b: In Fig. 15a we displayed the vertex-first projections of the 16-cell and Tesseract. Here we display, in the front row, their cell-first projections. Behind them we display the rectifications of these polychora, specifically rectifications of these projections. The projection of the rectified Tesseract (top right) is different from that shown in Fig. 15a: Instead of being centered on a Tetrahedron, it is centered on a Cuboctahedron. However, the rectification of the cell-first projection of the 16-cell is identical to the rectification of the vertex-first projection in Fig. 15a. This is because both are centered on Octahedra. There is a deeper, and rather beautiful reason for this, see Fig. 29.
This projection of the 16-cell is especially good for highlighting its 8 vertices separately. These represent half of the vertices of the Tesseract, its dual polytope, i.e., the 16-cell is a ``Demi-tesseract'', the 4-D Demi-hypercube (the 3-D version, the Tetrahedron is a ``Half-cube'', see the ``Stella Octangula'' model in Fig. 10). This particular projection also depicts two superposed 16-cells that together have all the vertices of the Tesseract.
Note: the two models on the right are not permanently in my office.
Fig. 16: Left: Yet again, the Rectified 16-cell. For each of the Tetrahedral cells of the 16-cell, this has their rectifications, 16 Octahedra. The 8 vertices of the 16-cell are here replaced by the latter's vertex figures, also Octahedra. This is therefore a regular polychoron bound by 24 Octahedra in total; it is more commonly known as the 24-cell, or Polyoctahedron. This polychoron has many interesting characteristics: It is a self-dual polychoron with no analogue in any dimension other than 4. Its 24 vertices are those of the Tesseract and the 16-cell combined, or the combination of three 16-cells (since the 16-cell is a Demitesseract). Also, this particular model also represents a regular polychoron compound, with three Tesseracts. For more on this see Figs. 31a and 31b.
The 24-cell can fill 4-D space, in what is known as the regular Icositetrachoric honeycomb. This is dual to the 16-cell honeycomb mentioned in the caption to Fig. 15a.
Right: A cuboctahedral cell-first projection of the Rectified 24-cell. For each Octahedron of the 24-cell the latter has 24 Cuboctahedra (the rectification of the Octahedron). For each of the 24 vertices of the 24-cell, the Rectified 24-cell has a Cube (the vertex figure of the 24-cell). This polychoron has 96 vertices, one for each edge of the 24-cell. You can see how to build this model in the Eusebeia page on the Rectified 24-cell (for this particular model, see the part on the "Cantellated 16-cell").
Both polychora have icositetrachoric symmetry.
Picture taken by Jason Wu.
Fig. 17: This is an Icosahedral cell-first projection of yet another convex uniform polychoron, the Snub 24-cell. This can be built from the Rectified 24-cell in Fig. 16 by diagonally bisecting all squares. If the diagonals are chosen correctly, then each of the 12 vertices of the Cuboctahedron now has 5 edges converging on it, i.e., it has been transformed into an irregular Icosahedron. The Cubes have been transformed into sets of 5 Tetrahedra, with only the central one being regular. Then we shorten the diagonals of these former squares in order to make two regular triangles. This generates one regular Icosahedron for each of the previous 24 Cuboctahedra, and five regular Tetrahedra for each of the 24 previous Cubes, making a total of 120 Tetrahedra. This operation preserved the number of vertices, which is still 96.
Because all cells are regular, this is also a semi-regular convex polychoron. This polychoron is neither regular nor a rectification, i.e., its Wythoff construction is very different from most polychora in this page. The vertices and edges of this model are a subset of those of the model of the 600-cell just below, i.e., this is a diminished 600-cell.
You can see how this might be built from Zometool parts by looking at the Eusebeia page on the Snub 24-cell. Also recommended is David Richter's page on the 24-cell and its snub.
Picture by Jason Wu.
Fig. 18: This is the vertex-first projection of a regular polychoron known as the 600-cell, or Polytetrahedron (see instructions in David Richter's list of Zometool projects). This is a 4-D analogue of the Icosahedron.
