[Math] A confusion about Axiom of Choice and existence of maximal ideals.

axiom-of-choiceaxiomsring-theory

The proof of the theorem of statement that every ring has a maximal ideal uses zorn's lemma or the axiom of choice.Now, the defintion of ring as well as the definition of maximal ideal don't depend on the axiom of choice.Is that true? So, given a ring I should be able to check whether it has maximal ideal or not without his axiom of choice. Am I right? So, how can the truth of this statement depend on Axiom of Choice? What is wrong with my assumptions?

To make my question more concrete, assuming that axiom of choice is false, can any one give me a ring which has no maximal ideal? How can a maximal ideal (assuming AoC is correct) turn into a non-maximal ideal just by assuming that AoC is false ?

PS. By rings I mean rings with identity.

Best Answer

The axiom of choice is in fact equivalent to the assertion that every commutative unital ring has a maximal ideal.

Since the negation of the axiom of choice is as non-constructive as the axiom of choice itself, we can only say that there exists a commutative unital ring without a maximal ideal when the axiom of choice fails. The sets which are involved in this process are non well-orderable.

It is important to understand that much like the axiom of choice only assures that certain objects exist; its negation has a very similar intangible action: it only assures that some sets cannot be well-ordered, some partial orders in which every chain is bounded will not have maximal elements, and some families of non-empty sets do not have a choice function. We have no means of knowing where these objects are without further assumptions.

It is consistent that the axiom of choice holds for every set you have ever dreamed using, but then fails acutely. In that universe you cannot imagine to actually find that set which cannot be well-ordered, or that ring without a maximal ideal, and so on. But it is also consistent that the axiom of choice fails "nearby" and the counterexamples appear in relatively familiar sets (objects related to, or defined from the real numbers for example).

The best way to actually "find" a ring without a maximal ideal is to follow the proof of how the existence of maximal ideals imply the axiom of choice. These proofs often consists of taking a family of non-empty sets, defining some ring (or whatever) and using the maximal ideal to prove the existence of a choice function. Therefore starting with a family of non-empty sets which does not have a choice function guarantees that the process fails and that the ring defined in such proof will not have any maximal ideals.

Added:

Some point that came up in my comment exchange with Trevor Wilson under his answer, is that the axioms are syntactical. They allow us to write proofs. It seems to me, upon re-reading this question that you think something like:

Take a universe of ZFC, $R$ is a unital ring and $I$ is a maximal ideal. Now just don't assume that AC holds. Now we can't prove that $I$ is a maximal ideal.

That's false for two main reasons:

  1. Once you fixed a universe of sets, the axiom of choice is either true or false in that universe. Even if you don't assume it, it has a truth value, and since we began from a universe of ZFC this truth value is indeed true. We just might not be able to write a proof from ZF that $I$ is maximal, but this is something that is still true in that universe of sets.

    So changing the assumptions does not necessarily mean that you have changed your universe of sets.

  2. If you have a definition for a ring (e.g. $\Bbb{R^R}$ with pointwise addition and multiplication) then the actual underlying set, and more importantly its subsets, may change between one universe of set theory and another. So for example we cannot prove from ZF that $\Bbb{R^R}$ has a maximal ideal because there are universes of set theory where this is false. But the underlying set $\Bbb{R^R}$ is very different between those universes, and again more importantly, its power set is different.

So to sum up all my answer (with its two additions), simply removing the assumption that the axiom of choice holds will not falsify the axiom of choice. It might be that you just won't be able to write a proof that there exists a maximal ideal in every unital ring.

If, however, you allow yourself to change the universe of set theory then it is possible that a certain ring will "lose" its maximal ideals, simply because we removed those sets from the universe (and in some cases did a whole revamp of the universe altogether).

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