# The Čech Complex and the Vietoris-Rips Complex

It’s about time we got back to computational topology. Previously in this series we endured a lightning tour of the fundamental group and homology, then we saw how to compute the homology of a simplicial complex using linear algebra.

What we really want to do is talk about the inherent shape of data. Homology allows us to compute some qualitative features of a given shape, i.e., find and count the number of connected components or a given shape, or the number of “2-dimensional holes” it has. This is great, but data doesn’t come in a form suitable for computing homology. Though they may have originated from some underlying process that follows nice rules, data points are just floating around in space with no obvious connection between them.

Here is a cool example of Thom Yorke, the lead singer of the band Radiohead, whose face was scanned with a laser scanner for their music video “House of Cards.”

Radiohead’s Thom Yorke in the music video for House of Cards (click the image to watch the video).

Given a point cloud such as the one above, our long term goal (we’re just getting started in this post) is to algorithmically discover what the characteristic topological features are in the data. Since homology is pretty coarse, we might detect the fact that the point cloud above looks like a hollow sphere with some holes in it corresponding to nostrils, ears, and the like. The hope is that if the data set isn’t too corrupted by noise, then it’s a good approximation to the underlying space it is sampled from. By computing the topological features of a point cloud we can understand the process that generated it, and Science can proceed.

But it’s not always as simple as Thom Yorke’s face. It turns out the producers of the music video had to actually degrade the data to get what you see above, because their lasers were too precise and didn’t look artistic enough! But you can imagine that if your laser is mounted on a car on a bumpy road, or tracking some object in the sky, or your data comes from acoustic waves traveling through earth, you’re bound to get noise. Or more realistically, if your data comes from thousands of stock market prices then the process generating the data is super mysterious. It changes over time, it may not follow any discernible pattern (though speculators may hope it does), and you can’t hope to visualize the entire dataset in any useful way.

But with persistent homology, so the claim goes, you’d get a good qualitative understanding of the dataset. Your results would be resistant to noise inherent in the data. It also wouldn’t be sensitive to the details of your data cleaning process. And with a dash of ingenuity, you can come up with a reasonable mathematical model of the underlying generative process. You could use that model to design algorithms, make big bucks, discover new drugs, recognize pictures of cats, or whatever tickles your fancy.

But our first problem is to resolve the input data type error. We want to use homology to describe data, but our data is a point cloud and homology operates on simplicial complexes. In this post we’ll see two ways one can do this, and see how they’re related.

## The Čech complex

Let’s start with the Čech complex. Given a point set $X$ in some metric space and a number $\varepsilon > 0$, the Čech complex $C_\varepsilon$ is the simplicial complex whose simplices are formed as follows. For each subset $S \subset X$ of points, form a $(\varepsilon/2)$-ball around each point in $S$, and include $S$ as a simplex (of dimension $|S|$) if there is a common point contained in all of the balls in $S$. This structure obviously satisfies the definition of a simplicial complex: any sub-subset $S' \subset S$ of a simplex $S$ will be also be a simplex. Here is an example of the epsilon balls.

An example of a point cloud (left) and a corresponding choice of (epsilon/2)-balls. To get the Cech complex, we add a k-simplex any time we see a subset of k points with common intersection.  [Image credit: Robert Ghrist]

Let me superscript the Čech complex to illustrate the pieces. Specifically, we’ll let $C_\varepsilon^{j}$ denote all the simplices of dimension up to $j$. In particular, $C_\varepsilon^1$ is a graph where an edge is placed between $x,y$ if $d(x,y) < \varepsilon$, and $C_{\varepsilon}^2$ places triangles (2-simplices) on triples of points whose balls have a three-way intersection.

A topologist will have a minor protest here: the simplicial complex is supposed to resemble the structure inherent in the underlying points, but how do we know that this abstract simplicial complex (which is really hard to visualize!) resembles the topological space we used to make it? That is, $X$ was sitting in some metric space, and the union of these epsilon-balls forms some topological space $X(\varepsilon)$ that is close in structure to $X$. But is the Čech complex $C_\varepsilon$ close to $X(\varepsilon)$? Do they have the same topological structure? It’s not a trivial theorem to prove, but it turns out to be true.

The Nerve Theorem: The homotopy types of $X(\varepsilon)$ and $C_\varepsilon$ are the same.

