A planar graph is one that can be drawn in the plane with no crossing edges. Planar graphs have been investigated for a long time, and it is known (Fáry's theorem) that any planar graph can be drawn in such a way that all edges are straight line segments.
But what can you do when a graph is not planar? I and my co-authors have been studying drawings in which the edges are grouped into layers (shown here as colors) such that no two edges in the same layer cross. It turns out (unlike for planar graphs) that it makes a difference whether you require the edges to be straight line segments. The minimum number of layers in a drawing, if you allow curved edges, is a well studied concept known as the thickness of the graph. The minimum number of layers in a drawing with straight edges is a less well studied concept which we call the geometric thickness.
The following drawing shows that the geometric thickness of the complete graph on twelve vertices (K12) is at most three:
The central hexagon of this drawing is a little hard to see, so here is a closer view:
The pattern used for this drawing can be generalized to show that, for any n, the graph Kn can be drawn in at most ceiling(n/4) layers [Eppstein et al, JGAA 2000]. We also have a lower bound that at least n/5.646 layers are needed. Since Kn is known to have n/6-layer drawings with curved edges, this lower bound shows that geometric thickness is not the same concept as thickness.
The following table shows what we know about lower bounds and upper bounds for the number of layers required for complete graphs Kn.
|n||1 - 4||5 - 8||9 - 12||13 - 14||15 - 16||17 - 20||21 - 24||25 - 26||27 - 28||29 - 31||32|
If one splits each edge of a complete graph (or any other graph) into a two-edge path, the resulting subdivided graph has geometric thickness two. The following picture shows a drawing for K8:
Interestingly, a Ramsey-theoretic argument shows that the book thickness of subdivisions of Kn is not bounded by any constant. So, there are graphs with geometric thickness two and arbitrarily large book thickness [Eppstein, 2001]. A very similar Ramsey-theoretic argument, applied to graphs formed by starting with n points and adding a new point adjacent to each triple of the n points, shows that there are also graphs with thickness three and arbitrarily large geometric thickness [Eppstein, Contemp. Math. 2004].
In our JGAA 2000 paper we also investigated the geometric thickness of complete bipartite graphs. When a is much smaller than b, the geometric thickness of Ka,b is just a/2, but complete bipartite graphs in which both sides are of nearly equal size are more interesting. For instance, here is a two-layer drawing of the complete bipartite graph K6,6:
K1,n and K2,n are planar (one layer), and K3,n and K4,n always require exactly two layers, so the first nontrivial cases are K5,n and K6,n. K5,n requires two layers for n < 8 and three layers for n > 11. For 9 < n < 10 we don't know whether two or three layers are needed. A drawing of K5,8 is below. K6,n requires three layers for n > 8. For K6,7 we don't know whether two or three layers are needed.
Below we show a two-layer drawing of a six-dimensional hypercube. More generally the d-cube has geometric thickness at most ceiling(d/3) [Eppstein, GD 2004].
Graphs with maximum degree up to four require geometric thickness at most two [Duncan et al., SCG 2004]. How does the geometric thickness behave for bounded degree graphs with higher degree bounds? Some progress has been made by [Dujmović et al., GD 2004] who showed that the geometric thickness is O(1) for bounded-degree interval graphs, co-comparability graphs, and AT-free graphs, and that it is O(log n) for bounded-degree bounded-treewidth graphs. Duncan [CCCG 2009] showed that the geometric treewidth is also O(log n) for graphs that can be decomposed into the union of two bounded-treewidth graphs, without degree restrictions.
However, Barát, Matoušek, and Wood [math.CO/0509150] have shown that larger constant bounds on degree need not imply bounded thickness for arbitrary graphs. In particular, they use counting arguments to prove the existence of Δ-regular graphs for any Δ > 8 with geometric thickness Ω(nc) for some c > 0, where c depends on Δ and approachs 1/2 in the limit of large Δ.
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