This polychoron can be built from the Snub 24-cell in Fig. 17 by adding on ``top'' of each of the latter's 24 Icosahedra a 4-D ``Pyramid'' consisting of an Icosahedral ``base'' and 20 regular Tetrahedra as ``sides''. The 24 apices of these Pyramids have the same arrangement as the vertices of a 24-cell. Together with the previous 96 vertices of the Snub 24-cell, they add to a total of 120 vertices. The 120 Tetrahedra of the Snub 24-cell are now joined by the 24 × 20 Tetrahedra = 480 Tetrahedra of the new 4-D ``Pyramids'', making a total of 600 Tetrahedral cells. The apices of the new ``Pyramids'' have Icosahedral vertex figures, this is true for all other vertices as well because the 600-cell is regular.
The high degree of symmetry of the 600-cell implies that there is nothing special about the new 24 vertices added to the Snub 24-cell to form the 600-cell. Just as the latter have the same arrangement as the vertices of the 24-cell, the previous 96 vertices can be decomposed into 4 groups of 24 vertices, each also with the same arrangement as the vertices of a 24-cell. This means that from the set of 120 vertices of the 600-cell we can also build a set of five 24-cells, the latter is a self-dual regular polychoron compound and is shown in Figs. 32a and 32b.
Picture by Jason Wu.
Fig. 19: This is the cell-first projection of the dual of the 600-cell, the 120-cell, or Polydodecahedron. This is bound by 120 Dodecahedra, it has 600 vertices with Tetrahedral vertex figures; this is a 4-D analogue of the Dodecahedron. This model is larger than the set sold by Zometool as the hyperdo by φ, the famous golden ratio, φ (it is also φ times smaller than the Jumbo version of the hyperdo).
Picture taken by Jason Wu.
Fig. 20: The icosahedral cell-first projection of the Rectified 600-cell. The 600 Tetrahedra of the 600-cell are here replaced by their rectifications, which are Octahedra. In the position of each of the 120 vertices of the 600-cell 120 Icosahedra (the vertex figure of the 600-cell) appeared. Because all cells are regular, this is the last of the three semi-regular polychora.
You can see how this might be built from Zometool parts by looking at the Eusebeia page on the Rectified 600 cell. Also recommended is David Richter's page on the rectified 600-cell. This model is no longer in my office.
Fig. 21: The icosidodecahedral cell-first projection of the Rectified 120-cell. The Dodecahedra of the 120-cell are here replaced by their rectifications, Icosidodecahedra. In the position of each of the 600 vertices of the 120-cell, 600 Tetrahedra (the vertex figure of the 120-cell) appeared. You can see how this might be built from Zometool parts by looking at the Eusebeia page on the Rectified 120 cell. This model is no longer in my office.
Picture taken by Jason Wu.
The last 4 polychora have hexacosichoric symmetry.
Fig. 22: Another diminished 600-cell, the Grand antiprism. This was the last uniform convex polychoron to be discovered (in 1964) by John H. Conway and Michael Guy; they also proved that there are no more undiscovered uniform convex polychora. This is an anomalous uniform polychoron in the sense that it has no Wythoffian construction. Its structure also helps understanding the structure of the 600-cell, in particular the fact that the latter can be decomposed in two disjoint perpendicular rings of 150 Tetrahedra each, separated by a Clifford torus of 100 Tetrahedra with the two sets of 100 Tetrahedra that border that torus on each side. In the Grand Antiprism, both tetrahedral rings have been removed and replaced with rings of 10 Pentagonal Antiprisms; in this model we're looking through one of these rings.
You can see how this might be built from Zometool parts by looking at the Eusebeia page on the Grand antiprism.
Picture by Aris Noutsos. Linked picture by Jason Wu.
