# Archive for January, 2010

## Hexiamonds on an Octahedron

January 26th, 2010

Here’s an interesting problem that seems not to have gotten as much attention as I think it deserves. The twelve hexiamonds contain a total of 72 triangular units. A regular octahedron with edges 3 units long can fit 9 triangles on each of its 8 faces, exactly enough to tile with the hexiamonds. Some individual solutions to this problem have been found. A solution at Livio Zucca’s site bears the label “Adrian Struyk 1963?” so we may assume the problem has been around at least since then. Another solution by Michael Dowle is here.

Notice that you can unfold an octahedron to produce a net in the form of an octiamond. This provides another source for solutions. The octahedron has 11 different octiamond nets. Pieter Torbijn found that 24 enlarged octiamonds could be tiled with the hexiamonds; of these, 5 are nets of an octahedron.

As far as I know, nobody has made an exhaustive computerized search for solutions to this problem. You can be the first!

#4: How many distinct ways can the hexiamonds tile an octahedron?

The octahedron has a large amount of symmetry compared to any planar figure that these pieces can tile. It has 48 automorphisms, or ways to map the solid onto itself. This would indicate a relatively small number of different solutions, since many solutions will be mappings of each other over the various ways of rotating and reflecting the octahedron. On the other hand, the shape lacks external borders, which ought to greatly increase the number of possibilities.

Some solutions have a piece that wraps around and caps a vertex. This could be considered an aesthetic flaw, because it would be impossible to tell which hexiamond the capping piece is just from knowing what triangles it occupies; you must also know its edges.

There are 7 different pieces that can cap a vertex, one of which, the pistol, can cap it in two distinct ways. Notice that due to the symmetry of the shape it makes when it caps a vertex, only the orientation of the sphinx is a riddle; its identity is never in question.

#5: How many solutions have a capped vertex?

#6: What is the largest number of vertices that can be capped in a solution? The ideal would be for all six vertices to be capped with all of the above hexiamonds except the sphinx.

The octahedron has twelve edges, the same as the number of hexiamonds. This suggests another problem:

#7: Is it possible to tile the octahedron so that each of the twelve hexiamonds is folded across exactly one of the edges of the octahedron?

## 2-coloring Pentomino Packings

January 24th, 2010

I like to collect pentomino coloring problems.

So it should come as little surprise that I was intrigued by the cover of Puzzle Fun 16. Puzzle Fun was a ‘zine produced by Rodolfo Marcelo Kurchan in the ’90s covering a variety of polyomino problems. I missed out on subscribing to it myself, and the Puzzle Fun website languished for a decade after new issues stopped appearing.

A few months ago, Kurchan put the content of all of the back issues of Puzzle Fun online.

Puzzle Fun 16 focused on pentomino packing problems. Packing problems differ from tiling problems in that empty space is allowed, and the goal is to minimize the amount of empty space required. Packing, usefully, makes some kinds of problems possible to solve that would not be solvable as tiling problems.

One such puzzle type is packing polyforms that are 2-colorable, (that is, one can use two colors to color every piece such that no piece touches another of the same color.) This is the puzzle type I saw on the cover of Puzzle Fun 16.

The problem itself was this: Place two sets of pentominoes in the smallest possible rectangle such that no pentomino touches another in the same set. [PF problem #549]

A solution was printed in Puzzle Fun 18:

(Solution by Hector San Segundo.)

I should note that this problem implies strict coloring: pieces are not allowed to touch even at corners. I am more interested in non-strict coloring, which is generally the default in coloring problems, and I am interested in colorings of a single set of pentominoes. (Which all of the problems on my pentomino coloring page are.)

#1: Place a 2-colorable (non-strict coloring) set of pentominoes in the smallest possible rectangle. My best attempt has 65 squares:

Filling out the matrix of variations gives two more problems:

#2: As #1, but with a strict coloring.

#3: As in PF #549, but with a non-strict coloring.

(The following problem in PF 16 (#550) was a variation on #549 minimizing perimeter rather than area, but this is less interesting to me.)

http://www.puzzlefun.com.ar/

## Welcome!

January 20th, 2010

I’ve been playing with polyominoes and other recreational math topics for quite a while, and sharing my results on an email list, and on my own website. It has occurred to me that this is the 21st century, and I should get a public blog for this material. Also, since I am now selling puzzles that I’ve designed and made, having a blog may, I hope, be a way of generating interest in them so that I can make and sell more.

Other subjects that I judge may be interesting to the same sorts of people who are into recreational math, (like interactive fiction writing, which I’ve been meaning to get back into) may come up here too. We’ll just have to see where this leads.