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Second and third isomorphism theorem

  1. Mar 2, 2010 #1
    I've recently encountered some forms of the second and third isomorphism theorem, but I don't quite get them. Could anyone explain in a bit of details please? I guess my thought was not in the right direction or something.
    (Second isomorphism theorem) Let A be a subring and I an ideal of the ring R.
    Show that there is an isomorphism of rings A/(A ∩ I) ∼= (A + I)/I
    where A + I = {a + i : a ∈ A, i ∈ I}.
    (Third isomorphism theorem) Let A be an ideal of the ring R. Show that if B is an
    ideal of R that contains A then B/A is an ideal of R/A. Moreover the map B → B/A
    is a bijection from the set of ideals of R containing A to the set of ideals of R/A. Show
    that if B is an ideal of R that contains A then there is an isomorphism of rings
    (R/A)/(B/A) ∼= R/B
    Any help is greatly appreciated here!
     
  2. jcsd
  3. Mar 3, 2010 #2
    Where does the theorem stop making sense? Also what book are you using and have you encountered these theorems in group theory?
     
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