# Prime ideal $\implies$ maximal in a Boolean ring

I want to show that a prime ideal in a non-unital Boolean ring $$B$$ is maximal ideal.

If the ring contains unity then it is easy. As Boolean rings are commutative, for a prime ideal $$P$$ the ring $$B/P$$ is both integral domain aswell as Boolean ring. The only non trivial integral domain and Boolean ring is $$\mathbb{Z}_2$$ which is a field, so $$P$$ is maximal in $$B$$.

But there are Boolean rings without unity. For example consider all the finite subsets of real numbers with symmetric difference and intersection; this is a Boolean ring without unity. (This is also the subring of $$\mathcal{P}(\mathbb{R})$$, the power set of real numbers under the same operations).

Generally for rings with unity, there is a technique to show that an ideal $$I$$ is maximal ideal. We consider an ideal which strictly contains $$I$$ and somehow show that it contains unity, so that it is an improper ideal. In the above case this way doesn't work. I'm looking for a technique which can be applied in a wider settings for similar problems such as in non Commutative rings etc..

• @NoahSchweber What exactly is Unital ring? I am not familiar with this terminology. Apr 20 at 18:35
• Unital rings are required to have multiplicative identities (= units) and homomorphisms of unital rings are required to preserve these identities; non-unital rings don't have these features. The term "ring" used without qualification is a bit ambiguous; my understanding is that it usually but not always means "unital ring" these days (with the term "rng" being sometimes used for non-unital rings, see the second paragraph of the wiki page). Apr 20 at 18:36

Note that $$P$$ being prime is the same as $$B/P$$ being a ring without nontrivial zero divisors, so multiplication by a fixed nonzero element is injective.

We will show that a nonzero boolean ring $$B$$ without nontrivial zero divisors has a unit, so it is in fact isomorphic to $$\mathbb Z/2\mathbb Z$$:

Let $$j\in B$$ be some nonzero element, then $$j$$ is the multiplicative identity:
For all $$x\in B$$, we have $$xj=xjj$$ therefore $$x=xj$$ (since multiplication by $$j$$ is injective), similarly we have $$jjx=jx$$ so $$jx=x$$. (This also shows that there is only one nonzero element, since the multiplicative identity is unique)

The point is that in a ring with no nontrivial zero divisors, the only idempotent elements are $$0$$ and the multiplicative identity.

• How no non trivial zero divisors imply that multiplication is injective? For example take integers which have no non trivial zero divisor a but multiplication is not injective : $4\times 3 = 6 \times 2$ Apr 21 at 11:59
• @quantum_spin I meant that the maps $\lambda_a,\rho_a :R \to R$ defined by $\lambda_a(x)=ax$, $\rho_a(x)=xa$ are injective for all nonzero elements $a\in R$ (this what I meant by "multiplication by nonzero elements"), I guess the correct term would be that multiplication is cancellative. Apr 21 at 12:40
• There is a confusion; When $P$ is prime the ring $B/P$ does not have non trivial zero divisors, so it is a field with two elements. In my opinion that does not necessarily imply that $P$ is maximal as $B$ may not have multiplicative identity. Apr 21 at 17:31
• It implies that $P$ is maximal due to the correspondence theorem, namely ideals of $B$ containing $P$ correspond to ideals of $B/P$, but if $B/P$ is a field it is simple, so there are no ideals of $B$ containing $P$ (other than $B$ or $P$), hence $P$ is maximal Apr 21 at 17:46
• Thank you. Now I am getting clarity. Apr 21 at 18:24