Axiom of set theory
In
mathematics
, the
axiom of choice
, abbreviated
AC
or
AoC
, is an
axiom
of
set theory
equivalent to the statement that
a
Cartesian product
of a collection of non-empty sets is non-empty
. Informally put, the axiom of choice says that given any collection of sets, each containing at least one element, it is possible to construct a new set by choosing one element from each set, even if the collection is
infinite
. Formally, it states that for every
indexed family
of
nonempty
sets, there exists an indexed set
such that
for every
. The axiom of choice was formulated in 1904 by
Ernst Zermelo
in order to formalize his proof of the
well-ordering theorem
.
[1]
In many cases, a set created by choosing elements can be made without invoking the axiom of choice, particularly if the number of sets from which to choose the elements is finite, or if a canonical rule on how to choose the elements is available ? some distinguishing property that happens to hold for exactly one element in each set. An illustrative example is sets picked from the natural numbers. From such sets, one may always select the smallest number, e.g. given the sets {{4, 5, 6}, {10, 12}, {1, 400, 617, 8000}}, the set containing each smallest element is {4, 10, 1}. In this case, "select the smallest number" is a
choice function
. Even if infinitely many sets are collected from the natural numbers, it will always be possible to choose the smallest element from each set to produce a set. That is, the choice function provides the set of chosen elements. But no definite choice function is known for the collection of all non-empty subsets of the real numbers. In that case, the axiom of choice must be invoked.
Bertrand Russell
coined an analogy: for any (even infinite) collection of pairs of shoes, one can pick out the left shoe from each pair to obtain an appropriate collection (i.e. set) of shoes; this makes it possible to define a choice function directly. For an
infinite
collection of pairs of socks (assumed to have no distinguishing features), there is no obvious way to make a function that forms a set out of selecting one sock from each pair without invoking the axiom of choice.
[2]
Although originally controversial, the axiom of choice is now used without reservation by most mathematicians,
[3]
and is included in the standard form of
axiomatic set theory
,
Zermelo?Fraenkel set theory
with the axiom of choice (ZFC). One motivation for this is that a number of generally accepted mathematical results, such as
Tychonoff's theorem
, require the axiom of choice for their proofs. Contemporary set theorists also study axioms that are not compatible with the axiom of choice, such as the
axiom of determinacy
. The axiom of choice is avoided in some varieties of
constructive mathematics
, although there are varieties of constructive mathematics in which the axiom of choice is embraced.
Statement
[
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]
A
choice function
(also called selector or selection) is a function
f
, defined on a collection
X
of nonempty sets, such that for every set
A
in
X
,
f
(
A
) is an element of
A
. With this concept, the axiom can be stated:
Axiom
—
For any set
X
of nonempty sets, there exists a choice function
f
that is defined on
X
and maps each set of
X
to an element of that set.
Formally, this may be expressed as follows:
Thus, the
negation
of the axiom may be expressed as the existence of a collection of nonempty sets which has no choice function. Formally, this may be derived making use of the logical equivalence of
to
.
Each choice function on a collection
X
of nonempty sets is an element of the
Cartesian product
of the sets in
X
. This is not the most general situation of a Cartesian product of a
family
of sets, where a given set can occur more than once as a factor; however, one can focus on elements of such a product that select the same element every time a given set appears as factor, and such elements correspond to an element of the Cartesian product of all
distinct
sets in the family. The axiom of choice asserts the existence of such elements; it is therefore equivalent to:
- Given any family of nonempty sets, their Cartesian product is a nonempty set.
Nomenclature
[
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]
In this article and other discussions of the Axiom of Choice the following abbreviations are common:
- AC ? the Axiom of Choice. More rarely, AoC is used.
[4]
- ZF ?
Zermelo?Fraenkel set theory
omitting the Axiom of Choice.
- ZFC ? Zermelo?Fraenkel set theory, extended to include the Axiom of Choice.
Variants
[
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]
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:
- Given any set
X
, if the empty set is not an element of
X
and the elements of
X
are
disjoint
, then there exists a set
C
such that its intersection with any of the elements of
X
contains exactly one element.
[5]
This can be formalized in first-order logic as:
- ∀x (
- ∃o (o ∈ x ∧ ¬∃n (n ∈ o)) ∨
- ∃a ∃b ∃c (a ∈ x ∧ b ∈ x ∧ c ∈ a ∧ c ∈ b ∧ ¬(a = b)) ∨
- ∃c ∀e (e ∈ x → ∃a (a ∈ e ∧ a ∈ c ∧ ∀b ((b ∈ e ∧ b ∈ c) → a = b))))
Note that P ∨ Q ∨ R is logically equivalent to (¬P ∧ ¬Q) → R.
In English, this first-order sentence reads:
- Given any set
X
,
- X
contains the empty set as an element or
- the elements of
X
are not pairwise disjoint or
- there exists a set
C
such that its intersection with any of the elements of
X
contains exactly one element.
