# Isomorph homework question

1. Mar 8, 2008

### proxyIP

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

http://i25.tinypic.com/j8i4278.jpg

2. Relevant equations&3. The attempt at a solution

isomoprism is bijective

i have no clue whatsoever..im gonna research a little bit now
ill appreciate any help

Last edited: Mar 8, 2008
2. Mar 8, 2008

### HallsofIvy

Staff Emeritus
You are given that F is an isomorphism from X to Y and G is an isomorphism from Y->Z. You are asked to show that G$\circ$F is an isomorphism from X to Z. I can guess that X, Y, and Z are vector spaces since another part of the problem talks about a "spanning set" but you should have said that! Proving an isomorphism of groups, rings, fields, etc. is quite different. An isomorphis is, as you say, bijective: you must prove this is surjective: that if z is any member of Z, then there exist x in X such that G$\circ$F(x)= z. Since G is an isomorphism from Y to Z it is surjective: what does that tell you? Once you have that, you know that F is surjective from X to Y. Use that.

An isomorphism must be injective also. If x1 and x2 are such that G$\circ$F(x1)= G$\circ$(x2) then you must prove that x1= x2. Use can use the fact that F and G are each injective to prove that. Of course, a isomorphism must also "preserve" the operations. You need to show that G$\circ$F(au+ bv)= aG$\circ$F(u)+ bG$\circ$F(v).

As for b) using the definition of "span" together with that last statement: G$\circ$F(au+ bv)= aG$\circ$F(u)+ bG$\circ$F(v) should be enough.