This week I discuss a question related to my research on graph theory. The question lies at the intersection of topology and graph searching games.

You might want to have a pad and paper for doodling nearby while you read. Or at least some donuts and coffee.

### Life on the surface of a donut

Life would seem mostly ordinary if you lived on the surface of the donut, as you could travel around in any direction and eventually come back to where you started. But there would be differences noticeable from life on the surface of a sphere (or imperfect oblate spheroid) like Earth. For one, if you made a lasso around the great equator, you couldn’t contract it to a point. A similar lasso around Earth’s equator would contract to either of the North or South Poles.

Another difference between spheres and donuts is the kinds of graphs you can* draw* on their surface. *Planar graphs* are those that can be drawn in the plane so their edges only touch at the nodes. For a more concrete idea, think of maps with dots at the capital cities and edges present if the countries share a border.

However, many graphs are not planar; for example, the complete graph on five nodes K_{5} isn’t planar. No matter how you try and draw it, at least one pair of edges overlap.

On a donut, however, you can draw K_{5} with no edge crossings. Seeing is believing.

This kind of mathematical discussion takes us into the realm of *topological graph theory*, which studies graphs drawn on surfaces like spheres and donuts. There are different kinds of surfaces, but they all boil down to essentially a sphere with some holes (at least the orientable ones do and I’m not going to get into the non-orientable case here). The number of holes is the *genus* of a surface.

Graphs also have a genus. If a graph can be drawn without edge crossings on a surface with genus g (and nothing smaller than g), then the graph has genus g. For example, a planar graph has genus 0, while K_{5} has genus 1.

### Cops and Robbers

People find it funny when I tell them I play games in my mathematical research.

In the game of *Cops and Robbers *played on graphs, there is a bad guy (the *robber*) loose in a network that a group of good guys (the *cops*) are trying to capture. There are specified rules about how this can occur. The cops go first, choosing a set of nodes to occupy. The robber goes next and chooses a vertex, alternating moves with the cops at ticks of the clock. The players can only move from one node to another along edges; no teleporting allowed! The cops win if they can land on the node occupied by the robber. If the robber can avoid being captured forever, then the robber wins.

For example, consider a cycle with at least four nodes. You should convince yourself one cop can’t capture a robber, but two cops can do so.

Of course, we could place a cop at each node of our graph and win, but that is no fun. The *cop number* of a graph is the minimum number of cops we need to guarantee a win. So, the cop number of a cycle with at least four vertices is 2.

As an exercise, show that the cop number of the Petersen graph is 3.

Finding the cop number is not an easy problem and that can be made precise. It is **NP**-complete to compute the cop number on a general graph, meaning there is likely no efficient algorithm to solve it for all graphs. In a formal sense, Cops and Robbers is **EXPTIME**-complete which means it can be as complex as Chess played on an arbitrarily large, but finite, board.

We don’t even know how large the cop number can be. A graph is *connected* if every pair of nodes is joined by a path. *Meyniel’s conjecture* says the cop number of a connected graph is at most n^{1/2}, where n is the number of nodes of the graph. We seem far from being able to prove Meyniel’s conjecture, and it is likely the toughest problem in this area of mathematics right now.

### Cops on donuts

An interesting question arises when we consider the cop number of graphs on surfaces. We know from early work by Aigner and Fromme in 1983 that planar graphs have cop number at most 3.

This is far from obvious! For example, how do three cops capture the robber on this graph?

Bernd Schroeder conjectured that the cop number of a graph with genus g is at most g +3. The conjecture holds for genus 0 graphs (that is, the planar ones), and he showed it holds for genus 1 graphs. In general, the best upper bound we have for the cop number of genus g graphs is ⌊3/2g⌋ +3. Getting rid of the 3/2 seems to be a quite tough mathematical problem.

There you have it: we’ve arrived at a deep mathematical question about cops chasing robbers on donuts with many holes.

Isn’t mathematics infinitely amazing?

Anthony Bonato

[…] or surround them. Often, such problems are posed within the context of a combinatorial game like Cops and Robbers. Inspired by other conferences like GRASTA, GRASCan is small and invite-only, with talks and time […]

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[…] We call this game Hyperopic Cops and Robbers, as hyperopia is the condition of being farsighted. The minimum number of cops needed to guarantee a win in a graph G is the hyperopic cop number, denoted by cH(G). If the cops capture the robber in Hyperopic Cops and Robbers, then they do so in the classical game of Cops and Robbers; hence, c(G) ≤ cH(G), where c(G) is the cop number of G. For more on the cop number, see this previous post. […]

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[…] Discrete mathematics and graph theory, in particular, is at the heart of understanding social networks like Facebook, how proteins interact in our cells, and how information flows through our e-mails, phones, and websites. We use it to model stock correlations in financial mathematics and in our understanding of how molecules fit together. Discrete mathematics interfaces with all of pure and applied mathematics, from modelling to coloring, to searching networks in fun and deep games such as Cops and Robbers. […]

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