# Math Help - counter example

1. ## counter example

i just need a counter example, this has been bothering me all day...!

If R and S are isomorphic commutative rings, then any ring homomorphism $f: R \to S$ is an isomorphism.

I wanted to say....

if $m \geq 2$, then the map $f:\mathbb{Z} \to \mathbb{Z}_m,$ given by $f(n) = [n]$, is not injective, but it is surjective....... but my two rings don't seem to be isomophic..

2. Originally Posted by ux0
i just need a counter example, this has been bothering me all day...!

If R and S are isomorphic commutative rings, then any ring homomorphism $f: R \to S$ is an isomorphism.
for any ring $R$ define the map $f: R[x] \longrightarrow R[x]$ by $f(a_0 + a_1 x + \cdots + a_n x^n)=a_0.$

3. The trivial map, sending all things to the (respective) identity, is an not an isomorphism between a ring and itself.