Fig. 23a: This model represents the vertex-first projection of two other regular Schläfli-Hess polychora which share the same edge arrangement, the Small stellated 120-cell and the Great grand 120-cell (see instructions in David Richter's dedicated page).
Fig. 23b: This is how it looks like in my office. Picture by Jason Wu.
As shown in the caption to Fig. 23c (later in this page), this model also represents one of the two regular compounds of 25 24-cells and one of the two regular compounds of 75 Tesseracts.
Fig. 24a: This model represents the vertex-first projections of four Schläfli-Hess polychora that share the same edge arrangement: The Grand 600-cell, the Great stellated 120-cell, the Grand stellated 120 and the Great icosahedral 120-cell.
Fig. 24b: This is how it looks like in my office. Picture by Jason Wu.
According to David Richter's page on this polytope, the edges in this model are images of the edges of the quasicrystalline projection of Gosset's eight-dimensional 421 polytope, the largest finite member of the k21 polytope family mentioned above. More about this family below.
Fig. 25a: The last of the Schläfli-Hess polychora, the most complex of the 16 regular polychora, the final stellation of the 120-cell: The Great grand stellated 120-cell. Here it is seen from a 5-fold symmetry axis.
Fig. 25b: Here seen from a 3-fold symmetry axis.
This model can use 20 very long yellow struts, no longer sold by Zometool (but easily found on eBay). These will be directly aligned with the 3-fold symmetry axis shown in this Figure. This saves 40 regular-sized struts and 20 balls and slightly improves the appearance of the model.
See also David Richter's page on this model.
With this model, the list of regular polychora is complete!
Fig. 26a: Stellation relations between the 120-cell, 600-cell and the Schläfli-Hess polychora; these provide an explanation of their names. As in Fig. 5, the vertical scale indicates the density and the two-sided gold arrows denote duality; note that the Great 120-cell and the Grand stellated 120-cell are self-dual. To a cell in a polychoron corresponds a vertex in its dual (the line from the centre to the vertex is perpendicular to the 3-D plane of the cell), to a face in the polychoron corresponds a perpendicular edge in its dual going through the face's centre, to an edge in the polychoron corresponds a face in its dual in a perpendicular plane and to a vertex in the polychoron corresponds a cell in its dual.
The dark blue arrows denote edge stellation, which has the same meaning as in 3-D. In the case of the 120-cell, this operation produces the Small stellated 120-cell. Each of the 120 Dodecahedra of the 120-cell was edge-stellated into a Small stellated dodecahedron (Fig. 4); the latter has only 12 vertices (one for each face of the Dodecahedron) arranged as in the Icosahedron. Likewise, the Small stellated 120-cell has only 120 vertices - one for each cell of the 120-cell, arranged as in the 600-cell. By the rules of duality, the dual polychoron, the Icosahedral 120-cell, will also have 120 cells and 120 vertices.
The light blue arrows denote greatening; as in 3-D this means replacing a face by an identical, larger one in the same plane. Finally, the green arrows denote a new operation for polychora, aggrandizing (replacing cells by identical larger ones in the same 3-d ``plane'').
Analogously to the 3-D case, none of these operations changes the number of cells or their 3-d ``planes''. Therefore, all stellations of the 120-cell (the polychora on the ``cubic'' chain starting on the 120-cell) must have 120 cells as well. As explained 2 paragraphs above, this is also true for the Icosahedral 120-cell, so it is also true for the latter's stellation, the Great icosahedral 120-cell. The Grand 600-cell is the only stellation of the 600-cell, so it is the only Schläfli-Hess polychoron with 600 cells. This makes it the 4-d equivalent of the Great Icosahedron in Fig. 4. Adapted from Conway, Burgiel and Goodman-Strauss (see references below).
Fig. 26b: Faceting relations between the 120-cell, 600-cell and the Schläfli-Hess polychora. As in Fig.5 these operations can be seen as the duals of the stellation operations in Fig. 26a, i.e., they are like a mirror image of the latter.
