Tag Archives: Embeddability

F ≤ 4E

1. E ≤ 3V

Let G be a simple planar graph with V vertices and E edges. It follows from Euler’s theorem that

3V

In fact, we have (when V is at least 3,) that E 3V – 6.

To see this,  denote by F the number of regions or faces determined by G (in other words, the number of connected components in the complement of the embedded graph). Euler’s theorem asserts that

E – V + F = 2

V – E + F = 2

and now note that every face must have at least three edges and every edge is contained in two faces and therefore 2E \ge 3F, so 6=3V – 3E + 3F ≤ 3V – 3E +2E.

2. F  4E

Now let K be a two-dimensional simplicial complex and suppose that K can be embedded in R^4. Denote by E the number of edges of K and by F the number of 2-faces of K.

Here is a really great conjecture:

Conjecture:

4E

A weaker version which is also widely open and very interesting is:

For some absolute constant C,

C E

Remarks: The conjecture extends to higher dimensions. If K is an r-dimensional simplicial complex that can be embedded into R^{2r} then the conjecture is that

f_r(K) \le C_rf_{r-1}(K),

Where C_r is a constant depending on r.  Here f_i(K) is the number of i-dimensional faces of K. A stronger statement is that C_r= r+2. The conjecture also extends to polyhedral complexes and more general form of complexes. In the conjecture ’embed’ refers to a topological embedding.