We won’t remind the readers about homotopy theory, but suffice it to say that when two topological spaces have the same homotopy type, then homology can’t distinguish them. In other words, if homotopy type is too coarse for a discriminator for our dataset, then persistent homology will fail us for sure.

So this theorem is a good sanity check. If we want to learn about our point cloud, we can pick a $\varepsilon$ and study the topology of the corresponding Čech complex $C_\varepsilon$. The reason this is called the “Nerve Theorem” is because one can generalize it to an arbitrary family of convex sets. Given some family $F$ of convex sets, the nerve is the complex obtained by adding simplices for mutually overlapping subfamilies in the same way. The nerve theorem is actually more general, it says that with sufficient conditions on the family $F$ being “nice,” the resulting Čech complex has the same topological structure as $F$.

The problem is that Čech complexes are tough to compute. To tell whether there are any 10-simplices (without additional knowledge) you have to inspect all subsets of size 10. In general computing the entire complex requires exponential time in the size of $X$, which is extremely inefficient. So we need a different kind of complex, or at least a different representation to compensate.

## The Vietoris-Rips complex

The Vietoris-Rips complex is essentially the same as the Čech complex, except instead of adding a $d$-simplex when there is a common point of intersection of all the $(\varepsilon/2)$-balls, we just do so when all the balls have pairwise intersections. We’ll denote the Vietoris-Rips complex with parameter $\varepsilon$ as $VR_{\varepsilon}$.

Here is an example to illustrate: if you give me three points that are the vertices of an equilateral triangle of side length 1, and I draw $(1/2)$-balls around each point, then they will have all three pairwise intersections but no common point of intersection.

Three balls which intersect pairwise, but have no point of triple intersection. With appropriate parameters, the Cech and V-R complexes are different.

So in this example the Vietoris-Rips complex is a graph with a 2-simplex, while the Čech complex is just a graph.

One obvious question is: do we still get the benefits of the nerve theorem with Vietoris-Rips complexes? The answer is no, obviously, because the Vietoris-Rips complex and Čech complex in this triangle example have totally different topology! But everything’s not lost. What we can do instead is compare Vietoris-Rips and Čech complexes with related parameters.

Theorem: For all $\varepsilon > 0$, the following inclusions hold

$\displaystyle C_{\varepsilon} \subset VR_{\varepsilon} \subset C_{2\varepsilon}$

So if the Čech complexes for both $\varepsilon$ and $2\varepsilon$ are good approximations of the underlying data, then so is the Vietoris-Rips complex. In fact, you can make this chain of inclusions slightly tighter, and if you’re interested you can see Theorem 2.5 in this recent paper of de Silva and Ghrist.

Now your first objection should be that computing a Vietoris-Rips complex still requires exponential time, because you have to scan all subsets for the possibility that they form a simplex. It’s true, but one nice thing about the Vietoris-Rips complex is that it can be represented implicitly as a graph. You just include an edge between two points if their corresponding balls overlap. Once we want to compute the actual simplices in the complex we have to scan for cliques in the graph, so that sucks. But it turns out that computing the graph is the first step in other more efficient methods for computing (or approximating) the VR complex.

Let’s go ahead and write a (trivial) program that computes the graph representation of the Vietoris-Rips complex of a given data set.

import numpy
def naiveVR(points, epsilon):
points = [numpy.array(x) for x in points]
vrComplex = [(x,y) for (x,y) in combinations(points, 2) if norm(x - y) < 2*epsilon]
return numpy.array(vrComplex)


Let’s try running it on a modestly large example: the first frame of the Radiohead music video. It’s got about 12,000 points in $\mathbb{R}^4$ (x,y,z,intensity), and sadly it takes about twenty minutes. There are a couple of ways to make it more efficient. One is to use specially-crafted data structures for computing threshold queries (i.e., find all points within $\varepsilon$ of this point). But those are only useful for small thresholds, and we’re interested in sweeping over a range of thresholds. Another is to invoke approximations of the data structure which give rise to “approximate” Vietoris-Rips complexes.

## Other stuff

In a future post we’ll implement a method for speeding up the computation of the Vietoris-Rips complex, since this is the primary bottleneck for topological data analysis. But for now the conceptual idea of how Čech complexes and Vietoris-Rips complexes can be used to turn point clouds into simplicial complexes in reasonable ways.

