Pach's "Animals": What if genus $> 0$ ?

2011-01-02T23:13:10Z

Janos Pach asked a deep question 23 years ago (1988) that remains unsolved today:

Can every animal—a topological ball in $\mathbb{R^3}$ composed of unit cubes glued face-to-face—be reduced to a single unit cube by adding and deleting cubes, while always maintaining the animal (ball) property?

("Animal" was an apparently original coinage of Janos's.) I and my students quickly found irreducible animals, i.e., balls of unit cubes from which no cube can be removed without destroying the topological-ball property. Here is one of 119 cubes due to Tom Shermer (which I exploded vertically for visualization):

Essentially all our irreducible examples are based on Bing's House with Two Rooms (unbeknownst to us at the time). So if Pach's question has a positive answer, it requires adding cubes as well as deleting. This history is recounted in Günter Ziegler's Lectures on Polytopes, Springer, 1995, p.276. His non-shellability Theorem 8.15 (p.243) is based on these irreducible animals.

So, I finally come to my question, which is essentially a question of shellability.:

Can every (embedded) object constructed by gluing unit cubes face-to-face, regardless of genus, be reduced to a single unit cube by adding and deleting cubes, while always maintaining that the surface is a 2-manifold?

This is exactly Pach's question, but with the ball-requirement removed. All the irreducible animals I know rely on violating the topological-ball requirement for their irreducibility; so it is (remotely) possible that reduction alone suffices(!). I am tempted to introduce a new genera to encompass Plantae & Animalia; but I resist.

Any pointers that may lead me to information on the generalization of Pach's question would be greatly appreciated. Thanks!

Addendum, 11May11 (original posting 2Jan11). The problem is now solved (positively): Every animal can be reduced by adding and deleting cubes. The proof is contained in two papers, the second of which appeared as a tech report in May 2010: "A solution to the animal problem," by Akira Nakamura. Here is the PDF. The first paper, an earlier 2006 tech report, is called simply, "B-Problem," by Akira Nakamura, Kenichi Morita, and Katsunobu Imai. Here is its PDF. I would summarize but I do not yet understand the papers, which are presented in terms of "digital topology."

Answer by Gil Kalai for Pach's "Animals": What if genus $> 0$ ?

2011-01-03T17:15:00Z

Regarding Pach's original question: Bing's house deals with the analogous question of collapsibility. It is an example of a contractible non-collapsible space. For that problem it is known that if you allow both collapses and anti-collapses, every contractable space can be reduced to a point. This follows from "Simple homotopy theory". I dont know if this result of simple homolopy theory extends to animals built from cubes but they might.

The modified higher genus question seems easier since you allow steps that changes the topology of the animal.

One can ask a stronger question if when you fixed the genus g you can pass between two animals of genus g by such steps. The analogous question for collapsibility seems to relate to simple homotopy invariants of surface groups but I dont know what they are.

Pach's "Animals": What if genus $> 0$ ? - MathOverflow

Pach's "Animals": What if genus $> 0$ ?

Answer by Gil Kalai for Pach's "Animals": What if genus $> 0$ ?