# How many group homomorphisms are there from $\Bbb Z_{20}$ to $\Bbb Z_{10}$? [duplicate]

Can someone please explain what determines whether or not you can have a group homomorphism from one set to another, and then what determines how many you can have?

• The general process for finding group homomorphisms $G\to H$ is to consider a presentation of the group $G$. Namely, we can say $G$ is generated by a set of elements $X$ which satisfy a set of "relations" $R$ (and we write $G=\langle X\mid R\rangle$). Then any group homomorphism $G\to H$ corresponds to a function $X\to H$ in which the images of the generators satisfy the same relations in $H$ as they do in $G$. In the case of two cyclic groups $\Bbb Z_n$ and $\Bbb Z_m$, we know $\Bbb Z_n$ is generated by a single element satisfying $ng=0$ (in additive notation).
– anon
Commented Jul 14, 2017 at 4:22
• Thus a group homomorphism $\Bbb Z_n\to\Bbb Z_m$ corresponds to a function $\phi:\{g\}\to\Bbb Z_m$ such that $\phi(g)\in\Bbb Z_m$ also satisfies $n\phi(g)=0$. In other words, there is exactly one group homomorphism $\Bbb Z_n\to\Bbb Z_m$ for every solution to $nx=0$ in the group $\Bbb Z_m$.
– anon
Commented Jul 14, 2017 at 4:24

## 3 Answers

Are you talking group or ring homomorphisms? It would help to remember that $\mathbb Z/20\mathbb Z$ is generated by $1$ and whatever you map $1$ to should satisfy that adding itself twenty times yields 0. Can you proceed?

• I am talking about group homorphisms. So I can map 1 to any generator of Z10, so there are 4 possible group homomorphisms? If so, can you tell me what determines the number of onoto group homomorphisms? Commented Jul 13, 2017 at 18:40
• Why does it have to be a generator? Whatever the candidate is, say $x$, it just needs to satisfy $20x=0$ in $\mathbb Z/10\mathbb Z$. How many such $x$ can you think of? Commented Jul 13, 2017 at 18:42
• Oh, that would be all of them, because each would be a multiple of 10? Is that the correct way to describe what is happening? Commented Jul 13, 2017 at 19:01
• But what is the general method of determining this? What if the sets were something like Z13 and Z4? What is the general thought process I can use to figure this out for sets of any two orders? Commented Jul 13, 2017 at 19:11
• Yes, I think so. For the Z20, Z10 examples, does this result mean that the order of every element in Z10 divides 10? If so, is that a general result in abstract algebra that applies to all groups, or groups of a certain kind? Commented Jul 13, 2017 at 19:24

The kernel of every homomorphism $\mathbb{Z}\to\mathbb{Z}/10\mathbb{Z}$ contains $10\mathbb{Z}$, hence also $20\mathbb{Z}$.

How many distinct homomorphisms $\mathbb{Z}\to\mathbb{Z}/10\mathbb{Z}$ are there?

What do the homomorphism theorems say?

Here observe that order of the element should divide both group's order And gcd(20,10)=10 Divisors of 10 are 1,2,5,10 Now try to find those elements in Z10 with these orders and check which of them will give group homomorphism In short the idea is to first find the gcd and ans is 10. No. Of onto homomorphism will be phi(10)=4 No. Of one one homomorphism will be phi(20)=8