# Homology statistics for 2-manifolds

## Tags: projects, research

For a new (forthcoming) research project, I have to brush up my knowledge about homology groups and triangulations. This constitutes an excellent opportunity to extend Aleph, my (also forthcoming) library for exploring persistent homology and its uses.

As a quick example, I decided to calculate some statistics about the homology of 2-manifolds. Thanks to the work of Frank H. Lutz, a list of triangulations with up to 9 vertices, as well as a list of triangulations with 10 vertices, exist. After adding support for the appropriate data format in Aleph, I was able to easily read a family of triangulations and subsequently process them. Even though Aleph’s main purpose is the calculation of persistent homology, ordinary homology calculations work just as well—ordinary homology being a special case of persistent homology.

As a starting point for further mathematical ruminations, let us take a look at the distribution of “Betti numbers” of 2-manifolds. More precisely, we analyse the distribution of the first Betti number of the manifold—according to Poincaré duality, the zeroth and second Betti number have to equal, and we know the zeroth Betti number to be 1 for a manifold.

Without further ado, here is a tabulation of first Betti numbers for triangulations with up to 9 vertices:

Value Number of occurrences
0 73
1 154
2 313
3 133
4 37
5 2

The results are interesting: most triangulations appear to have the homology of a torus, i.e. a first Betti number of 2, followed by the homology of the real projective plane with a first Betti number of 1. Betti numbers larger than 3 are extremely rare. Intuitively, this makes sense—the triangulation only consists of at most 9 vertices, so there are natural limits on how high the Betti number can become.

For triangulations with 10 vertices, another distribution arises:

Value Number of occurrences
0 233
1 1210
2 6571
3 11784
4 14522
5 7050
6 1042
7 14

Here, the mode of Betti numbers is a value of 4, with 14522 occurrences out of 42426 triangulations. The homology type of this signature is the one of a genus-4 surface. Again, higher values get progressively less likely because only 10 vertices are permitted.

I am not yet sure what to make of this, but it sure is a nice test case and application scenario for Aleph. One of the next blog posts will give more details about this calculation, and its implementation using the library. Stay tuned!