# Lecture 5.6: The Sylow theorems

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1 Lecture 5.6: The Sylow theorems Matthew Macauley Department of Mathematical Sciences Clemson University Math 4120, Modern Algebra M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 1 / 17

2 Overview The Sylow theorems are about one question: What finite groups are there Early on, we saw five families of groups: cyclic, dihedral, abelian, symmetric, alternating. Later, we classified all (finitely generated) abelian groups. But what other groups are there, and what do they look like For example, for a fixed order G, we may ask the following questions about G: 1. How big are its subgroups 2. How are those subgroups related 3. How many subgroups are there 4. Are any of them normal There is no one general method to answer this for any given order. However, the Sylow Theorems, developed by Norwegian mathematician Peter Sylow ( ), are powerful tools that help us attack this question. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 2 / 17

3 Recall from last time Definition A p-group is a group whose order is a power of a prime p. A p-group that is a subgroup of a group G is a p-subgroup of G. Notational convention Througout, G will be a group of order G = p n m, with p m. That is, p n is the highest power of p dividing G. There are three Sylow theorems, and loosely speaking, they describe the following about a group s p-subgroups: 1. Existence: In every group, p-subgroups of all possible sizes exist. 2. Relationship: All maximal p-subgroups are conjugate. 3. Number: There are strong restrictions on the number of p-subgroups a group can have. Together, these place strong restrictions on the structure of a group G with a fixed order. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 3 / 17

4 Our unknown group of order 200 Throughout our two lectures on the Sylow theorems, we will have a running example, a mystery group M of order 200. M =200 e Using only the fact that M = 200, we will unconver as much about the structure of M as we can. We actually already know a little bit. Recall Cauchy s theorem: Cauchy s theorem If p is a prime number dividing G, then G has an element g of order p. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 4 / 17

5 Our mystery group of order 200 Since our mystery group M has order M = = 200, Cauchy s theorem tells us that: M has an element a of order 2; M has an element b of order 5; Also, by Lagrange s theorem, a b = {e}. M =200 a = 2 a b e b2 b = 5 b 4 b 3 M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 5 / 17

6 The 1 st Sylow Theorem: Existence of p-subgroups First Sylow Theorem G has a subgroup of order p k, for each p k dividing G. Also, every p-subgroup with fewer than p n elements sits inside one of the larger p-subgroups. The First Sylow Theorem is in a sense, a generalization of Cauchy s theorem. Here is a comparison: Cauchy s Theorem First Sylow Theorem If p divides G, then... If p k divides G, then... There is a subgroup of order p There is a subgroup of order p k which is cyclic and has no non-trivial proper subgroups. which has subgroups of order 1, p, p 2... p k. G contains an element of order p G might not contain an element of order p k. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 6 / 17

7 The 1 st Sylow Theorem: Existence of p-subgroups Proof The trivial subgroup {e} has order p 0 = 1. Big idea: Suppose we re given a subgroup H < G of order p i < p n. We will construct a subgroup H of order p i+1. By the normalizer lemma, H N G (H), and the order of the quotient group N G (H)/H is a multiple of p. By Cauchy s Theorem, N G (H)/H contains an element (a coset!) of order p. Call this element ah. Note that ah is cyclic of order p. Claim: The preimage of ah under the quotient q : N G (H) N G (H)/H is the subgroup H we seek. The preimages q 1 (H), q 1 (ah), q 1 (a 2 H),..., q 1 (a p 1 H) are all distinct cosets of H in N G (H), each of size p i. Thus, the preimage H = q 1 ( ah ) contains p H = p i+1 elements. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 7 / 17

8 The 1 st Sylow Theorem: Existence of p-subgroups Here is a picture of how we found the group H = q 1 ( ah ). q N G (H) H g 1H H a 3 H g 2H g 3H a 2 H ah g 4H q 1 g 1 a3 H g 2 H g 3 ah a 2 H g 4 N G (H) H ah Since H = p i, the subgroup H = p 1 k=0 a k H contains p H = p i+1 elements. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 8 / 17

9 Our unknown group of order 200 We now know a little bit more about the structure of our mystery group of order M = : M has a 2-subgroup P 2 of order 2 3 = 8; M has a 5-subgroup P 5 of order 25 = 5 2 ; Each of these subgroups contains a nested chain of p-subgroups, down to the trivial group, {e}. M = a = 2 a b e b2 b = 5 25 b 4 b 3 M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 9 / 17

10 The 2 nd Sylow Theorem: Relationship among p-subgroups Definition A subgroup H < G of order p n, where G = p n m with p m is called a Sylow p-subgroup of G. Let Syl p (G) denote the set of Sylow p-subgroups of G. Second Sylow Theorem Any two Sylow p-subgroups are conjugate (and hence isomorphic). Proof Let H < G be any Sylow p-subgroup of G, and let S = G/H = {Hg g G}, the set of right cosets of H. Pick any other Sylow p-subgroup K of G. (If there is none, the result is trivial.) The group K acts on S by right-multiplication, via φ: K Perm(S), where φ(k) = the permutation sending each Hg to Hgk. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 10 / 17

