Sylow subgroups exist

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Statement

Let be a Finite group (?) and be a prime number. Then, there exists a -Sylow subgroup (?) of : a subgroup whose order is a power of and whose index is relatively prime to .

Note that when does not divide the order of , the -Sylow subgroup is trivial, so the statement gives interesting information only when divides the order of . This statement is often viewed as a part of a more general statement called Sylow's theorem.

Related facts

Other parts of Sylow's theorem

Stronger forms of existence

Analogues for Hall subgroups

The analogous statement does not hold for Hall subgroups. A Hall subgroup is a subgroup whose order and index are relatively prime. A -Hall subgroup is a Hall subgroup such that the set of primes dividing its order is contained in , and the set of primes dividing its index is disjoint from .

Analogues in other algebraic structures

Facts used

  1. Lucas' theorem (more specifically, Lucas' theorem prime power case)
  2. Fundamental theorem of group actions: There is a bijection between the coset space of the stabilizer of an element and the orbit of that element. In particular, the size of the orbit equals the index of the stabilizer.
  3. Lagrange's theorem
  4. Class equation of a group
  5. Cauchy's theorem for abelian groups
  6. Central implies normal: Any subgroup inside the center is normal.

Proof

This proof uses a tabular format for presentation. Provide feedback on tabular proof formats in a survey (opens in new window/tab) | Learn more about tabular proof formats|View all pages on facts with proofs in tabular format

Proof by action on subsets

Given: A finite group of order where is prime, is a nonnegative integer, and does not divide .

To prove: has a subgroup of order .

Proof: Here are some observations regarding this action:

Step no. Construction/assertion Facts used Previous steps used Explanation
1 Let be the set of all subsets of of size .
2 The size of is relatively prime to Fact (1) The size of is the binomial coefficient , and an appeal to Lucas's theorem (fact (1)) reveals that its value is relatively prime to .
3 acts on by left multiplication. First, note that a group acts on the set of all its subsets by left multiplication. Further, since the left multiplication maps are bijective, they preserve the sizes of subsets. In particular, any subset of size gets mapped to a subset of size . Thus, we can restrict the action to the set of subsets of size .
4 The action of on has at least one orbit, say , whose size is relatively prime to Step (2) The size of equals the sum of sizes of the orbits under the -action. Since the total is relatively prime to , at least one of the numbers has to be relatively prime to .
5 Let be a member of such an orbit and be the stabilizer of in .
6 The index of in equals the size of Fact (2) Steps (4), (5) Follows directly from fact (2).
7 The index of in is relatively prime to . Steps (4), (6) By step (4), the size of is relatively prime to . By step (6), this equals the index of . Thus, the index of in is relatively prime to .
8 The index of in divides . Fact (3) Step (6) By fact (3) (Lagrange's theorem), the index of in divides the order of , which is . By step (6), the index of in is relatively prime to . Hence, it must divide .
9 The index of in is at least . Steps (1), (4), (5) The union of , is the whole group . Since each has size , there are at least translates of in . Thus, has size at least .
10 The index of in is exactly , so is a -Sylow subgroup of . Steps (8), (9) The index of in divides and is at least . The only way both of these could be true is if it equals .

Interestingly, the only nontrivial result we use here (Lucas' theorem) can itself be proved using group theory by doing the above proof in reverse taking a cyclic group (although a purely algebraic proof also exists). ,

Proof by conjugation action

Given: A finite group of order , where is prime, is a nonnegative integer, and does not divide .

To prove: has a subgroup of order .

Proof: We prove the claim by induction on the order of . Specifically, we assume that the result is true for all groups of order smaller than the order of .

For the case , the proof is direct since the trivial group is a subgroup of order . Thus, we assume . In particular, divides the order of .

Consider the class equation of (fact (4)):

where are the conjugacy classes of non-central elements and is an element of for each .

We consider two cases:

Case that divides the order of  :

Step no. Construction/assertion Facts used Previous steps used Explanation
1 There exists a normal subgroup of order in Facts (5), (6) Since is Abelian, fact (5) yields that it has a subgroup of order . Since is in the center, is normal in (by fact (6)). Thus, is a normal subgroup of of order .
2 has a -Sylow subgroup, in particular, a subgroup of order Step (1) Since has order , has order . This is strictly smaller than the order of , so induction applies and we get a -Sylow subgroup, whose order is .
3 Let be the quotient map. Then is a -Sylow subgroup of Steps (1), (2) Indeed, the order of is the product of the order of and the order of , which is therefore .

Case that does not divide the order of

Step no. Construction/assertion Facts used Previous steps used Explanation
1 There exists such that does not divide the index of in Since divides the order of , cannot divide the index of every , otherwise the class equation would yield that divides the order of .
2 is a proper subgroup of whose order is a multiple of , so is the largest power of dividing its order. Fact (3) Step (1) Since is non-central, is proper in . Further, since is relatively prime to , Lagrange's theorem (fact (3)) yields that the order of is , which is a multiple of .
3 contains a subgroup of order Step (2) This follows by the induction hypothesis.
4 contains a subgroup of order , and hence a -Sylow subgroup. Step (3) A subgroup of order in is also a subgroup of order in .