I’m not sure how, but many of my Mathematics A To Z essays seem to circle around algebra. I mean abstract algebra, not the kind that involves petty concerns like ‘x’ and ‘y’. In abstract algebra we worry about letters like ‘g’ and ‘h’. For special purposes we might even have ‘e’. Maybe it’s that the subject has a lot of familiar-looking words. For today’s term, I’m doing an algebra term, and one that wasn’t requested. But it’ll make my life a little easier when I get to a word that was requested.
Also, I lied when I said this was an abstract algebra word. At least I was imprecise. The word appears in a fairly wide swath of mathematics. But abstract algebra is where most mathematics majors first encounter it. And the other uses hearken back to this. If you understand what an algebraist means by “homomorphism” then you understand the essence of what someone else means by it.
One of the things mathematicians study a lot is mapping. This is matching the things in one set to things in another set. Most often we want this to be done by some easy-to-understand rule. Why? Well, we often want to understand how one group of things relates to another group. So we set up maps between them. These describe how to match the things in one set to the things in another set. You may think this sounds like it’s just a function. You’re right. I suppose the name “mapping” carries connotations of transforming things into other things that a “function” might not have. And “functions”, I think, suggest we’re working with numbers. “Mappings” sound more abstract, at least to my ear. But it’s just a difference in dialect, not substance.
A homomorphism is a mapping that obeys a couple of rules. What they are depends on the kind of things the homomorphism maps between. I want a simple example, so I’m going to use groups.
A group is made up of two things. One is a set, a collection of elements. For example, take the whole numbers 0, 1, 2, and 3. That’s a good enough set. The second thing in the group is an operation, something to work like addition. For example, we might use “addition modulo 4”. In this scheme, addition (and subtraction) work like they do with ordinary whole numbers. But if the result would be more than 3, we subtract 4 from the result, until we get something that’s 0, 1, 2, or 3. Similarly if the result would be less than 0, we add 4, until we get something that’s 0, 1, 2, or 3. The result is an addition table that looks like this:
So let me call G the group that has as its elements 0, 1, 2, and 3, and that has addition be this modulo-4 addition.
Now I want another group. I’m going to name it H, because the alternative is calling it G2 and subscripts are tedious to put on web pages. H will have a set with the elements 0, 1, 2, 3, 4, 5, 6, and 7. Its addition will be modulo-8 addition, which works the way you might have guessed after looking at the above. But here’s the addition table:
G and H look a fair bit like each other. Their sets are made up of familiar numbers, anyway. And the addition rules look a lot like what we’re used to.
We can imagine mapping from one to the other pretty easily. At least it’s easy to imagine mapping from G to H. Just match a number in G’s set — say, ‘1’ — to a number in H’s set — say, ‘2’. Easy enough. We’ll do something just as daring in matching ‘0’ to ‘1’, and we’ll map ‘2’ to ‘3’. And ‘3’? Let’s match that to ‘4’. Let me call that mapping f.
But f is not a homomorphism. What makes a homomorphism an interesting map is that the group’s original addition rule carries through. This is easier to show than to explain.
In the original group G, what’s 1 + 2? … 3. That’s easy to work out. But in H, what’s f(1) + f(2)? f(1) is 2, and f(2) is 3. So f(1) + f(2) is 5. But what is f(3)? We set that to be 4. So in this mapping, f(1) + f(2) is not equal to f(3). And so f is not a homomorphism.
Could anything be? After all, G and H have different sets, sets that aren’t even the same size. And they have different addition rules, even if the addition rules look like they should be related. Why should we expect it’s possible to match the things in group G to the things in group H?
Let me show you how they could be. I’m going to define a mapping φ. The letter’s often used for homomorphisms. φ matches things in G’s set to things in H’s set. φ(0) I choose to be 0. φ(1) I choose to be 2. φ(2) I choose to be 4. φ(3) I choose to be 6.
And now look at this … φ(1) + φ(2) is equal to 2 + 4, which is 6 … which is φ(3). Was I lucky? Try some more. φ(2) + φ(2) is 4 + 4, which in the group H is 0. In the group G, 2 + 2 is 0, and φ(0) is … 0. We’re all right so far.
One more. φ(3) + φ(3) is 6 + 6, which in group H is 4. In group G, 3 + 3 is 2. φ(2) is 4.
If you want to test the other thirteen possibilities go ahead. If you want to argue there’s actually only seven other possibilities do that, too. What makes φ a homomorphism is that if x and y are things from the set of G, then φ(x) + φ(y) equals φ(x + y). φ(x) + φ(y) uses the addition rule for group H. φ(x + y) uses the addition rule for group G. Some mappings keep the addition of things from breaking. We call this “preserving” addition.
This particular example is called a group homomorphism. That’s because it’s a homomorphism that starts with one group and ends with a group. There are other kinds of homomorphism. For example, a ring homomorphism is a homomorphism that maps a ring to a ring. A ring is like a group, but it has two operations. One works like addition and the other works like multiplication. A ring homomorphism preserves both the addition and the multiplication simultaneously.
And there are homomorphisms for other structures. What makes them homomorphisms is that they preserve whatever the important operations on the strutures are. That’s typically what you might expect when you are introduced to a homomorphism, whatever the field.