Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quotient Groups and their Index

  1. May 28, 2010 #1
    As a way to keep busy in between semesters I decided to work my way through Algebra by Dummit and Foote in order to prepare for the fall. Working my way through quotient groups is proving to be quite difficult and as a result I'm stuck on an exercise that looks simple, but I just don't know where to start. Any ideas how to prove that given H[tex]\leq[/tex]K[tex]\leq[/tex]G |G:H|=|G:K||K:H|?
  2. jcsd
  3. May 28, 2010 #2
    Have they not gone through Lagrange's theorem yet?
  4. May 30, 2010 #3
    Who said any of the groups were finite?
  5. May 31, 2010 #4
    good point.
  6. Jun 1, 2010 #5


    User Avatar

    Lattice isomorphism theorem. Assuming those are normal subgroups, (G/K)/(K/H) is isomorphic to G/H.

    Of course, it's not hard to just count the number of cosets. G = g_1K + ... + g_nK where n = [G:K]. K = k_1H + ... + k_mH, where m = [H:K]. Then G = g_1(k_1 + ... + k_m)H + ... + g_n(k_1+...+k_m)H, where I have abused notation a little. This shows [G:H]<=[G:K][K:H]. Now think about the other direction.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook