*This post is authored by Eran Nevo. (It is the first in a series of five posts.)*

**Peter McMullen**

## The g-conjecture

What are the possible face numbers of triangulations of spheres?

There is only one zero-dimensional sphere and it consists of 2 points.

The one-dimensional triangulations of spheres are simple cycles, having vertices and edges, where .

The 2-spheres with vertices have edges and triangles, and can be any integer . This follows from Euler formula.

For higher-dimensional spheres the number of vertices doesn’t determine all the face numbers, and for spheres of dimension a characterization of the possible face numbers is only conjectured! This problem has interesting relations to other mathematical fields, as we shall see.

A good place to read more about it is Stanley’s book `Combinatorics and Commutative algebra‘. First let’s fix some notation.

### vectors

A collection of subsets of is called a (finite abstract) **simplicial complex** if it is closed under inclusion, i.e. implies . Note that if is not empty (which we will assume from now on) then , and is of dimension . The elements of are called **faces**. The -**vector** (face vector) of is where . For example, if is the -simplex then .

From now on, let’s assume that is a simplicial -sphere, meaning that its geometric realization is homeomorphic to the -dimensional sphere. What are the relations among the ‘s? We know that the reduced Euler characteristic is . There are more linear relations, called the Dehn-Sommerville relations. They have a nicer form when expressed in terms of the -**vector** of , defined by

We see that there are invertible maps .

What’s called “Stanley’s trick” is a convenient way to practically compute one from the other, as illustrated in the difference table below, taken from Ziegler’s book `Lectures on Polytopes’, p.251:

**1**

1 **6**

1 5 **12**

1 4 7 ** 8**

h= (1 3 3 1)

Here, we start with the -vector of the Octahedron (1,6,12,8) (bold face entries) and take differences as shown in this picture to end with the -vector (1,3,3,1).

We see that . More is true. The Dehn-Sommerville relations state that is symmetric, i.e. for every . This result can be proved combinatorially, for the larger family of Eulerian posets, and actually these relations span all the linear equalities among the -vector of .

What about inequalities?

### -vectors

We say that a vector is an -vector ( for Macaulay) if it is the -vector of a **multicomples**, i.e. of a collection of multisets (elements can repeat!) closed under inclusion. For example, is an -vector, as is demonstrated by the multicomplex , written in monomial notation – the exponent tells how many copies of to take. Macaulay gave a numerical characterization of such vectors. (The proof uses compression – see this post for a general description of the method.) We will revisit Macaulay theorem in the next part, when discussing **face rings**.

### -vector and the -theorem

Let , for . . By the Dehn-Sommerville relations, can be recovered from . McMullen asked whether is always an -vector, and conjectured that this the case if is the boundary of a simplicial polytope. He conjectured further that any -vector is the -vector of the boundary of a simplicial polytope.

A major result is the proof of this conjecture in the polytopal case, known as the -theorem, which gives a complete characterization of the -vectors of boundaries of simplicial polytopes. Billera and Lee constructed in 1979 a simplicial polytope with boundary satisfying for any given -vector . Stanley proved, in the same year, that if is the boundary of a convex polytope then is an -vector. We will discuss this proof in the next section.

The -conjecture (or, one version of it) is:

If is a simplicial sphere then is an -vector.

This conjecture is around since McMullen first asked it in 1970.

**Next time: how does Stanley’s proof go?**

Thanks, Eran! The g-conjecture for spheres is probably the single problem I worked longest and hardest to solve (without success), and several other people worked on it too. There are various interesting problems around it and it is connected to all sort of fascinating mathematics.

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