Since none of the stellation operations changes the number and hyperplane of the cells, all faceting operations preserve the vertex arrangements, as in 3 dimensions. Therefore, we see straight away that the eight polychora in the cubic chain starting on the 600-cell (the duals of eight polychora in the stellation chain of the 120-cell shown in Fig. 26a) have its vertex arrangement, i.e., they are all facetings of the 600-cell. As mentioned in Fig. 26a, this is also true for the Small stellated 120-cell, so its faceting, the Great grand 120-cell, must also have the vertex arrangement of the 600-cell.
The Great grand stellated 120-cell, one of the many stellations of the 120-cell, is the only regular faceting of the 120-cell, i.e., it is the only Schläfli-Hess polychoron with 600 vertices. It is the dual of the Grand 600-cell, one of the many facetings of the 600-cell that is its only regular stellation. This makes the Great grand stellated 120-cell a 4-D analogue of the Great stellated dodecahedron (see Fig. 4), which it has as cells.
We now describe the individual faceting operations. The solid red and gold lines are equivalent to: finding dual + edge stellating + finding dual. Since the edge stellation preserves the edge directions, the dual faceting operation preserves the face arrangements, which also imply the same edge arrangement. The polychora at the ends of these lines are not identical because their cells differ.
The dashed lines are equivalent to find dual + greaten + find dual. Since greatening preserves the type of face and its plane, the dual faceting operation preserves the edge arrangement. Since Zometool models only depict edge arrangements, this conservation law is what defines how the polychora are represented: The 4 polychora in the red circuit can be depicted by the model of the 600-cell (Fig. 18), the 2 polychora in the purple line are depicted by the model of the Small stellated 120-cell (Figs. 23a and b) and the 4 polychora in the gold circuit can be depicted by the model of the Grand 600-cell (Figs. 24a and b). The Zometool models of the 120-cell (Fig. 19) and the Great grand stellated 120-cell (Figs 25a and b) depict only those polychora.
The solid blue lines are equivalent to: find dual + aggrandize + find dual. Since aggrandizing preserves the cell type, the dual faceting operation preserves the vertex figure. The light blue lines show that each polychoron in the red family has a faceting in the yellow family with exactly the same vertex figure. The dark blue line shows that the only faceting of the 120-cell, the Great grand stellated 120-cell, also shares the former's vertex figure.
Fig. 27: These are the C-D diagrams of the regular polygons used in the polychora above, plus their generating groups. All these graphs are left-right symmetric, this means that all regular polygons are self-dual. The number of such diagrams is, of course, infinite.
Fig. 28: These are the C-D diagrams of the regular and quasi-regular polyhedra displayed above, plus their generating groups. On the left column we have the graphs for the groups, in the middle column we have the convex polyhedra, and in the right column we have the star polyhedra.
These diagrams include the polygonal diagrams in Fig. 27, such polygons are thus a part of the polyhedron. For instance, for the Cube, the only polygon graph that can be built with the red ball is the Square. However, for the Icosidodecahedron, we can form the graphs of the Pentagon and the Equilateral triangle with the red ball (since the graph can be read in both directions); thus the polyhedron is bound by both types of polygons. Two neighbouring white nodes indicate a regular vertex figure: from this, we can see immediately that the vertex figure of the Cube is a regular triangle.
At the start of the page we said that regular and quasi-regular polyhedra share the same Kaleidoscopic Wythoff construction. This fact is evident in the C-D diagrams by the fact that all polyhedron graphs have only one red ball, i.e., they are reflections of vertices of their Schwarz triangles. Other Wythoff constructions, which reflect points in the edges or even faces of the Schwarz triangles to create many uniform polyhedra, would require more than one red ball.
An important point to re-emphasize is that a particular polyhedron (like the Octahedron) can be represented by different diagrams. This is a reflection of the fact that the same polyhedron can be produced from different symmetries. These polyhedra have several other C-D diagrams, we present here those where only one red ball is needed; which directly implies regularity or quasi-regularity.
