# Idempotents in a ring without unity (rng) and no zero divisors.

Question: Given a ring without unity and with no zero-divisors, is it possible that there are idempotents other than zero?

Def: $a$ is idempotent if $a^2 = a$.

Originally the problem was to show that $1$ and $0$ are the only idempotents in a ring with unity and no zero-divisors, but I wonder what happens if we remove the unity condition.

I am trying to find a ring with idempotents not equal to $0$ or $1$. So far my biggest struggle has been coming up with examples of rings with the given properties.

Does anyone have any hints? How should I attack this problem?

• Every rng (a ring without unity) can be embedded in a ring, but I don't know if this can be done while preserving the no zero divisors property. – Jim Feb 4 '14 at 16:59
• @Jim It is actually mentioned here that one might have trouble preserving the no zero divisor property when embedding a rng into a ring. – Improve Feb 4 '14 at 20:17

Proposition: If a rng $R$ which does not have nonzero zero divisors, a nonzero idempotent of $R$ must be an identity for the ring.
Proof: Let $e$ be a nonzero idempotent. Since $e(er-r)=0=(re-r)e$ for all $r\in R$ and $e$ is nonzero, we conclude $er-r=0=re-r$, and so $e$ is an identity element.