Before we close we should mention that there are other ways to do this. I’ve chosen the algebraic flavor of topological data analysis due to my familiarity with algebra and the work based on this approach. The other approaches have a more geometric flavor, and are based on the Delaunay triangulation, a hallmark of computational geometry algorithms. The two approaches I’ve heard of are called the alpha complex and the flow complex. The downside of these approaches is that, because they are based on the Delaunay triangulation, they have poor scaling in the dimension of the data. Because high dimensional data is crucial, many researchers have been spending their time figuring out how to speed up approximations of the V-R complex. See these slides of Afra Zomorodian for an example.

Until next time!

# Homology Theory — A Primer

This series on topology has been long and hard, but we’re are quickly approaching the topics where we can actually write programs. For this and the next post on homology, the most important background we will need is a solid foundation in linear algebra, specifically in row-reducing matrices (and the interpretation of row-reduction as a change of basis of a linear operator).

Last time we engaged in a whirlwind tour of the fundamental group and homotopy theory. And we mean “whirlwind” as it sounds; it was all over the place in terms of organization. The most important fact that one should take away from that discussion is the idea that we can compute, algebraically, some qualitative features about a topological space related to “n-dimensional holes.” For one-dimensional things, a hole would look like a circle, and for two dimensional things, it would look like a hollow sphere, etc. More importantly, we saw that this algebraic data, which we called the fundamental group, is a topological invariant. That is, if two topological spaces have different fundamental groups, then they are “fundamentally” different under the topological lens (they are not homeomorphic, and not even homotopy equivalent).

Unfortunately the main difficulty of homotopy theory (and part of what makes it so interesting) is that these “holes” interact with each other in elusive and convoluted ways, and the algebra reflects it almost too well. Part of the problem with the fundamental group is that it deftly eludes our domain of interest: computing them is complicated!

What we really need is a coarser invariant. If we can find a “stupider” invariant, it might just be simple enough to compute efficiently. Perhaps unsurprisingly, these will take the form of finitely-generated abelian groups (the most well-understood class of groups), with one for each dimension. Now we’re starting to see exactly why algebraic topology is so difficult; it has an immense list of prerequisite topics! If we’re willing to skip over some of the more nitty gritty details (and we must lest we take a huge diversion to discuss Tor and the exact sequences in the universal coefficient theorem), then we can also do the same calculations over a field. In other words, the algebraic objects we’ll define called “homology groups” are really vector spaces, and so row-reduction will be our basic computational tool to analyze them.

Once we have the basic theory down, we’ll see how we can write a program which accepts as input any topological space (represented in a particular form) and produces as output a list of the homology groups in every dimension. The dimensions of these vector spaces (their ranks, as finitely-generated abelian groups) are interpreted as the number of holes in the space for each dimension.

## Recall Simplicial Complexes

In our post on constructing topological spaces, we defined the standard $k$-simplex and the simplicial complex. We recall the latter definition here, and expand upon it.

Definition: A simplicial complex is a topological space realized as a union of any collection of simplices (of possibly varying dimension) $\Sigma$ which has the following two properties:

• Any face of a simplex $\Sigma$ is also in $\Sigma$.
• The intersection of any two simplices of $\Sigma$ is also a simplex of $\Sigma$.

We can realize a simplicial complex by gluing together pieces of increasing dimension. First start by taking a collection of vertices (0-simplices) $X_0$. Then take a collection of intervals (1-simplices) $X_1$ and glue their endpoints onto the vertices in any way. Note that because we require every face of an interval to again be a simplex in our complex, we must glue each endpoint of an interval onto a vertex in $X_0$. Continue this process with $X_2$, a set of 2-simplices, we must glue each edge precisely along an edge of $X_1$. We can continue this process until we reach a terminating set $X_n$. It is easy to see that the union of the $X_i$ form a simplicial complex. Define the dimension of the cell complex to be $n$.

There are some picky restrictions on how we glue things that we should mention. For instance, we could not contract all edges of a 2-simplex $\sigma$ and glue it all to a single vertex in $X_0$. The reason for this is that $\sigma$ would no longer be a 2-simplex! Indeed, we’ve destroyed its original vertex set. The gluing process hence needs to preserve the original simplex’s boundary. Moreover, one property that follows from the two conditions above is that any simplex in the complex is uniquely determined by its vertices (for otherwise, the intersection of two such non-uniquely specified simplices would not be a single simplex).