Fig. 29: These are the C-D diagrams of the convex polychora presented above, plus their generating groups.
On the left we have most of the group graphs. In the middle we have the convex polychora displayed above. These include as sub-sets the polyhedral graphs in the middle column of Fig. 28. For instance, in the 16-cell, the only polyhedral graph that can be built with the red ball is the Tetrahedron, therefore the 16-cell only has Tetrahedra, and is therefore regular. In the case of the rectified 5-cell, however, we can build the graphs of the Octahedron and the Tetrahedron from the red ball; this means that that polychoron is bound by both types of polyhedra. Three neighbouring white nodes indicate a regular vertex figure (which happens, of course, only for regular polychora), with the equivalent of the red ball next to the actual red ball. Doing this for the 600-cell, for instance, we see that the vertex figure is an Icosahedron.
Most (but, unlike in Fig. 28, not all) of the polytope graphs presented here have the same Wythoff construction, i.e., they have one red ball. The exceptions are the diminished 600-cells: the Snub 24-cell (see Fig. 17), which has two green balls (indicating rotational but not reflexive symmetry) and the Grand antiprism. The latter does not appear in this Figure because (as mentioned in the caption to Fig. 22) it has no Wythoff construction. Many other convex uniform polychora are possible with the use of more red balls, but building all of them requires many parts, as one can see in David Richter's H4 page.
Studying these diagrams, we can see that the process of rectification of a regular polychoron with X-type of cell and Y-type of vertex figure generates a new polychoron for which the cells are rectified Xs (as many as the previous number of cells) and Ys (as many as the previous number of vertices). The number of vertices will be the same as the number of edges in the polychoron before.
Again, almost all of these polychora can be represented by multiple C-D diagrams; those presented here are those that require only one red ball. Note that the 24-cell is represented by 3 different diagrams in the figure, and the 16-cell by two. Some of these extra diagrams appear in the right column, we have, on top, the Demi-hypercubic group D4. In 4 dimensions, this type of symmetry does not yet produce new polytopes, this starts happening only in 5 dimensions. However, it has some interesting consequences, one of them being Triality.
This concept deserves a detailed explanation. In the case of duality in 3 dimensions, one can, by the definition of the meaning of duality, scale two concentric dual polyhedra in such a way that each edge from a polyhedron meets another (perpendicular) edge from the dual polyhedron at a point that is in the middle of both (the midpoint). Those two edges form a plane that is perpendicular to a third line joining the midpoint to the centre of those polyhedra. Rectifying those two dual polyhedra results in the same rectified polyhedron.
In the case of Triality in 4 dimensions, the situation is very similar, except that the perpendicular edges of three concentric ``trial'' polychora (16-cells for the only regular case) meet at their midpoints. These edges are in a 3-D ``plane'' that is itself perpendicular to a fourth direction, the line joining those midpoints to the centres of the trial polychora. Rectifying all three polychora results in the same rectified polychoron, the regular case being the 24-cell.
The same applies to projections: if we pick three ``trial'' projections of the 16-cell and rectify them, we obtain an invariant projection of the 24-cell. In this page, we show a triad of such trialities. The first we met already in Figs. 15a and 15b, triality is the reason for the identical rectifications of the vertex-first and cell-first projections of the 16-cell. The other two are shown in Figs. 30a and 30b. This shows the predictive power of the C-D graphs.
Fig. 30a: Three edge-first projections of the 16-cell, with the central edges of the projections loosely aligned along 3 perpendicular orthogonal axes in 3-d space. Rectifying all three projections results in the same projection of the 24-cell, which is shown in the centre. This is its the vertex-first (or ``pyritohedric'') projection, the dual of the cell-first projection in Figs. 15a, 15b, 16 and 31a.
Fig. 30b: The three ``prismatic'' projections of the 16-cell. Rectifying all three projections results in the same projection of the 24-cell, also known as its ``prismatic'' projection, this is shown in the centre.
