# Cardinality of the set of maximal ideals in a Boolean ring/algebra

If B is a Boolean ring is of uncountable cardinality c, does B have 2^c distinct maximal ideals ? Can you please give me a reference where this question is answered (hopefully) positively ? Thanks

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This particular question is easy to answer. Many related questions about the relationships between various cardinals associated with Boolean algebra (or Boolean rings), such as: cardinality, number of ultrafilters, density, cellularity, distributivity etc, can be found in

• Monk, J. Donald: Cardinal invariants on Boolean algebras. Progress in Mathematics, 142. Birkhäuser Verlag, Basel, 1996. 3-7643-5402-X. MR1393943
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Thanks for the reference –  Rangaswamy Dec 24 '12 at 20:55

This is false. We might as well count the number of ultrafilters, since an ultrafilter is the complement of a maximal ideal.

Let $X$ be a set of cardinality $c$. If $B$ is the set of subsets $S \subseteq X$ such that $S$ or its complement is finite, then $B$ is a Boolean algebra under the usual set-theoretic operations. If an ultrafilter $U \subset B$ contains a finite set $S$, then it is easy to see it must be principal. If it contains no finite set $S$, then it must be the ultrafilter of cofinite sets. So here the cardinality of the set of ultrafilters or of maximal ideals is $c$ again.

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Thanks for your answer and the example. I understand that the statement is true for atomless countable Boolean rings. –  Rangaswamy Dec 24 '12 at 20:55
Yes, there is only one such Boolean algebra up to isomorphism, and it is the free Boolean algebra on countably many generators. It is immediate that this has $c$ many ultrafilters in it. –  Todd Trimble Jan 1 '13 at 21:19

I think this is false—is it supposed to be the other way around?

The Stone representation theorem embeds $B$ in $\prod_{\mathfrak{p} \in \operatorname{Spec} B} \mathbb{F}_2$, where $\operatorname{Spec} B$ is the set of prime (or maximal) ideals of $B$. In particular, $c = \lvert B \rvert \le 2^{\lvert \operatorname{Spec} B\rvert}$.

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For infinite Boolean algebras you have $|B|\leq|Spec(B)|$, so the other inequality is not sharp except for finite Boolean algebras. –  Joseph Van Name Dec 22 '12 at 21:06