11 The 2 nd Sylow Theorem: All Sylow p-subgroups are conjugate Proof A fixed point of φ is a coset Hg S such that Hgk = Hg, k K Hgkg 1 = H, k K gkg 1 H, k K gkg 1 H gkg 1 = H. Thus, if φ has a fixed point Hg, then H and K are conjugate by g, and we re done! All we need to do is show that Fix(φ) p 0. By the p-group Lemma, Fix(φ) p S. Recall that S = [G, H]. Since H is a Sylow p-subgroup, H = p n. By Lagrange s Theorem, S = [G : H] = G H = pn m p n = m, p m. Therefore, Fix(φ) p m p 0. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 11 / 17

12 Our unknown group of order 200 We now know even more about the structure of our mystery group M, of order M = : If M has any other Sylow 2-subgroup, it is isomorphic to P 2; If M has any other Sylow 5-subgroup, it is isomorphic to P 5. M = If any other Sylow 2-subgroup exists, it is isomorphic to the first a = 2 a b e b 4 b2 b 3 b = 5 25 M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 12 / 17

13 The 3 rd Sylow Theorem: Number of p-subgroups Third Sylow Theorem Let n p be the number of Sylow p-subgroups of G. Then n p divides G and n p p 1. (Note that together, these imply that n p m, where G = p n m.) Proof The group G acts on S = Syl p (G) by conjugation, via φ: G Perm(S), where φ(g) = the permutation sending each H to g 1 Hg. By the Second Sylow Theorem, all Sylow p-subgroups are conjugate! Thus there is only one orbit, Orb(H), of size n p = S. By the Orbit-Stabilizer Theorem, Orb(H) }{{} Stab(H) = G = n p divides G. =n p M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 13 / 17

14 The 3 rd Sylow Theorem: Number of p-subgroups Proof (cont.) Now, pick any H Syl p (G) = S. The group H acts on S by conjugation, via θ : H Perm(S), where θ(h) = the permutation sending each K to h 1 Kh. Let K Fix(θ). Then K G is a Sylow p-subgroup satisfying We know that: h 1 Kh = K, h H H N G (K) G. H and K are Sylow p-subgroups of G, but also of N G (K). Thus, H and K are conjugate in N G (K). (2nd Sylow Thm.) K N G (K), thus the only conjugate of K in N G (K) is itself. Thus, K = H. That is, Fix(θ) = {H} contains only 1 element. By the p-group Lemma, n p := S p Fix(θ) = 1. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 14 / 17

15 Summary of the proofs of the Sylow Theorems For the 1st Sylow Theorem, we started with H = {e}, and inductively created larger subgroups of size p, p 2,..., p n. For the 2 nd and 3 rd Sylow Theorems, we used a clever group action and then applied one or both of the following: (i) Orbit-Stabilizer Theorem. If G acts on S, then Orb(s) Stab(s) = G. (ii) p-group Lemma. If a p-group acts on S, then S p Fix(φ). To summarize, we used: S2 The action of K Syl p (G) on S = G/H by right multiplication for some other H Syl p (G). S3a The action of G on S = Syl p (G), by conjugation. S3b The action of H Syl p (G) on S = Syl p (G), by conjugation. M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 15 / 17

16 Our unknown group of order 200 We now know a little bit more about the structure of our mystery group M, of order M = = 200: n 5 8, thus n 5 {1, 2, 4, 8}. But n 5 5 1, so n 5 = 1. n 2 25 and is odd. Thus n 2 {1, 5, 25}. We conclude that M has a unique (and hence normal) Sylow 5-subgroup P 5 (of order 5 2 = 25), and either 1, 5, or 25 Sylow 2-subgroups (of order 2 3 = 8). M =200 The only Sylow 5-subgroup is normal 8 4 a = 2 a b e b2 b = 5 25 There may be other Sylow 2-subgroups b 4 b 3 M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 16 / 17

17 Our unknown group of order 200 M =200 The only Sylow 5-subgroup is normal 8 4 a = 2 a b e b2 b = 5 25 There may be other Sylow 2-subgroups b 4 b 3 Suppose M has a subgroup isomorphic to D 4. This would be a Sylow 2-subgroup. Since all of them are conjugate, M cannot contain a subgroup isomorphic to Q 4, C 4 C 2, or C 8! In particular, M cannot even contain an element of order 8. (Why) M. Macauley (Clemson) Lecture 5.6: The Sylow theorems Math 4120, Modern Algebra 17 / 17

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