For a detailed explanation of these models, see David Richter's Triality with Zometool page. Note: These models are not permanently in my office.
Fig. 31a: Superposing the 3 orthogonal 16-cells in Fig. 30a, we obtain, on top right, a regular polychoron compound of three 16-cells. This is the first stellation of the 24-cell; this particular projection results from stellating the pyritohedric projection of the 24-cell. Since the latter has the Rhombic dodecahedron as its outer surface, this projection has, as its outer surface, the first stellation of the Rhombic dodecahedron; the latter is also known as ``Escher's polyhedron''.
Each of the individual Octahedra of the 24-cell was stellated into a ``Stella Octangula'', which has two Tetrahedra. Therefore, the compound of three 16-cells can also be seen as a compound of 24 ``Stella Octangula'', with a total of 48 Tetrahedral cells.
The green balls in this model represent false vertices (where 3 edges from 3 different 16-cells intersect in the middle - remember the triality!), the red balls superpositions of false and real vertices, and the white balls are real vertices. These ``false'' vertices are the real vertices of the rectification of these 16-cells - the pyritohedric projection of the 24-cell in Fig. 30a. The real vertices have the same vertex arrangement of a larger, cell-first projection of the 24-cell seen on the top left - this is the dual of the projection of the pyritohedric 24-cell. This is a consequence of the fact mentioned in the caption of Fig. 16 that the vertices of three 16-cells make the vertices of a 24-cell.
The dual of the edge-first projection of the 16-cell (lower right) is the face-first projection of the Tesseract (lower left). This means that the dual of the projection of the compound of three 16-cells shown above must be another regular compound, with three (face-first) tesseracts. Amazingly, that regular compound is well represented by the model of the 24-cell above left: each of its vertices belongs to two Tesseracts, and both the vertex and edge arrangements are identical to that of the cell-first projection of the 24-cell. With some attention, you might discern the 3 face-first Tesseracts in that model.
Note that combining these projections of the 16-cell and Tesseract, we obtain a compound with the same (real) vertices of both models above, but a mix of edges.
Fig. 31b: Combining one vertex-first projection of the 16-cell in Fig. 15a with two complementary superposed cell-first projections of the 16-cell in Fig. 15b, one obtains, on the left, a different projection of the compound of three 16-cells. This projection is the stellation of the cell-first projection of the 24-cell; since the latter has a Cuboctahedral envelope; its stellation's envelope must be a composite of a Cube and a Octahedron. The real vertices (the white balls) have the same vertex arrangement of the pyritohedric projection of the 24-cell (on the right) and of course the false vertices (in red) are the same as the cell-first projection of the 24-cell in Fig. 31a.
The dual of this projection of the compound of three 16-cells is the projection of the compound of three Tesseracts, also on the right, which looks exactly like the pyritohedric projection of the 24-cell. Indeed, we can see that this model looks exactly like a superposition of the projections of the Tesseract in Figs. 15a and 15b.
Fig. 31c: Combining the three prismatic projections of the 16-cell in Fig. 30b one obtains, on the right, the prismatic projection of the compound of three 16-cells.
The false vertices (in green) have the same arrangement as the vertices of the prismatic projection of the 24-cell shown on the left. Through each of the false green vertices goes a single edge of all three projections of the 16-cell in figure 30b. This shows that the prismatic projection of the 24-cell on the left really is the rectification of those three projections of the 16-cell. Furthermore, this also shows that this projection of the compound of three 16-cells is the stellation of the prismatic projection of the 24-cell. The red balls are edge intersections that appear because of the projection; they are not real edge intersections in 4 dimensions.
The real vertices of the compound of three 16-cells (white balls) have the same arrangement of the dual of the prismatic projection of the 24-cell, a projection of the 24-cell that cannot be built in the Zometool system.
For more details on this projection of the compound of three 16-cells, check David Richter's dedicated page. Note: these models are not permanently in my office.
