Möbius Deltahedra
Copyright1998 Peter Messer |
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Five non-convex deltahedra
which share
special symmetry properties |

A deltahedron is a polyhedron with
faces that are all equilateral triangles. There are an infinite number
of non-convex deltahedra and only eight convex deltahedra. The convex set
includes three familiar polyhedra: the regular tetrahedron, the regular
octahedron, and the regular icosahedron. The eight convex deltahedra are
commonly described in general published works on polyhedra [1].
We direct our attention to a unique set of five highly symmetric non-convex deltahedra which appear in the above image of six figures rendered by Rolf Asmund. The last figure in the bottom row represents a "stellated cube" (more technically an equilateral tetrakis hexahedron) which has a similar origin as the others but, as you will see later, it does not satisfy the special symmetry requirements of the other five cases. The symmetry relationships that distinguish our five cases evoke both mathematical interest and esthetic pleasure. Clicking on a figure in the image gives you a VRML display of it which you can rotate and examine in three dimensions. If you do not have a VRML plug-in viewer, you can obtain the free Cosmo 2.0 VRML viewer from SGI. The VRML models were created by Melinda Green using the coordinate data generated by Peter Messer. You can easily assemble physical models of these figures by using rigid equilateral triangles made of cardboard or plastic. As you shall see shortly, the pieces do not fall so easily into place if you are attempting an exact mathematical solution. Study each of the figures, look for similarities in symmetry, and make your conclusions before reading on. |
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The five deltahedra are characterized
by the following concise definition:
A deltahedron where each face is bounded by a Möbius triangle
of
the same symmetry kind. |
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If you are already familiar with
the subject of Möbius triangles, you should have no trouble visualizing
the various symmetry features of the five figures. You can also see George
Hart's excellent page for images and descriptions of the subject. Möbius
triangles
(2 3 are right spherical triangles which have
dihedral angles
p)p/ 2,
p/
3, p/
and which are the
fundamental domains for the three polyhedral symmetry groups. In decreasing
order they are icosahedral symmetry p(2 3 5), octahedral symmetry
(2
3 4), and tetrahedral symmetry
(2 3 3). Instead of pradians,
you may substitute
180 degrees. The sides of Möbius triangles
participate as reflection (mirror) planes in the corresponding symmetry
group. Each polyhedral symmetry group has a characteristic number of Möbius
triangles which are connected in a tessellating pattern that covers a sphere.
An excellent introduction that illustrates these triangle reflection units
is found in reference [2]. Excluded from our discussion is dihedral symmetry
(2
2 p).
Our definition states that each equilateral triangle face fits precisely
the same way inside the smallest space (Möbius triangle) bounded by
three mirror planes of the deltahedron’s symmetry group. Like a kaleidoscope,
you can generate the entire figure by sequentially reflecting the images
of a given face across the sides of a single Möbius triangle. These
mirror planes are not actually visible in the figures. You will note that
the mirror planes include only the sides of the equilateral triangles,
that is, they do not cut across the interior of the triangles. The top
row of Asmund’s image shows two figures with icosahedral symmetry. Each
is associated with 120, 48, and 24
for the decreasing polyhedral symmetry groups. Consequently, the rows in
the image are associated with 120, 48, and 24 faces,
respectively.
It should now be clear that other highly symmetric deltahedra (e.g. tetrahedron, octahedron, icosahedron) do not satisfy our concise definition because they contain mirror planes that cut across the faces. It turns out that none of the five special deltahedra can be constructed using Euclidean tools (straightedge and compass). This is attributed to the following conditions for generating the equilateral triangle faces. First, fix one vertex of a general (scalene) triangle on a rotation axis of the symmetry group. You are left with two other triangle vertices that slide simultaneously and independently along their respective axes until the distances between all three vertices are the same. You will quickly encounter three simultaneous non-linear equations as you attempt to calculate any of the metrical properties of these deltahedra. By eliminating variables you are at best left with a fourth degree polynomial equation in the desired metrical variable. The polynomial equation is observed to generate two imaginary roots and two real roots in these cases. The real roots turn out to be complicated expressions involving cubic radicals. That is why you see two non-constructible cases with octahedral symmetry and two non-constructible cases with icosahedral symmetry. However, one of the two real roots associated with the tetrahedral symmetry group consists only of square root radicals which of course is constructible. This constructible case appears as a stellated cube which, surprisingly, has full octahedral symmetry. How did this one case start with conditions of tetrahedral symmetry but then end up with octahedral symmetry? Recall that the tetrahedral Möbius triangle is a right isosceles
spherical triangle with angles
We shall now solve a set of equilateral triangle vertices ,
V_{3}
for each of the two cases of deltahedra with
V_{5}}120 faces. You should
then be able to apply similar methods to the remaining special deltahedra
with 48 or 24 faces.
Calculations are simplified if we properly orient the set of |
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the 2-fold rotation axis
coincides with the -axis and directed Z,
Z > 0the , Y > 0,
Z > 0the , X > 0.Z > 0 |
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We further simplify by using the
golden ratio and assigning the
"sliding factors" along the j 3-fold axis and along
the k5-fold axis. Triangle vertex on
the V_{2}2-fold axis is conveniently fixed at (0, 0, 1) which
lies unit distance from the origin. Triangle vertex which
slides on the V_{3}3-fold axis has general coordinates (0,
,
j/t.
Triangle vertex jt) which slides on the V_{5}5-fold
axis has general coordinates (,k 0, .The
general coordinates are left for the reader to confirm.
kt)Applying the formula in analytic geometry for the distance between two
points and then equating the squares of the lengths of the sides of equilateral
triangle |
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from which three simultaneous non-linear
equations can be derived. Any two of such equations are sufficient for
solving the factors and j. At this point
you must use standard iterative numerical methods for solving simultaneous
non-linear equations. You will find two solution sets for kand
j
which then can be substituted in the general set of vertices noted above.
kThe disadvantage of solving simultaneous equations is that you must
estimate starting values for . It is
often better to find a polynomial equation in one variable. With a symbolic
processor like MATHEMATICA it is easy to generate
a fourth degree polynomial equation in k. Next, an expression
for j in terms of kwould nicely bypass the
need for solving a second polynomial equation in j . The desired
two equations arek |
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A fourth degree polynomial equation
in can be solved algebraically without initial guesses.
Unfortunately the two real roots are very complicated expressions involving
cubic radicals and so they are not shown here. However, expressed in decimal
form the two solution sets j { , jk}are { 0.316383241…, 0.44300155…} and { 1.37056825…, 1.17639464…}
for
the first
and second case of the 120-faced deltahedron, respectively.
Finally, the equilateral triangle vertices {,
V_{2},
V_{3}
for the first and second case are approximatelyV_{5}} |
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{ {0, 0, 1}, {0, 0.19553559,
0.51191883}, {0.44300155, 0, 0.71679157} }
and { {0, 0, 1}, {0, 0.84705776, 2.21762601}, {1.17639464, 0, 1.90344651}
} |
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We succeeded in defining an initial
face for both cases of the 120-faced deltahedron. Because the deltahedra
have a specific orientation in a Cartesian coordinate system, the other
119
faces
can be mapped by applying sequential transformations (rotations, reflections)
under icosahedral symmetry.
1. Pugh, A. (1976). |