I'm sorry I broke this entry, pahio, I wanted to add illustrations for the graphics, but it's behaving weirdly page iamge vs html mode
(image misplaced, colors wrong, no equations being displayed)
y the way, a possible cause for all these pstricks and graphics problems might be that tetex version installed on planetmath is waytoo old (and thus breaks things dealing with new xkeyval package), but that's just a conjecture
f
G -----> H G
p \ /_ ----- ~ f(G)
\ / f ker f
G/ker f |
|
![[parent]](http://images.planetmath.org:8080/images/uparrow.png)