One of the advantages of the prismatic projection of the compound of three 16-cells is that is shows all 48 real and false vertices of the compound separately - no superposed vertices here. The vertex arrangement of the dual configuration of 24-cells represents the 48 root vectors of the F4 group. Like the 24-cell, this is a truly exceptional object.
Fig. 32a: With five 24-cells (one in pyritohedric projection, as in Fig. 30a, and four in prismatic projection, as in Fig. 30b, all built on a φ2 larger scale), we obtain a self-dual regular polychoron compound. In this picture, we're looking at its 3-d projection through a genuine three-fold symmetry axis. This compound has 5 × 24 = 120 vertices, which happen to have the same arrangement as the 120 vertices of the 600-cell in Fig. 18. This illustrates the fact mentioned in the caption of that figure that the vertices of the 600-cell can be decomposed in five sets, each with the same arrangement as the vertices of a 24-cell. The edges of this model are a sub-set of the edges of the Small stellated 120-cell in Figs. 23a and 23b.
Fig. 32b: The regular compound of five 24-cells, now seen through a five-fold ``Ghost symmetry'' axis. Note that since the edges and vertices of the compound of three Tesseracts coincide with those of a 24-cell, the model above also represents another regular compound, one with 15 Tesseracts. In that case, each of the 120 vertices is shared by two Tesseracts, instead of one 24-cell.
See detailed explanation of this model in David Richter's dedicated page, which includes a part count. Here we're building this model on a φ smaller scale than the model in David Richter's page. This has an interesting implication: If we can find some of the very long old struts no longer manufactured by Zometool (12 very long reds, 12 very long yellows), we can use each of them to replace a combination of two struts and one ball where the ball is not strictly necessary (i.e., a ball where thers is no intersection with other edges). Doing this, we can save 48 regular-sized struts and 24 balls compared to the part count in David Richter's page, and improve the appearance of the model.
Fig. 33a: Combining the pyritohedric projection of the compound of 3 16-cells (in Fig. 31a) with four prismatic projections of the compound of 3 16-cells (in Fig. 31c), all built on a φ2 larger scale than in those figures, we obtain the dual of the compound of 15 Tesseracts in Figs. 32a and b, the (also regular) compound of fifteen 16-cells. This is also the stellation of the compound of five 24-cells in Figs. 32a and b. Here we see it through the 3-fold symmetry axis. The 15 × 8 = 120 vertices are also those of a 600-cell. Interestingly, in this projection they are not arranged as the vertices of the projection of the 600-cell in Fig. 18, but as a sub-set of the 600 vertices of the 120-cell in Fig. 19. This compound can also be seen as a compound of 120 ``Stella Octangula''.
Fig. 33b: The compound of 15 16-cells, here seen through a 5-fold ``ghost symmetry'' axis.
See also David Richter's dedicated page, which has a detailed description and part count. As in the model of the compound of 5 24-cells, if we can find some of the very long old struts (in this case, 48 very long reds, 120 very long yellows and 72 very long blues), we can save 480 regular-sized struts and 240 balls compared to the part count in David Richter's page, and greatly improve the appearance of the model.
Fig. 32c: Rectifying the compound of 15 16-cells, we produce three superposed copies of the compound of 5 24-cells. As in Fig. 32b, we're looking at the model through one of its ghost symmetry axes. However, in this photo the strut colors are not as obvious, so the model resembles even more the model of the Small stellated 120-cell in Figs. 23a and 23b.
As remarked above, the edges of this model are a sub-set of the edges of the Small stellated 120-cell in Figs. 23a and 23b, while the vertices have the same arrangement as the latter (which are the same as the 600-cell). For each axis of ghost symmetry, we can superpose four other sets of five 24-cells, each set rotated relative to the previous by 72 degrees around that axis. The result is in the picture below:
Fig. 23c: This is another regular compound, one of the pair of dual compounds that has 25 24-cells. In this compound the 600 vertices still have the same arrangement as the 600-cell; i.e., they appear in 120 unique points, with each point representing the vertices of five different 24-cells. The 96 × 25 = 2400 edges will twice cover be the full set of 1200 edges of the Small stellated 120-cell, i.e., each edge of the Small stellated 120-cell is shared by two 24-cells. Thus the Zometool model of this compound of 25 24-cells is also the model of the Small stellated 120-cell in Figs. 23a and 23b!
