# 100 Years of Zermelos Axiom of Choice What was the Problem with It

The initial arrangement of points affects the nature of the perspective view. Having the points at infinity very far away will produce something closer to orthoprojection; having them close produces a more extreme perspective, which simulates a view from a vantage point extremely close to the board. Please give the construction a try! All you need is paper, pencil and a straightedge! Provide links below in the comments to photos of your creations.

Meanwhile, let me point you towards my follow post, How to draw infinite chessboards by hand in perfect perspective, using only a straightedge. The difference between the methods is that the method of this post is about subdividing a given board, and the other method is about generating arbitrarily large chessboards from a given unit square. In recent work, Alfredo Roque Freire and I have realized that the axiom of well-ordered replacement is equivalent to the full replacement axiom, over the Zermelo set theory with foundation.

In other words, the image of a well-ordered set under a first-order definable class function is a set. At first, I had found the well-ordered replacement theory a bit awkward, because one can only apply the replacement axiom with well-orderable sets, and without the axiom of choice, it seemed that there were not enough of these to make ordinary set-theoretic arguments possible.

### No proof of everything

The axiom of well-ordered replacement is equivalent to full replacement over Zermelo set theory with foundation. Well-ordered replacement is sufficient to prove that transfinite recursion along any well-order works as expected. One proves that every initial segment of the order admits a unique partial solution of the recursion up to that length, using well-ordered replacement to put them together at limits and overall. This is not immediate and requires careful proof. Perhaps they are unbounded in the ordinals? No, they are not, by the following argument.

We can now establish the collection axiom, using a similar idea. The new realization here is that we do not need the axiom of choice in that argument, since transfinite recursion implies well-ordered replacement, which gives us full replacement by the argument above.

The principle of transfinite recursion is equivalent to the replacement axiom over Zermelo set theory with foundation.

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## 100 Years of Zermelo’s Axiom of Choice: What was the Problem with It?

This will be a series of lectures on the philosophy of mathematics, given at Oxford University, Michaelmas term The lectures are mainly intended for undergraduate students preparing for exam paper , although all interested parties are welcome. My approach to the philosophy of mathematics tends to be grounded in mathematical arguments and ideas, treating philosophical issues as they arise organically.

The lectures will accordingly be organized around mathematical themes, in such a way that naturally brings various philosophical issues to light. Lecture 1. Numbers are perhaps the essential mathematical idea, but what are numbers? Lecture 2. Let us consider the problem of mathematical rigour in the development of the calculus. Informal continuity concepts and the use of infinitesimals ultimately gave way to formal epsilon-delta limit concepts, which provided a capacity for refined notions, such as uniform continuity, equicontinuity and uniform convergence. Nonstandard analysis resurrected the infinitesimal concept on a more secure foundation, providing a parallel development of the subject, which can be understood from various sweeping perspectives.

Lecture 3. Time permitting, we shall count into the transfinite ordinals. Lecture 4. Classical Euclidean geometry, accompanied by its ideal of straightedge and compass construction and the Euclidean concept of proof, is an ageless paragon of deductive mathematical reasoning. Yet, the impossibility of certain constructions, such as doubling the cube, trisecting the angle or squaring the circle, hints at geometric realms beyond Euclid, and leads one to the concept of constructible and non-constructible numbers.

The rise of non-Euclidean geometry, especially in light of scientific observations and theories suggesting that physical reality may not be Euclidean, challenges previous accounts of what geometry is about and changes our understanding of the nature of geometric and indeed mathematical ontology. New formalizations, such as those of Hilbert and Tarski, replace the old axiomatizations, augmenting and correcting Euclid with axioms on completeness and betweenness.

Lecture 5. What is proof? What is the relation between proof and truth? Is every mathematical truth, true for a reason? After clarifying the distinction between syntax and semantics, we shall discuss formal proof systems and highlight the importance of soundness, completeness and verifiability in any such system, outlining the central ideas used in proving the completeness theorem. The compactness theorem distills the finiteness of proofs into an independent purely semantic consequence.

Nonclassical logics, such as intuitionistic logic, arise naturally from formal systems by weakenings of the logical rules. Lecture 6. What is computability?