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:
- For any set
A
, the
power set
of
A
(with the empty set removed) has a choice function.
Authors who use this formulation often speak of the
choice function on A
, but this is a slightly different notion of choice function. Its domain is the power set 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
- Every set has a choice function.
[6]
which is equivalent to
- For any set
A
there is a function
f
such that for any non-empty subset
B
of
A
,
f
(
B
) lies in
B
.
The negation of the axiom can thus be expressed as:
- There is a set
A
such that for all functions
f
(on the set of non-empty subsets of
A
), there is a
B
such that
f
(
B
) does not lie in
B
.
Restriction to finite sets
[
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]
The usual 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. However, that particular case is a theorem of the Zermelo?Fraenkel set theory without the axiom of choice (ZF); it is easily proved by the
principle of finite induction
.
[7]
In the even simpler case of a collection of
one
set, a choice function just corresponds to an element, so this instance of the axiom of choice says that every nonempty set has an element; this holds trivially. The axiom of choice can be seen as asserting the generalization of this property, already evident for finite collections, to arbitrary collections.
Usage
[
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]
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
.
Examples
[
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]
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 add the axiom of choice to our axioms of set theory.
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 selection forms a legitimate set (as defined by the other ZF axioms of set theory)? 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. For example, the open
interval
(0,1) does not have a least element: if
x
is in (0,1), then so is
x
/2, and
x
/2 is always strictly smaller than
x
. 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, that is, rotations by angles which are rational multiples of
π
. Here
G
is countable while
S
is uncountable. Hence
S
breaks up into uncountably many orbits under
G
. Using the axiom of choice, we could pick a single point from each orbit, obtaining an uncountable subset
X
of
S
with the property that all of its translates by
G
are disjoint from
X
. 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 form a set from selecting a point in each orbit requires that one add the axiom of choice to our axioms of set theory. See
non-measurable set
for more details.
In classical arithmetic, the natural numbers are
well-ordered
: for every nonempty subset of the natural numbers, there is a unique least element under the natural ordering. In this way, one may specify a set from any given subset. 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." The problem then becomes that of constructing a well-ordering, which turns out to require the axiom of choice for its existence; every set can be well-ordered if and only if the axiom of choice holds.
Criticism and acceptance
[
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]
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 individual 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.
[8]
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.
[9]
Because there is no
canonical
well-ordering of all sets, a construction that relies on a well-ordering may not produce a canonical result, even if a canonical result is desired (as is often the case in
category theory
). 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.
[10]
One example is the
Banach?Tarski paradox
, which says that it is possible to decompose the 3-dimensional solid unit ball into finitely many pieces and, using only rotations and translations, reassemble the pieces into two solid balls each with the same volume as the original. The pieces in this decomposition, constructed using the axiom of choice, are
non-measurable sets
.
Moreover, paradoxical consequences of the axiom of choice for the
no-signaling principle
in physics have recently been pointed out.
[11]
Despite these seemingly paradoxical results, most mathematicians accept the axiom of choice as a valid principle for proving new results in mathematics. But the debate is interesting enough that it is considered notable 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 that requires the axiom of choice to be true.
Theorems of ZF hold true in any
model
of that theory, regardless of the truth or falsity of the axiom of choice in that particular model. The implications of choice below, including weaker versions of the axiom itself, are listed because they are not theorems of ZF. The Banach?Tarski paradox, for example, 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. Such statements can be rephrased as conditional statements?for example, "If AC holds, then the decomposition in the Banach?Tarski paradox exists." Such conditional statements are provable in ZF when the original statements are provable from ZF and the axiom of choice.
In constructive mathematics
[
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]
As discussed above, in the classical theory of ZFC, the axiom of choice enables
nonconstructive proofs
in which the existence of a type of object is proved without an explicit instance being constructed. In fact, in set theory and
topos theory
,
Diaconescu's theorem
shows that the axiom of choice implies the
law of excluded middle
. The principle is thus not available in
constructive set theory
, where non-classical logic is employed.
The situation is different when the principle is formulated in
Martin-Lof type theory
. There and higher-order
Heyting arithmetic
, the appropriate statement of the axiom of choice is (depending on approach) included as an axiom or provable as a theorem.
[12]
A cause for this difference is that the axiom of choice in type theory does not have the
extensionality
properties that the axiom of choice in constructive set theory does.
[13]
The type theoretical context is discussed further below.
Different choice principles have been thoroughly studied in the constructive contexts and the principles' status varies between different school and varieties of the constructive mathematics.
Some results in constructive set theory use the
axiom of countable choice
or the
axiom of dependent choice
, which do not imply the law of the excluded middle.
