# Normal subgroup of a product of simple groups

1. Jun 11, 2009

### john_nj

1. The problem statement, all variables and given/known data

This is an exercise from Jacobson Algebra I, which has me stumped.
Let G = G1 x G2 be a group, where G1 and G2 are simple groups.
Prove that every proper normal subgroup K of G is isomorphic to G1 or G2.

2. Relevant equations

3. The attempt at a solution
Certainly the intersection of K with G1 x {1} is normal, and so is isomorphic to the trivial group {1} or to G1. Similarly, the intersection of K with {1} x G2 is isomorphic to {1} or G2. But anyway this falls well short of a solution.

Thanks,

John
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 12, 2009

### Office_Shredder

Staff Emeritus
Well, you've basically got it. Try each of the three cases.... if both intersections are trivial, then what is K? If both intersections are the full group, then what is K? And if only one intersection is trivial, what is K?

3. Jun 12, 2009

### john_nj

Thank you. It's now clear.