Again, because the edges and vertices of the compound of three Tesseracts coincide with those of a 24-cell (with each vertex shared by two Tesseracts), this model has the same vertices and edges as one of the two regular compounds of 75 Tesseracts! This means that this model also represents one of these compounds of 75 Tesseracts, the one where each of the 120 vertices (arranged as those of the 600-cell) is shared by 10 Tesseracts.
Fig. 34: These are the C-D diagrams of the Schläfli-Hess polychora presented above (on the left column), plus those of their rectifications (on the right column). I have built models for all polychora on the left, but not for their rectifications (for lack of space, time and money...), except for one or two (see below).
These also include the star polyhedra diagrams in the right column of Fig. 28. From these diagrams, we can see, for instance, that the Small stellated 120-cell is bound by Small stellated dodecahedron cells (the only polyhedral graph that can be built from the red ball) and that this has a Dodecahedral vertex figure. Thus its dual, the Icosahedral 120-cell, has Icosahedra as cells, and Great dodecahedra as vertex figures. We can also see that, following the logic of rectification, the Rectified small stellated 120-cell has Dodecadodecahedra and Dodecahedra as cells, and that the Rectified icosahedral 120-cell has Icosidodecahedra and Great dodecahedra as cells.
Note that the edges of the latter rectified polychoron are identical to those of the Rectified 600-cell (Fig. 20); thus both can be represented by the same Zometool model. The identical rectification is a consequence of the conservation laws in Fig. 26b: the rectifications of two polytopes that share the same edge arrangement will necessarily share the same vertex arrangement, since the vertex of the rectification is the middle point of the edge of the original polytope. The rectifications of two polytopes that additionally share the same face arrangement (like the 600-cell and the Icosahedral 120-cell) will necessarily share the same edge arrangement.
The symmetry of graphs for the Great 120-cell and the Grand-stellated 120-cell indicates that these are, as remarked above, self-dual.
Fig. 35: C-D diagrams for some of the higher-dimensional polytopes. In the centre we see the infinite Demi-hypercubic family. All polytopes of this family in 3 and 4 dimensions are also generated by the B3 and B4 groups. In 5 dimensions, the D5 family starts generating unique symmetries that are not present in the An or Bn families.
We also see the k21 family evolving along the diagonal. This ends at 8 dimensions, with the 421 polytope. The next member of the family is an infinite tesselation filling 8-dimensional space.
Fig. 36: A Zometool model of the 421 polytope. The 240 vertices project here as the vertices of two concentric 600-cells, with one of them being larger than the other by a factort of φ. It is not possible to project all the edges into a 3-D Zometool model, so we must choose a sub-set. In this case, we chose the edges of the projections of the two concentric 600-cells, exactly as in David Richter's model. Here we can see that projecting higher-dimensional figures in three dimensions leads to a concentration of vertices and edges near the centre.
This model required many old very long struts, no longer manufactored by Zometool: 120 very long yellows, 120 very long blues, and 60 very long reds. Alternatively, the model can be made on a smaller scale.
Fig. 37: Print on my door: The orthographic projection of the 8-dimensional 421 polytope onto the Coxeter plane where its projection has maximal symmetry. This 2-D projection highlights even more the concetration of vertices towards the centre, As its 240 vertices represent the root vectors of the simple Lie group E8, the polytope is sometimes referred to as the ``E8 polytope''. Using a complex number coordinate system, it can also be constructed as a 4-dimensional regular complex polytope known as the Witting polytope.
Image by Claudio Rocchini (Own work), via Wikimedia Commons.