How the Axiom of Choice Gives Sizeless Sets - Infinite Series

Meanwhile, the distinction between computable decidability and computable enumerability, highlighted by the undecidability of the halting problem, shows that not all mathematical problems can be solved by machine, and a vast hierarchy looms in the Turing degrees, an infinitary information theory. Complexity theory refocuses this on the realm of feasible computation, with the still-unsolved P vs.

NP problem standing in the background of nearly every serious issue in theoretical computer science. Lecture 7. We shall describe several proofs of the first incompleteness theorem, via the halting problem, via self-reference, and via definability. Ultimately, one is led to the inherent hierarchy of consistency strength underlying all mathematical theories.

Lecture 8. Set theory. We shall discuss the emergence of set theory as a foundation of mathematics. The development of forcing solved many stubborn questions and illuminated a ubiquitous independence phenomenon, feeding into philosophical issues concerning the criteria by which one should add new axioms to mathematics and the question of pluralism in mathematical foundations. Given a countable model of set theory, we study the structure of its generic multiverse, the collection of its forcing extensions and ground models, ordered by inclusion.

Mostowski showed that any finite poset embeds into the generic multiverse while preserving the nonexistence of upper bounds. We obtain several improvements of his result, using what we call the blockchain construction to build generic objects with varying degrees of mutual genericity.

The method accommodates certain infinite posets, and we can realize these embeddings via a wide variety of forcing notions, while providing control over lower bounds as well. We also give a generalization to class forcing in the context of second-order set theory, and exhibit some further structure in the generic multiverse, such as the existence of exact pairs.

This is joint work with Russell Miller and Kameryn Williams.

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The rearrangement number. Blass, J. Brendle, W. Brian, J. Hamkins, M. Hardy, and P. Conference Program Conference web page Here are the notes I used for my response. In this article and other discussions of the Axiom of Choice the following abbreviations are common:. There are many other equivalent statements of the axiom of choice. These are equivalent in the sense that, in the presence of other basic axioms of set theory, they imply the axiom of choice and are implied by it.

One variation avoids the use of choice functions by, in effect, replacing each choice function with its range. This guarantees for any partition of a set X the existence of a subset C of X containing exactly one element from each part of the partition. Another equivalent axiom only considers collections X that are essentially powersets of other sets:. Authors who use this formulation often speak of the choice function on A , but be advised that this is a slightly different notion of choice function.

Its domain is the powerset of A with the empty set removed , and so makes sense for any set A , whereas with the definition used elsewhere in this article, the domain of a choice function on a collection of sets is that collection, and so only makes sense for sets of sets. With this alternate notion of choice function, the axiom of choice can be compactly stated as.

The statement of the axiom of choice does not specify whether the collection of nonempty sets is finite or infinite, and thus implies that every finite collection of nonempty sets has a choice function.

## Linked bibliography for the SEP article "The Axiom of Choice" by John L. Bell - PhilPapers

However, that particular case is a theorem of the Zermelo—Fraenkel set theory without the axiom of choice ZF ; it is easily proved by mathematical induction. The axiom of choice can be seen as asserting the generalization of this property, already evident for finite collections, to arbitrary collections. Until the late 19th century, the axiom of choice was often used implicitly, although it had not yet been formally stated.

For example, after having established that the set X contains only non-empty sets, a mathematician might have said "let F s be one of the members of s for all s in X " to define a function F. In general, it is impossible to prove that F exists without the axiom of choice, but this seems to have gone unnoticed until Zermelo. Not every situation requires the axiom of choice. For finite sets X , the axiom of choice follows from the other axioms of set theory. In that case it is equivalent to saying that if we have several a finite number of boxes, each containing at least one item, then we can choose exactly one item from each box.

Clearly we can do this: We start at the first box, choose an item; go to the second box, choose an item; and so on. The number of boxes is finite, so eventually our choice procedure comes to an end. The result is an explicit choice function: a function that takes the first box to the first element we chose, the second box to the second element we chose, and so on. A formal proof for all finite sets would use the principle of mathematical induction to prove "for every natural number k , every family of k nonempty sets has a choice function.

The nature of the individual nonempty sets in the collection may make it possible to avoid the axiom of choice even for certain infinite collections. For example, suppose that each member of the collection X is a nonempty subset of the natural numbers. Every such subset has a smallest element, so to specify our choice function we can simply say that it maps each set to the least element of that set. This gives us a definite choice of an element from each set, and makes it unnecessary to apply the axiom of choice.

