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## Example: Examples of Properties of Group Homomorphisms

We want to verify the properties of group homomorphisms for the examples given for group homomorphisms.

### Ad Example 1

• Verifying the property (1) $f(e_G)=e_H$:
• The neutral element of $G=(\mathbb Z,+)$ is $e_G=0$ and the same neutral element has $H=(\{3n\mid n\in\mathbb Z\}, +)$, i.e. $e_H=0$. Thus $$f(0)=3\cdot 0=0.$$
• Verifying the property (2) $f(x^{-1})=f(x)^{-1}$ for all $x\in G$:
• Let $x\in(\mathbb Z,+)$. Then $x^{-1}=-x.$
• Then $f(-x)=3\cdot(-x)=-3x$ which is the inverse element of $3x\in (\{3n\mid n\in\mathbb Z\},+).$

### Ad Example 2

• Verifying the property (1) $f(e_G)=e_H$:
• Verifying the property (2) $f(x^{-1})=f(x)^{-1}$ for all $x\in G$:

### Ad Example 3

• Verifying the property (1) $f(e_G)=e_H$:
• The neutral element of $G=(\mathbb R,+)$ is $e_G=0$ and the neutral element of $H=(\operatorname{GL}(2,\mathbb R),\cdot)$ is $$e_H=\pmatrix{1&0\\0&1}.$$ We verify for the in the example defined group homomorphism $$\rho(0)=\pmatrix{\cos(0)&-\sin(0)\\\sin(0)&\cos(0)}=\pmatrix{1&0\\0&1}.$$
• Verifying the property (2) $f(x^{ -1})=f(x)^{ -1}$ for all $x\in G$:
• Let $x\in(\mathbb R,+)$. Then $x^{ -1}=-x.$ We verify by the eveness of cosine and the oddness of sine
$$\rho(-x)=\pmatrix{\cos( -x)&-\sin( -x)\\\sin( -x)&\cos( -x)}=\pmatrix{\cos(-x)&\sin(x)\\-\sin(x)&\cos(x)},$$ which is the inverse element of $H.$ This can be verified by the Pythagorean identity $$\pmatrix{\cos(x)&-\sin(x)\\\sin(x)&\cos(x)}\cdot \pmatrix{\cos(x)&\sin(x)\\-\sin(x)&\cos(x)}=\pmatrix{1&0\\0&1}.$$

| | | | created: 2020-06-28 08:43:40 | modified: 2020-06-28 10:29:23 | by: bookofproofs | references: [677]

### Bibliography (further reading)

[677] Modler, Florian; Kreh, Martin: “Tutorium Algebra”, Springer Spektrum, 2013