# Ring help

• Mar 4th 2007, 12:02 PM
Ring help
Two questions:
1. If R is a ring then is every element of R either a unit or zero-divisor.

COnversely

2. IF R is a ring, then there is no elements of R which is both a unit and a zero-divisor.
• Mar 4th 2007, 04:44 PM
ThePerfectHacker
Quote:

Two questions:
1. If R is a ring then is every element of R either a unit or zero-divisor.

Is R a finite ring?

Because, if it is then we can prove it as follows:

If x is a zero-divisor the proof is complete.

If x is not a zero-divisor then consider the finite set,
{a_1x,a_2x,...,a_nx} where a_i are the non-zero elements of the ring (assuming it is non-trivial).
Then, we can show none are equal to each other.
(Excercise).
Then by Dirichlet's Pigeonhole Principle, is an enumeration of all non-zero elements in R. Hence we can find an a_i such that a_i x = 1. Thus, x is a unit.
• Mar 4th 2007, 07:32 PM
ThePerfectHacker
Quote:

Originally Posted by TheNextDirichlet
Is R a finite ring?

It has to be.
Because, for example, consider, Z.
The integers under addition and multiplication.

The element 2 in Z, it neither a unit nor zero divisor :eek:
(However, for all finite rings this is true).
• Mar 5th 2007, 06:18 AM