The difficulty appears when there is no natural choice of elements from each set. If we cannot make explicit choices, how do we know that our set exists? For example, suppose that X is the set of all non-empty subsets of the real numbers. First we might try to proceed as if X were finite. If we try to choose an element from each set, then, because X is infinite, our choice procedure will never come to an end, and consequently, we shall never be able to produce a choice function for all of X.

Next we might try specifying the least element from each set. But some subsets of the real numbers do not have least elements. So this attempt also fails. Additionally, consider for instance the unit circle S , and the action on S by a group G consisting of all rational rotations. Here G is countable while S is uncountable. The set of those translates partitions the circle into a countable collection of disjoint sets, which are all pairwise congruent. Since X is not measurable for any rotation-invariant countably additive finite measure on S , finding an algorithm to select a point in each orbit requires the axiom of choice.

See non-measurable set for more details. The reason that we are able to choose least elements from subsets of the natural numbers is the fact that the natural numbers are well-ordered : every nonempty subset of the natural numbers has a unique least element under the natural ordering. One might say, "Even though the usual ordering of the real numbers does not work, it may be possible to find a different ordering of the real numbers which is a well-ordering.

Then our choice function can choose the least element of every set under our unusual ordering. A proof requiring the axiom of choice may establish the existence of an object without explicitly defining the object in the language of set theory. For example, while the axiom of choice implies that there is a well-ordering of the real numbers, there are models of set theory with the axiom of choice in which no well-ordering of the reals is definable. Similarly, although a subset of the real numbers that is not Lebesgue measurable can be proved to exist using the axiom of choice, it is consistent that no such set is definable.

The axiom of choice proves the existence of these intangibles objects that are proved to exist, but which cannot be explicitly constructed , which may conflict with some philosophical principles. This has been used as an argument against the use of the axiom of choice. Another argument against the axiom of choice is that it implies the existence of objects that may seem counterintuitive. The pieces in this decomposition, constructed using the axiom of choice, are non-measurable sets. Despite these seemingly paradoxical facts, most mathematicians accept the axiom of choice as a valid principle for proving new results in mathematics.

The debate is interesting enough, however, that it is considered of note when a theorem in ZFC ZF plus AC is logically equivalent with just the ZF axioms to the axiom of choice, and mathematicians look for results that require the axiom of choice to be false, though this type of deduction is less common than the type which requires the axiom of choice to be true. It is possible to prove many theorems using neither the axiom of choice nor its negation; such statements will be true in any model of ZF, regardless of the truth or falsity of the axiom of choice in that particular model.

The restriction to ZF renders any claim that relies on either the axiom of choice or its negation unprovable. For example, the Banach—Tarski paradox is neither provable nor disprovable from ZF alone: it is impossible to construct the required decomposition of the unit ball in ZF, but also impossible to prove there is no such decomposition. Similarly, all the statements listed below which require choice or some weaker version thereof for their proof are unprovable in ZF, but since each is provable in ZF plus the axiom of choice, there are models of ZF in which each statement is true.

Statements such as the Banach—Tarski paradox can be rephrased as conditional statements, for example, "If AC holds, then the decomposition in the Banach—Tarski paradox exists. As discussed above, in ZFC, the axiom of choice is able to provide " nonconstructive proofs " in which the existence of an object is proved although no explicit example is constructed.

ZFC, however, is still formalized in classical logic. The axiom of choice has also been thoroughly studied in the context of constructive mathematics, where non-classical logic is employed. The status of the axiom of choice varies between different varieties of constructive mathematics. A choice function exists in constructive mathematics, because a choice is implied by the very meaning of existence. Thus the axiom of choice is not generally available in constructive set theory. The decision must be made on other grounds. One argument given in favor of using the axiom of choice is that it is convenient to use it because it allows one to prove some simplifying propositions that otherwise could not be proved.

Many theorems which are provable using choice are of an elegant general character: every ideal in a ring is contained in a maximal ideal , every vector space has a basis , and every product of compact spaces is compact. Without the axiom of choice, these theorems may not hold for mathematical objects of large cardinality. The proof of the independence result also shows that a wide class of mathematical statements, including all statements that can be phrased in the language of Peano arithmetic , are provable in ZF if and only if they are provable in ZFC.

