Quote (feanur @ Jan 29 2016 10:53pm)
Let (R,+,x) a ring.
Let I a non-empty set (countable or not) and let Ji ( i ∈ I ) a collection of ideals of R.
I assume here we're talking about bilateral ideals (but this doesn't change anything in the end).
Let J = ∩ Ji, for i ∈ I.
We want to prove that J is an ideal of R, ie :
- J is an additive subgroub of R,
- for every j in J, for every r in R, jr and rj are in J.
For every i in I, Ji is an additive subgroub of R, hence 0 lies in Ji for every i in I : 0 lies in J.
Let x, y in J. Hence x and y belong to every Ji, thus x-y is in every Ji, since they all are subgroubs of (R,+).
This shows that J is an additive subgroub of R.
Now let j in J, and r in R.
Since j is in J, it is in every Ji. Since every Ji is an ideal, jr and rj are in Ji, for every i in I : jr and rj are in J.
So, J is an ideal of R.
this is similar to wat i was thinking of doing but wasnt sure if it deals with uncountable collection of ideals