The first isomorphism theorem is probably the most important result one will learn in a first course on algebra. We will state and prove the theorem for groups. There is, of course, an analogous result for rings. We will state the theorem for rings (the proof is identical to that for groups) and then look at an example using the theorem.

Theorem. If \varphi : G \to H is a homomorphism of groups, then \text{ker}(\varphi) is a normal subgroup of G and G/\text{ker}(\varphi)\cong\varphi(G).

The proof is not terribly complicated; in fact, it is rather “computational”, meaning, we just have to verify a few definitions. It is just a matter of checking that the kernel satisfies the definition of normal and then defining a function from G/\text{ker}(\varphi) to \varphi(G) and checking that the function satisfies the definition of an isomorphism. The “hardest” part of the proof, I think, is actually defining a function from G/\text{ker}(\varphi) to \varphi(G). However, as we will see, the choice of such a function is quite natural.

Proof. First, we will show that the kernel of \varphi is a normal subgroup of G. We check the following:

Nonempty. Since \varphi is a homomorphism, it must map the identity in G to the identity in H. Thus, 1_G\in\text{ker}(\varphi) which shows that the kernel is nonempty.

Closure. Let a and b be in the kernel of \varphi. We want to show that ab^{-1} is in the kernel too. We compute that


Thus, ab^{-1} is indeed in the kernel of \varphi, which shows that \text{ker}(\varphi) is a subgroup of G.

Normal. Let g be an element of G and let k be an element in the kernel of \varphi. It suffices to show that gkg^{-1} is also in the kernel. We have that


Thus, gkg^{-1} is in the kernel of \varphi as desired. This shows that \text{ker}(\varphi) is a normal subgroup of G.

Next, we will show that G/\text{ker}(\varphi) is isomorphic to \varphi(G). Define \psi : G/\text{ker}(\varphi) \to \varphi(G) by \psi(g\text{ker}(\varphi))=\varphi(g). I claim that \psi is a well-defined isomorphism. Indeed, we check the following:

Well-Defined. Suppose that a\text{ker}(\varphi)=b\text{ker}(\varphi). Then a=bk for some element k in the kernel of \varphi. We wish to show that \psi(a\text{ker}(\varphi))=\psi(b\text{ker}(\varphi)). Evaluating \psi(a\text{ker}(\varphi)) we see,


Thus, \psi is well defined.

Homomorphism. Let a\text{ker}(\varphi) and b\text{ker}(\varphi) be cosets in the factor group G/\text{ker}(\varphi). By multiplying cosets we observe that a\text{ker}(\varphi)b\text{ker}(\varphi)=ab\text{ker}(\varphi). Consequentially,

\psi(a\text{ker}(\varphi)b\text{ker}(\varphi))=\varphi(ab) =\varphi(a)\varphi(b) =\psi(a\text{ker}(\varphi))\psi(b\text{ker}(\varphi)).

Thus, \psi is a homomorphism.

Injective. Suppose that \psi(a\text{ker}(\varphi))=\psi(b\text{ker}(\varphi)). We wish to show that a\text{ker}(\varphi)=b\text{ker}(\varphi). Consider the following,


Since \varphi(b) is an element of H and H is a group, \varphi(b)^{-1} exists. Therefore,


which shows that ab^{-1} is in the kernel of \varphi. It follows that a\text{ker}(\varphi)=b\text{ker}(\varphi) and so \psi is injective.

Surjective. Let x be an element in the image of \varphi. Then there exists an element g in G such that \varphi(g)=x. Thus, \psi(g\text{ker}(\varphi))=\varphi(g)=x which shows that \psi is surjective.

We conclude that \psi is a bijective homomorphism, hence an isomorphism. \Box

We now state the analogous result for rings.

Theorem. If \varphi : R \to S is a homomorphism of rings, then \text{ker}(\varphi) is an ideal of R and R/\text{ker}(\varphi)\cong\varphi(R).

Example. Let R be a commutative ring with unity. Prove that R[x] / \langle x \rangle \cong R.

Define the function \varphi:R[x]\to R to be the evaluation homomorphism \varphi(f(x))=f(0) for any f(x) in R[x]. By the first isomorphism theorem, it suffices to check that \varphi is a surjective homomorphism with \text{ker}\varphi=\langle x \rangle. We check the following:

Homomorphism. Let f(x) and g(x) be in R[x]. We compute that



Thus, \varphi is indeed a ring homomorphism.

Surjective. Let r be an element in R. Then f(x)=x+r is in R[x] and \varphi(f(x))=f(0)=0+r=r which shows that \varphi is surjective.

Kernel. Let f(x) be in the kernel of \varphi. Then f(0)=0 which means that x is a factor of f(x) and so f(x) is in the ideal \langle x \rangle. Therefore, \text{ker}\varphi\subseteq\langle x \rangle. Now, let g(x) be in the ideal \langle x \rangle. Then g(x)=xf(x) for some f(x) in R[x]. Thus, \varphi(g(x))=g(0)=0f(0)=0 which shows that g(x) is in the kernel of \varphi. Therefore, \langle x \rangle\subseteq\text{ker}\varphi. We conclude that \text{ker}\varphi=\langle x \rangle.

By the first isomorphism theorem we have that R[x] / \langle x \rangle \cong R. \Box