When one attempts to solve problems in this class, it makes no difference whether ZF or ZFC is employed if the only question is the existence of a proof. The axiom of choice is not the only significant statement which is independent of ZF. The axiom of constructibility and the generalized continuum hypothesis each imply the axiom of choice and so are strictly stronger than it. The axiom of global choice follows from the axiom of limitation of size.

In fact, Zermelo initially introduced the axiom of choice in order to formalize his proof of the well-ordering theorem. There are several results in category theory which invoke the axiom of choice for their proof. These results might be weaker than, equivalent to, or stronger than the axiom of choice, depending on the strength of the technical foundations. For example, if one defines categories in terms of sets, that is, as sets of objects and morphisms usually called a small category , or even locally small categories, whose hom-objects are sets, then there is no category of all sets , and so it is difficult for a category-theoretic formulation to apply to all sets.

There are several weaker statements that are not equivalent to the axiom of choice, but are closely related. One example is the axiom of dependent choice DC. These axioms are sufficient for many proofs in elementary mathematical analysis , and are consistent with some principles, such as the Lebesgue measurability of all sets of reals, that are disprovable from the full axiom of choice. Other choice axioms weaker than axiom of choice include the Boolean prime ideal theorem and the axiom of uniformization.

The former is equivalent in ZF to the existence of an ultrafilter containing each given filter, proved by Tarski in One of the most interesting aspects of the axiom of choice is the large number of places in mathematics that it shows up. There are several historically important set-theoretic statements implied by AC whose equivalence to AC is open. The partition principle, which was formulated before AC itself, was cited by Zermelo as a justification for believing AC.

In Russell declared PP to be equivalent, but whether the Partition Principle implies AC is still the oldest open problem in set theory, and the equivalences of the other statements are similarly hard old open problems. In every known model of ZF where choice fails, these statements fail too, but it is unknown if they can hold without choice.

Now, consider stronger forms of the negation of AC. Strengthened negations may be compatible with weakened forms of AC. Solovay , cannot be proved in ZFC itself, but requires a mild large cardinal assumption the existence of an inaccessible cardinal. The much stronger axiom of determinacy , or AD, implies that every set of reals is Lebesgue measurable, has the property of Baire, and has the perfect set property all three of these results are refuted by AC itself. In the NF axiomatic system, the axiom of choice can be disproved.

There are models of Zermelo-Fraenkel set theory in which the axiom of choice is false. In type theory , a different kind of statement is known as the axiom of choice. Unlike in set theory, the axiom of choice in type theory is typically stated as an axiom scheme , in which R varies over all formulas or over all formulas of a particular logical form.

The axiom of choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma? This is a joke: although the three are all mathematically equivalent, many mathematicians find the axiom of choice to be intuitive, the well-ordering principle to be counterintuitive, and Zorn's lemma to be too complex for any intuition. The Axiom of Choice is necessary to select a set from an infinite number of pairs of socks, but not an infinite number of pairs of shoes.

The observation here is that one can define a function to select from an infinite number of pairs of shoes by stating for example, to choose a left shoe. Without the axiom of choice, one cannot assert that such a function exists for pairs of socks, because left and right socks are presumably indistinguishable. This quote comes from the famous April Fools' Day article in the computer recreations column of the Scientific American , April Among boots we can distinguish right and left, and therefore we can make a selection of one out of each pair, namely, we can choose all the right boots or all the left boots; but with socks no such principle of selection suggests itself, and we cannot be sure, unless we assume the multiplicative axiom, that there is any class consisting of one sock out of each pair.

Russell generally used the term "multiplicative axiom" for the axiom of choice.

• Linked bibliography for the SEP article "The Axiom of Choice" by John L. Bell.
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Referring to the ordering of a countably infinite set of pairs of objects, he wrote:. There is no difficulty in doing this with the boots. Within each pair, take the left boot first and the right second, keeping the order of the pair unchanged; in this way we obtain a progression of all the boots. But with the socks we shall have to choose arbitrarily, with each pair, which to put first; and an infinite number of arbitrary choices is an impossibility.