[chestervonwinch]

So, let me try to understand: Let's take the order 1 case. If I pick a value on y-axis and hold it constant, this refers to a particular "1D hole". As I move left to right, increasing the radius, the graph is colored black if this particular hole is present for the given radius and not-colored otherwise. Is it not misleading, then, to label the y-axis as the Betti number since this is a single, global number?

> the 0-th dimension case is relatively uninteresting

It was explained to me that the zeroth order Betti numbers have applications for clustering.

[wmsiler]

Your description is correct. And to call the y-axis the Betti number is a bit misleading. If you are looking at a barcode for 1-dimensional holes, then at a given radius, the number of bars over that radius is the first Betti number (at that radius). So Betti number counts the number of holes, but the barcode graph is keeping track of each hole's "lifetime" as the radius changes.

> It was explained to me that the zeroth order Betti numbers have applications for clustering.

That is correct, so perhaps "uninteresting" was too strong :) The 0-th Betti number counts the number of connected components of the space. So if we are at radius, say, 1 and the 0-th Betti number is 3, then we know the data points can be put into 3 "dense" clusters. By dense, I mean that for every two data points A and B in the cluster, there is a sequence of data points that you could step on going from A to B where each step has distance at most 1. I don't know if that explanation made any sense.