# Thread: Prove that two rings are isomorphic

1. ## Prove that two rings are isomorphic

Let R be a ring with a unit element. Using its elements, we define a ring T by defining operations ! and @ on T such that a ! b = a+b+1 and a@b = ab+a+b where the addition and multiplication on right hand side of these relations are those of R. (Note that the ring T forms a group with the operation !). Prove that the rings R and T are isomorphic.

2. I am thinking that a good place to start is to figure out what are the neutral elements with respect to the operations ! and @ in T: For what element x in the ring R will x ! a = a for all a? For what element y in R will y@a = a for all a in R? (Notice that both ! and @ are commutative, so it suffices to check that x and y work from the left).

Then you could find inverses with respect to !, i.e. for a given a, what element b satisfied that a ! b = x, where x is the neutral element from before?

3. Hi,

Thank you for your post. I have found out the identity elements for ! and @ are -1 and 0 respectively. But I do not know how finding the identity elements or inverses would help me establish an isomorphism from R to T. The method I am trying to use is to establish the result is to establish a homomorphism from R onto T and then prove the kernel of this homomorphism is -1 (the identity element of !). Please correct my approach if it is wrong.

4. I agree with the identity elements you found. Though you probably meant the kernel of the homomorphism to be {0} (remember the kernel is a subset of R), and not {-1}. Let me tell you my approach.

So given those identity elements, you know that 0 in R must map to -1 in T, and that 1 in R must map to 0 in T. In fact, this suggests that we try mapping any x in R to x-1 in T.

Does this give a ring homomorphism? Call the map f. Then

f(x+y) = x+y-1,

while

f(x) ! f(y) = (x-1)!(y-1) = x-1+y-1+1 = x+y-1,

the same thing as above.

What about f(x*y) = xy-1. Is this equal to f(x)&f(y)?

Finally you just need to check that f is a bijection.

5. Hi! Thanks for your idea. I get how to do the sum now