# Recall: Properties of Homomorphisms

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1 Recall: Properties of Homomorphisms Let φ : G Ḡ be a homomorphism, let g G, and let H G. Properties of elements Properties of subgroups 1. φ(e G ) = eḡ 1. φ(h) Ḡ. 2. φ(g n ) = (φ(g)) n for all n Z. 2. H cyclic φ(h) cyclic. 3. If g is finite, φ(g) divides g. 3. H Abelian φ(h) Abelian. 4. Ker(φ) G 4. H G φ(h) φ(g) In fact, Ker(φ) G 5. φ(a) = φ(b) aker(φ) = bker(φ) 6. φ(g) = ḡ φ 1 (ḡ) = gker(φ) 7. K Ḡ φ 1 ( K) G. 8. K Ḡ φ 1 ( K) G. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

2 Properties of Homomorphisms Let φ : G Ḡ be a homomorphism, let g G, and let H G. Properties of elements Properties of subgroups 1. φ(e G ) = eḡ 1. φ(h) Ḡ. 2. φ(g n ) = (φ(g)) n for all n Z. 2. H cyclic φ(h) cyclic. 3. If g is finite, φ(g) divides g. 3. H Abelian φ(h) Abelian. 4. Ker(φ) G 4. H G φ(h) φ(g) In fact, Ker(φ) G 5. φ(a) = φ(b) 5. Ker(φ) = n φ is an n-to-1 aker(φ) = bker(φ) map 6. φ(g) = ḡ φ 1 (ḡ) = gker(φ) 7. K Ḡ φ 1 ( K) G. 8. K Ḡ φ 1 ( K) G. Ker(φ) = {e G } φ is φ onto and Ker(φ) = {e G } φ an isomorphism. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

3 Properties of Homomorphisms Let φ : G Ḡ be a homomorphism, let g G, and let H G. Properties of elements Properties of subgroups 1. φ(e G ) = eḡ 1. φ(h) Ḡ. 2. φ(g n ) = (φ(g)) n for all n Z. 2. H cyclic φ(h) cyclic. 3. If g is finite, φ(g) divides g. 3. H Abelian φ(h) Abelian. 4. Ker(φ) G 4. H G φ(h) φ(g) In fact, Ker(φ) G 5. φ(a) = φ(b) aker(φ) = bker(φ) 5. Ker(φ) = n φ is an n-to-1 map 6. φ(g) = ḡ φ 1 (ḡ) = gker(φ) 6. H = n φ(h) divides n 7. K Ḡ φ 1 ( K) G. 8. K Ḡ φ 1 ( K) G. Ker(φ) = {e G } φ is φ onto and Ker(φ) = {e G } φ an isomorphism. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

4 In Class Work 1. Prove that (A B)/(A {e}) is isomorphic to B, by identifying a homomorphism from A B B that has A {e} as its kernel. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

5 1. Prove that (A B)/(A {e}) is isomorphic to B. Define φ : A B B by φ((a, b)) = b. From previous in-class work, we already know that φ is a well-defined onto homomorphism. In fact, we know (more or less) from Friday what the kernel is. Ker(φ) = A {e}. The easiest way to do this problem is to use the 1st isomorphism theorem. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

6 1. Prove that (A B)/(A {e}) is isomorphic to B. Define φ : A B B by φ((a, b)) = b. From previous in-class work, we already know that φ is a well-defined onto homomorphism. In fact, we know (more or less) from Friday what the kernel is. Ker(φ) = A {e}. The easiest way to do this problem is to use the 1st isomorphism theorem. The 1st Isomorphism Thm tells us A B/Ker(φ) φ(a B), or A B/Ker(φ) B, since φ is onto. Thus, since Ker(φ) = A {e}, A B/A {e} B. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

7 We need to find φ 1 (9). Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

8 We need to find φ 1 (9). Thm 10.1 Property 5 states: if φ(g) = ḡ, then φ 1 (ḡ) = gker(φ). Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

9 We need to find φ 1 (9). Thm 10.1 Property 5 states: if φ(g) = ḡ, then φ 1 (ḡ) = gker(φ). In Z30, the operation is addition mod 30. Thm 10.1 Prop 5 thus becomes if φ(g) = ḡ, then φ 1 (ḡ) = g + Ker(φ), mod 30. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

10 We need to find φ 1 (9). Thm 10.1 Property 5 states: if φ(g) = ḡ, then φ 1 (ḡ) = gker(φ). In Z30, the operation is addition mod 30. Thm 10.1 Prop 5 thus becomes if φ(g) = ḡ, then φ 1 (ḡ) = g + Ker(φ), mod 30. We know that φ(23) = 9 Thus φ 1 (9) = 23 + Ker(φ) = 23 + {0, 10, 20} mod 30 Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

11 We need to find φ 1 (9). Thm 10.1 Property 5 states: if φ(g) = ḡ, then φ 1 (ḡ) = gker(φ). In Z30, the operation is addition mod 30. Thm 10.1 Prop 5 thus becomes if φ(g) = ḡ, then φ 1 (ḡ) = g + Ker(φ), mod 30. We know that φ(23) = 9 Thus φ 1 (9) = 23 + Ker(φ) = 23 + {0, 10, 20} mod 30 = {23, 3, 13}, Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

12 We need to find φ 1 (9). Thm 10.1 Property 5 states: if φ(g) = ḡ, then φ 1 (ḡ) = gker(φ). In Z30, the operation is addition mod 30. Thm 10.1 Prop 5 thus becomes if φ(g) = ḡ, then φ 1 (ḡ) = g + Ker(φ), mod 30. We know that φ(23) = 9 Thus φ 1 (9) = 23 + Ker(φ) = 23 + {0, 10, 20} mod 30 = {23, 3, 13}, so the elements that map to 9 are 3, 13, and 23. Math 321-Abstract (Sklensky) In-Class Work November 22, / 6

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