We also have to remember that we’re imposing a specific ordering on the vertices of a simplex. In particular, if we label the vertices of an $n$-simplex $0, \dots, n$, then this imposes an orientation on the edges where an edge of the form $\left \{ i,j \right \}$ has the orientation $(i,j)$ if $i < j$, and $(j,i)$ otherwise. The faces, then, are “oriented” in increasing order of their three vertices. Higher dimensional simplices are oriented in a similar way, though we rarely try to picture this (the theory of orientations is a question best posted for smooth manifolds; we won’t be going there any time soon). Here are, for example, two different ways to pick orientations of a 2-simplex:

Two possible orientations of a 2-simplex.

It is true, but a somewhat lengthy exercise, that the topology of a simplicial complex does not change under a consistent shuffling of the orientations across all its simplices. Nor does it change depending on how we realize a space as a simplicial complex. These kinds of results are crucial to the welfare of the theory, but have been proved once and we won’t bother reproving them here.

As a larger example, here is a simplicial complex representing the torus. It’s quite a bit more complicated than our usual quotient of a square, but it’s based on the same idea. The left and right edges are glued together, as are the top and bottom, with appropriate orientations. The only difficulty is that we need each simplex to be uniquely determined by its vertices. While this construction does not use the smallest possible number of simplices to satisfy that condition, it is the simplest to think about.

A possible realization of the torus as a simplicial complex. As an exercise, the reader is invited to label the edges and fill in the orientations on the simplices to be consistent across the entire complex. Remember that the result should coincide with our classical construction via the quotient of the disk, so some of the edges on the sides will coincide with those on the opposite sides, and the orientations must line up.

Taking a known topological space (like the torus) and realizing it as a simplicial complex is known as triangulating the space. A space which can be realized as a simplicial complex is called triangulable.

The nicest thing about the simplex is that it has an easy-to-describe boundary. Geometrically, it’s obvious: the boundary of the line segment is the two endpoints; the boundary of the triangle is the union of all three of its edges; the tetrahedron has four triangular faces as its boundary; etc. But because we need an algebraic way to describe holes, we want an algebraic way to describe the boundary. In particular, we have two important criterion that any algebraic definition must satisfy to be reasonable:

1. A boundary itself has no boundary.
2. The property of being boundariless (at least in low dimensions) coincides with our intuitive idea of what it means to be a loop.

Of course, just as with homotopy these holes interact in ways we’re about to see, so we need to be totally concrete before we can proceed.

## The Chain Group and the Boundary Operator

In order to define an algebraic boundary, we have to realize simplices themselves as algebraic objects.  This is not so difficult to do: just take all “formal sums” of simplices in the complex. More rigorously, let $X_k$ be the set of $k$-simplices in the simplicial complex $X$. Define the chain group $C_k(X)$ to be the $\mathbb{Q}$-vector space with $X_k$ for a basis. The elements of the $k$-th chain group are called k-chainon $X$. That’s right, if $\sigma, \sigma'$ are two $k$-simplices, then we just blindly define a bunch of new “chains” as all possible “sums” and scalar multiples of the simplices. For example, sums involving two elements would look like $a\sigma + b\sigma'$ for some $a,b \in \mathbb{Q}$. Indeed, we include any finite sum of such simplices, as is standard in taking the span of a set of basis vectors in linear algebra.

Just for a quick example, take this very simple simplicial complex:

We’ve labeled all of the simplices above, and we can describe the chain groups quite easily. The zero-th chain group $C_0(X)$ is the $\mathbb{Q}$-linear span of the set of vertices $\left \{ v_1, v_2, v_3, v_4 \right \}$. Geometrically, we might think of “the union” of two points as being, e.g., the sum $v_1 + v_3$. And if we want to have two copies of $v_1$ and five copies of $v_3$, that might be thought of as $2v_1 + 5v_3$. Of course, there are geometrically meaningless sums like $\frac{1}{2}v_4 - v_2 - \frac{11}{6}v_1$, but it will turn out that the algebra we use to talk about holes will not falter because of it. It’s nice to have this geometric idea of what an algebraic expression can “mean,” but in light of this nonsense it’s not a good idea to get too wedded to the interpretations.

