# Example: Examples of Kernels and Images of Group Homomorphisms

(related to Definition: Group Homomorphism)

We will now build the respective group homomorphisms for the examples given for group homomorphisms.

• Kernel $\operatorname{ker}(f):= \{g\in G\mid f(g)=e_H\}$:
• The neutral element of $H=(\{3n\mid n\in\mathbb Z\}, +)$ is $e_H=0$. The only element of $x\in G=(\mathbb Z, +)$ for which $f(x)=3x=0$ is $x=0.$ Thus $\{0\}$ is the kernel of $f.$
• Image $\operatorname{im}(f):=f[G]=\{f(g)\in H\mid g\in G\}$:
• The image $f[G]=\{3x\mid x\in\mathbb Z\}$ are all multiples of $3$, i.e. $f[G]=H.$

• Kernel $\operatorname{ker}(f):= \{g\in G\mid f(g)=e_H\}$:
• The neutral element of $H=(\mathbb R^*, \cdot)$ is $e_H=1$. The only element of $x\in G=(\mathbb R,+)$ for which $\exp(x)=1$ is $x=0.$ Thus $\{0\}$ is the kernel of $\exp.$
• Image $\operatorname{im}(\exp):=f[G]=\{f(g)\in H\mid g\in G\}$:
• The image $\exp[\mathbb R]=\mathbb R^+$ are all positive real numbers, i.e. $\exp[\mathbb R]=\mathbb R^+.$

• Kernel $\operatorname{ker}(f):= \{g\in G\mid f(g)=e_H\}$:
• The neutral element of $H$ is $$e_H=\pmatrix{1&0\\0&1}.$$ The elements of $x\in (\mathbb R,+)$ for which $\rho(x)=e_H$ are all integer multiples of $2\pi k,$ $k\in\mathbb Z.$ This follows from special values of cosine and sine and their periodicity. Thus $\{2\pi k\mid k\in\mathbb Z\}$ is the kernel of $\rho.$
• Image $\operatorname{im}(\exp):=f[G]=\{f(g)\in H\mid g\in G\}$:
• The image $\rho[\mathbb R]=\mathbb R^+$ are all matrices of $H=(\operatorname{GL}(2,\mathbb R),\cdot).$

Github: ### References

#### Bibliography

1. Modler, Florian; Kreh, Martin: "Tutorium Algebra", Springer Spektrum, 2013