Both of these prisms have 28-pointed stars as bases. I made them using *Stella 4d: Polyhedron Navigator*, which you can try for free right here.

# Tag Archives: prism

# A Compound of Four Triangular Prisms

I made this compound using *Stella 4d*, which you may try for free at this website.

# A Spiral of Fifteen Regular Prisms Which Could Continue Indefinitely

I made this using *Stella 4d*, which you can try for free at http://www.software3d.com/Stella.php.

# A Compound of Ten Hexagonal Prisms

I created this polyhedral compound by faceting a great rhombicosidodecahedron, using *Stella 4d*. You can try this program for free at http://www.software3d.com/Stella.php.

# Twelve Pentagrammic Prisms

I made this by augmentation (with prisms) of the faces of a small stellated dodecahedron, using *Stella 4d*, which you can try for free at this website.

# A Compound of Six Pentadecagonal Prisms

This chiral polyhedral compound was generated from a partial faceting of the polyhedron shown in the last post here, using *Stella 4d*‘s faceting function, plus its “try to make faces regular” operation afterwards. Making the six-prism compound in the first place was suggested by Tony Hartley, on Facebook, where I posted a link to that last post in a mathematical group for discussion. Thanks, Tony!

If you’d like to try *Stella* for yourself, the site to visit for a free trial download is http://www.software3d.com/Stella.php.

# Thirteen Images, Each, of Jynx, the Black Kitten, on Two Hendecagonal Prisms

The above hendecagonal prism shows what Jynx is like when he’s in “kyperkitten” mode. (If you have a kitten, you know what that means.) It’s also rotating rapidly in an effort to make those who fear black cats, and/or the number thirteen, feel even more jumpy, in the hope that Jynx and I can, by working together, startle them into rationality.

On the other hand, Jynx does sometimes like to just lounge around, and watch the world go by — so I’ll show him in “tiredcat” mode as well.

Software credit: I used *Stella 4d: Polyhedron Navigator* to make these images, a program which is available at this website.

# Infinite Families of Polyhedra

Many polyhedra appear in finite sets. The most well-known example of such a set is the five Platonic solids. Many know them from role-playing games.

Other finite sets include the thirteen Archimedean, four Kepler-Poinsot, thirteen Catalan, and 92 Johnson solids, and the eight convex deltahedra, among others. There is even a finite set, the near-misses, with an unknown number of members, due to the “fuzziness” of its definition. The symmetrohedra is another set with “fuzzy” criteria, but there are still only so many symmetrohedra to be found. We simply haven’t found them all yet, or, if we have, we don’t know that we have, but it would not be reasonable to claim that infinitely many await discovery.

However, not all groups of polyhedra are finite. Some polyhedra appear, instead, in infinite families. What is needed to generate such infinite families (at the cost of some forms of symmetry, compared to, say, Platonic or Archimedean solids) is the use of *bases* — special polyhedral faces which play a stronger role in the determination of that polyhedron’s shape than do the other faces. For the familar prism, there are two bases.

In a pentagonal prism, the bases are pentagons, as seen above. For a pentagonal prism, n = 5, for n is simply the number of sides of the base. The smallest value of n which is possible, 3, yields a triangular prism. There’s no upper limit for n, either. Here’s a regular hexacontagonal prism, where each base has sixty sides.

Obviously, n can be increased without limit, although for very large values of n, the prism will be hard to distinguish from a cylinder.

Another infinite family may be found by taking the dual of each prism. This is the dual of the pentagonal prism:

Taking a dual of a prism produces a dipyramid, with its n-gonal base hidden between the puramids, but with no guarantee that the triangular faces will be regular — and in this case, they are not. It is possible for a pentagonal dipyramid to have only regular faces . . .

. . . but regular faces will not work if n = 6 (because the dipyramid collapses to zero height), or indeed any number other than 3, 4, or 5. Therefore, dipyramids will not be an infinite family unless non-equilateral triangles are permitted as faces.

Half of a dipyramid, of course, is simply a pyramid, with the single base now visible:

Like the dipyramids, and for the same reason, there are only three pyramids (n = 3, 4, or 5) which can have all faces regular. An infinite family of right, regular pyramids do exist, though, if isosceles triangles are permitted as lateral faces.

While pyramids and dipyramids have only one base each, the already-described prisms have two. Prisms also can maintain regularity of all faces, no matter how large n becomes, unlike pyramids and dipyramids. Moreover, prisms can be transformed into another infinite family of regular-faced polyhedra by rotating one base by one half of one-nth of a rotation, relative to the other base, and replacing the n square lateral faces with 2n equilateral triangles. These polyhedra are called antiprisms. The pentagonal antiprism looks like this:

Antiprisms with all faces regular do not have the “only 3, 4, and 5” limitation that affected the pyramids and dipyramids. For example, here is one with all faces regular, and dodecagonal bases:

One more infinite family of polyhedra may be found, using the antiprisms: their duals. The dual of the pentagonal antiprism looks like this:

There is no regularity of faces for any members of this family with n > 3, for their faces are kites. (When n = 3, the kites become squares, and the polyhedron formed is simply a cube.) Unfortunately, “kiteohedron” looks ridiculous, and sounds worse, so efforts have been made to find better names for these polyhedra. “Deltohedra” has been used, but is too easily confused with the deltahedra, which are quite different. The best names yet invented for this infinite family of kite-faced polyhedra are, in my opinion, the antidipyramids, and the trapezohedra.

(Note: the rotating polyhedral images above were generated using *Stella 4d: Polyhedron Navigator*, available at http://www.software3d.com/Stella.php.)

## Variations On Octagonal Prisms

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I made these with *Stella 4d*, a program you can find at http://www.software3d.com/Stella.php.

## Variations On Decagonal Prisms

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I made these with *Stella 4d*, a program you can find at http://www.software3d.com/Stella.php.