Likewise, $C_1(X)$ is the linear span of the set $\left \{ e_1, e_2, e_3, e_4, e_5 \right \}$ with coefficients in $\mathbb{Q}$. So we can talk about a “path” as a sum of simplices like $e_1 + e_4 - e_5 + e_3$. Here we use a negative coefficient to signify that we’re travelling “against” the orientation of an edge. Note that since the order of the terms is irrelevant, the same “path” is given by, e.g. $-e_5 + e_4 + e_1 + e_3$, which geometrically is ridiculous if we insist on reading the terms from left to right.

The same idea extends to higher dimensional groups, but as usual the visualization grows difficult. For example, in $C_2(X)$ above, the chain group is the vector space spanned by $\left \{ \sigma_1, \sigma_2 \right \}$. But does it make sense to have a path of triangles? Perhaps, but the geometric analogies certainly become more tenuous as dimension grows. The benefit, however, is if we come up with good algebraic definitions for the low-dimensional cases, the algebra is easy to generalize to higher dimensions.

So now we will define the boundary operator on chain groups, a linear map $\partial : C_k(X) \to C_{k-1}(X)$ by starting in lower dimensions and generalizing. A single vertex should always be boundariless, so $\partial v = 0$ for each vertex. Extending linearly to the entire chain group, we have $\partial$ is identically the zero map on zero-chains. For 1-simplices we have a more substantial definition: if a simplex has its orientation as $(v_1, v_2)$, then the boundary $\partial (v_1, v_2)$ should be $v_2 - v_1$. That is, it’s the front end of the edge minus the back end. This defines the boundary operator on the basis elements, and we can again extend linearly to the entire group of 1-chains.

Why is this definition more sensible than, say, $v_1 + v_2$? Using our example above, let’s see how it operates on a “path.” If we have a sum like $e_1 + e_4 - e_5 - e_3$, then the boundary is computed as

$\displaystyle \partial (e_1 + e_4 - e_5 - e_3) = \partial e_1 + \partial e_4 - \partial e_5 - \partial e_3$
$\displaystyle = (v_2 - v_1) + (v_4 - v_2) - (v_4 - v_3) - (v_3 - v_2) = v_2 - v_1$

That is, the result was the endpoint of our path $v_2$ minus the starting point of our path $v_1$. It is not hard to prove that this will work in general, since each successive edge in a path will cancel out the ending vertex of the edge before it and the starting vertex of the edge after it: the result is just one big alternating sum.

Even more importantly is that if the “path” is a loop (the starting and ending points are the same in our naive way to write the paths), then the boundary is zero. Indeed, any time the boundary is zero then one can rewrite the sum as a sum of “loops,” (though one might have to trivially introduce cancelling factors). And so our condition for a chain to be a “loop,” which is just one step away from being a “hole,” is if it is in the kernel of the boundary operator. We have a special name for such chains: they are called cycles.

For 2-simplices, the definition is not so much harder: if we have a simplex like $(v_0, v_1, v_2)$, then the boundary should be $(v_1,v_2) - (v_0,v_2) + (v_0,v_1)$. If one rewrites this in a different order, then it will become apparent that this is just a path traversing the boundary of the simplex with the appropriate orientations. We wrote it in this “backwards” way to lead into the general definition: the simplices are ordered by which vertex does not occur in the face in question ($v_0$ omitted from the first, $v_1$ from the second, and $v_2$ from the third).

We are now ready to extend this definition to arbitrary simplices, but a nice-looking definition requires a bit more notation. Say we have a k-simplex which looks like $(v_0, v_1, \dots, v_k)$. Abstractly, we can write it just using the numbers, as $[0,1,\dots, k]$. And moreover, we can denote the removal of a vertex from this list by putting a hat over the removed index. So $[0,1,\dots, \hat{i}, \dots, k]$ represents the simplex which has all of the vertices from 0 to $k$ excluding the vertex $v_i$. To represent a single-vertex simplex, we will often drop the square brackets, e.g. $3$ for $[3]$. This can make for some awkward looking math, but is actually standard notation once the correct context has been established.

Now the boundary operator is defined on the standard $n$-simplex with orientation $[0,1,\dots, n]$ via the alternating sum

$\displaystyle \partial([0,1,\dots, n]) = \sum_{k=0}^n (-1)^k [0, \dots, \hat{k}, \dots, n]$

It is trivial (but perhaps notationally hard to parse) to see that this coincides with our low-dimensional examples above. But now that we’ve defined it for the basis elements of a chain group, we automatically get a linear operator on the entire chain group by extending $\partial$ linearly on chains.

