If H and K are subgroups of ' order p, show that H=K\ \text{or}\ H \cap K=1 My thought wou

Annette Sabin

Annette Sabin

Answered question

2022-01-14

If H and K are subgroups of ' order p, show that H=K or HK=1
My thought would be the Lagrange Theorem. If H is a subgroup of order p of group G, then |H| divides |G|. If |G| isdivided by two subgroups with the same order, then the result is the same. The condition where the subgroups are are thesame makes sense (mostly), but the other condition makesno sense.
I honestly have no idea how one would prove this.
Edit: Prime group is cyclic. Cyclic group generated by a single element. If the groups aren't the same, (H=K), then the only element they have in common is the identity element (in this case 1)?

Answer & Explanation

lalilulelo2k3eq

lalilulelo2k3eq

Beginner2022-01-15Added 38 answers

Use the primality of p and Lagrange's theorem to conclude that for all hH such that h1 we have
H={h0,h1,h2,,hp1}.
In an entirely analogous way for all kK such that k1 we have
K={k0,k1,k2,,kp1}.
If uHK and u1 then
K={u0,u1,u2,,up1} and H={u0,u1,u2,,up1}
Terry Ray

Terry Ray

Beginner2022-01-16Added 50 answers

Suppose HK. We have that HK is a subgroup of both H and K. By Lagrange, either |HK|=p or |HK|=1, but if the former, then we must have H=HK=K, a contradiction;
Answer: Hence |HK|=1, which can only happen if HK=1.

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