Definition: The k-cycles on $X$ are those chains in the kernel of $\partial$. We will call k-cycles boundariless. The k-boundaries are the image of $\partial$.

We should note that we are making a serious abuse of notation here, since technically $\partial$ is defined on only a single chain group. What we should do is define $\partial_k : C_k(X) \to C_{k-1}(X)$ for a fixed dimension, and always put the subscript. In practice this is only done when it is crucial to be clear which dimension is being talked about, and otherwise the dimension is always inferred from the context. If we want to compose the boundary operator in one dimension with the boundary operator in another dimension (say, $\partial_{k-1} \partial_k$), it is usually written $\partial^2$. This author personally supports the abolition of the subscripts for the boundary map, because subscripts are a nuisance in algebraic topology.

All of that notation discussion is so we can make the following observation: $\partial^2 = 0$. That is, every chain which is a boundary of a higher-dimensional chain is boundariless! This should make sense in low-dimension: if we take the boundary of a 2-simplex, we get a cycle of three 1-simplices, and the boundary of this chain is zero. Indeed, we can formally prove it from the definition for general simplices (and extend linearly to achieve the result for all simplices) by writing out $\partial^2([0,1,\dots, n])$. With a keen eye, the reader will notice that the terms cancel out and we get zero. The reason is entirely in which coefficients are negative; the second time we apply the boundary operator the power on (-1) shifts by one index. We will leave the full details as an exercise to the reader.

So this fits our two criteria: low-dimensional examples make sense, and boundariless things (cycles) represent loops.

## Recasting in Algebraic Terms, and the Homology Group

For the moment let’s give boundary operators subscripts $\partial_k : C_k(X) \to C_{k-1}(X)$. If we recast things in algebraic terms, we can call the k-cycles $Z_k(X) = \textup{ker}(\partial_k)$, and this will be a subspace (and a subgroup) of $C_k(X)$ since kernels are always linear subspaces. Moreover, the set $B_k(X)$ of k-boundaries, that is, the image of $\partial_{k+1}$, is a subspace (subgroup) of $Z_k(X)$. As we just saw, every boundary is itself boundariless, so $B_k(X)$ is a subset of $Z_k(X)$, and since the image of a linear map is always a linear subspace of the range, we get that it is a subspace too.

All of this data is usually expressed in one big diagram: each of the chain groups are organized in order of decreasing dimension, and the boundary maps connect them.

Since our example (the “simple space” of two triangles from the previous section) only has simplices in dimensions zero, one, and two, we additionally extend the sequence of groups to an infinite sequence by adding trivial groups and zero maps, as indicated. The condition that $\textup{im} \partial_{k+1} \subset \textup{ker} \partial_k$, which is equivalent to $\partial^2 = 0$, is what makes this sequence a chain complex. As a side note, every sequence of abelian groups and group homomorphisms which satisfies the boundary requirement is called an algebraic chain complex. This foreshadows that there are many different types of homology theory, and they are unified by these kinds of algebraic conditions.

Now, geometrically we want to say, “The holes are all those cycles (loops) which don’t arise as the boundaries of higher-dimensional things.” In algebraic terms, this would correspond to a quotient space (really, a quotient group, which we covered in our first primer on groups) of the k-cycles by the k-boundaries. That is, a cycle would be considered a “trivial hole” if it is a boundary, and two “different” cycles would be considered the same hole if their difference is a k-boundary. This is the spirit of homology, and formally, we define the homology group (vector space) as follows.

Definition: The $k$-th homology group of a simplicial complex $X$, denoted $H_k(X)$, is the quotient vector space $Z_k(X) / B_k(X) = \textup{ker}(\partial_k) / \textup{im}(\partial_{k+1})$. Two elements of a homology group which are equivalent (their difference is a boundary) are called homologous.

The number of $k$-dimensional holes in $X$ is thus realized as the dimension of $H_k(X)$ as a vector space.

The quotient mechanism really is doing all of the work for us here. Any time we have two holes and we’re wondering whether they represent truly different holes in the space (perhaps we have a closed loop of edges, and another which is slightly longer but does not quite use the same edges), we can determine this by taking their difference and seeing if it bounds a higher-dimensional chain. If it does, then the two chains are the same, and if it doesn’t then the two chains carry intrinsically different topological information.

For particular dimensions, there are some neat facts (which obviously require further proof) that make this definition more concrete.

• The dimension of $H_0(X)$ is the number of connected components of $X$. Therefore, computing homology generalizes the graph-theoretic methods of computing connected components.
• $H_1(X)$ is the abelianization of the fundamental group of $X$. Roughly speaking, $H_1(X)$ is the closest approximation of $\pi_1(X)$ by a $\mathbb{Q}$ vector space.

Now that we’ve defined the homology group, let’s end this post by computing all the homology groups for this example space:

This is a sphere (which can be triangulated as the boundary of a tetrahedron) with an extra “arm.” Note how the edge needs an extra vertex to maintain uniqueness. This space is a nice example because it has one-dimensional homology in dimension zero (one connected component), dimension one (the arm is like a copy of the circle), and dimension two (the hollow sphere part). Let’s verify this algebraically.

Let’s start by labelling the vertices of the tetrahedron 0, 1, 2, 3, so that the extra arm attaches at 0 and 2, and call the extra vertex on the arm 4. This completely determines the orientations for the entire simplex, as seen below.

Indeed, the chain groups are easy to write down:

$\displaystyle C_0(X) = \textup{span} \left \{ 0,1,2,3,4 \right \}$

$\displaystyle C_1(X) = \textup{span} \left \{ [0,1], [0,2], [0,3], [0,4], [1,2], [1,3],[2,3],[2,4] \right \}$

$\displaystyle C_2(X) = \textup{span} \left \{ [0,1,2], [0,1,3], [0,2,3], [1,2,3] \right \}$

We can easily write down the images of each $\partial_k$, they’re just the span of the images of each basis element under $\partial_k$.

$\displaystyle \textup{im} \partial_1 = \textup{span} \left \{ 1 - 0, 2 - 0, 3 - 0, 4 - 0, 2 - 1, 3 - 1, 3 - 2, 4 - 2 \right \}$

The zero-th homology $H_0(X)$ is the kernel of $\partial_0$ modulo the image of $\partial_1$. The angle brackets are a shorthand for “span.”

$\displaystyle \frac{\left \langle 0,1,2,3,4 \right \rangle}{\left \langle 1-0,2-0,3-0,4-0,2-1,3-1,3-2,4-2 \right \rangle}$

Since $\partial_0$ is actually the zero map, $Z_0(X) = C_0(X)$ and all five vertices generate the kernel. The quotient construction imposes that two vertices (two elements of the homology group) are considered equivalent if their difference is a boundary. It is easy to see that (indeed, just by the first four generators of the image) all vertices are equivalent to 0, so there is a unique generator of homology, and the vector space is isomorphic to $\mathbb{Q}$. There is exactly one connected component. Geometrically we can realize this, because two vertices are homologous if and only if there is a “path” of edges from one vertex to the other. This chain will indeed have as its image the difference of the two vertices.

We can compute the first homology $H_1(X)$ in an analogous way, compute the kernel and image separately, and then compute the quotient.

$\textup{ker} \partial_1 = \textup{span} \left \{ [0,1] + [0,3] - [1,3], [0,2] + [2,3] - [0,3], [1,2] + [2,3] - [1,3], [0,1] + [1,2] - [0,2], [0,2] + [2,4] - [0,4] \right \}$

It takes a bit of combinatorial analysis to show that this is precisely the kernel of $\partial_1$, and we will have a better method for it in the next post, but indeed this is it. As the image of $\partial_2$ is precisely the first four basis elements, the quotient is just the one-dimensional vector space spanned by $[0,2] + [2,4] - [0,4]$. Hence $H_1(X) = \mathbb{Q}$, and there is one one-dimensional hole.

Since there are no 3-simplices, the homology group $H_2(X)$ is simply the kernel of $\partial_2$, which is not hard to see is just generated by the chain representing the “sphere” part of the space: $[1,2,3] - [0,2,3] + [0,1,3] - [0,1,2]$. The second homology group is thus again $\mathbb{Q}$ and there is one two-dimensional hole in $X$.

So there we have it!

## Looking Forward

Next time, we will give a more explicit algorithm for computing homology for finite simplicial complexes, and it will essentially be a variation of row-reduction which simultaneously rewrites the matrix representations of the boundary operators $\partial_{k+1}, \partial_k$ with respect to a canonical basis. This will allow us to simply count entries on the digaonals of the two matrices, and the difference will be the dimension of the quotient space, and hence the number of holes.

Until then!