Errett Bishop
, who is notable for developing a framework for constructive analysis, argued that an axiom of choice was constructively acceptable, saying
A choice function exists in constructive mathematics, because a choice is implied by the very meaning of existence.
[14]
Although the axiom of countable choice in particular is commonly used in constructive mathematics, its use has also been questioned.
[15]
Independence
[
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]
It has been known since as early as 1922 that the axiom of choice may fail in a variant of ZF with
urelements
, through the technique of
permutation models
introduced by
Abraham Fraenkel
and developed further by
Andrzej Mostowski
.
The basic technique can be illustrated as follows: Let
x
n
and
y
n
be distinct urelements for
n
=1, 2, 3...
, and build a model where each set is symmetric under the interchange
x
n
↔
y
n
for all but a finite number of
n
. Then the set
X
= {{
x
1
,
y
1
}, {
x
2
,
y
2
}, {
x
3
,
y
3
}, ...}
can be in the model but sets such as
{
x
1
,
x
2
,
x
3
, ...}
cannot, and thus
X
cannot have a choice function.
In 1938,
[18]
Kurt Godel
showed that the
negation
of the axiom of choice is not a theorem of ZF by constructing an
inner model
(the
constructible universe
) that satisfies ZFC, thus showing that ZFC is consistent if ZF itself is consistent. In 1963,
Paul Cohen
employed the technique of
forcing
, developed for this purpose, to show that, assuming ZF is consistent, the axiom of choice itself is not a theorem of ZF. He did this by constructing a much more complex model that satisfies ZF¬C (ZF with the negation of AC added as axiom) and thus showing that ZF¬C is consistent. Cohen's model is a
symmetric model
, which is similar to permutation models, but uses "generic" subsets of the natural numbers (justified by forcing) in place of urelements.
[19]
Together these results establish that the axiom of choice is
logically independent
of ZF. The assumption that ZF is consistent is harmless because adding another axiom to an already inconsistent system cannot make the situation worse. Because of independence, the decision whether to use the axiom of choice (or its negation) in a proof cannot be made by appeal to other axioms of set theory. It must be made on other grounds.
One argument in favor of using the axiom of choice is that it is convenient because it allows one to prove some simplifying propositions that otherwise could not be proved. Many theorems provable using choice are of an elegant general character: the cardinalities of any two sets are comparable, every nontrivial
ring
with unity has a
maximal ideal
, every
vector space
has a
basis
, every
connected graph
has a
spanning tree
, and every
product
of
compact spaces
is compact, among many others. Frequently, the axiom of choice allows generalizing a theorem to "larger" objects. For example, it is provable without the axiom of choice that every vector space of finite dimension has a basis, but the generalization to all vector spaces requires the axiom of choice. Likewise, a finite product of compact spaces can be proven to be compact without the axiom of choice, but the generalization to infinite products (
Tychonoff's theorem
) requires the axiom of choice.
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.
[20]
Statements in this class include the statement that
P = NP
, the
Riemann hypothesis
, and many other unsolved mathematical problems. When attempting 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. It is possible, however, that there is a shorter proof of a theorem from ZFC than from ZF.
The axiom of choice is not the only significant statement that is independent of ZF. For example, the
generalized continuum hypothesis
(GCH) is not only independent of ZF, but also independent of ZFC. However, ZF plus GCH implies AC, making GCH a strictly stronger claim than AC, even though they are both independent of ZF.
Stronger axioms
[
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]
The
axiom of constructibility
and the
generalized continuum hypothesis
each imply the axiom of choice and so are strictly stronger than it. In class theories such as
Von Neumann?Bernays?Godel set theory
and
Morse?Kelley set theory
, there is an axiom called the
axiom of global choice
that is stronger than the axiom of choice for sets because it also applies to proper classes. The axiom of global choice follows from the
axiom of limitation of size
. Tarski's axiom, which is used in
Tarski?Grothendieck set theory
and states (in the vernacular) that every set belongs to
some
Grothendieck universe
, is stronger than the axiom of choice.
Equivalents
[
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]
There are important statements that, assuming the axioms of
ZF
but neither AC nor ¬AC, are equivalent to the axiom of choice.
[21]
The most important among them are
Zorn's lemma
and the
well-ordering theorem
. In fact, Zermelo initially introduced the axiom of choice in order to formalize his proof of the well-ordering theorem.
- Set theory
- Tarski's theorem about choice
: For every infinite set
A
, there is a
bijective map
between the sets
A
and
A
×
A
.
- Trichotomy
: If two sets are given, then either they have the same cardinality, or one has a smaller cardinality than the other.
- Given two non-empty sets, one has a surjection to the other.
- Every
surjective function
has a
right inverse
.
- The
Cartesian product
of any family of nonempty sets is nonempty. In other words, every family of nonempty sets has a choice function (
i.e.
a function which maps each of the nonempty sets to one of its elements).
- Konig's theorem
: Colloquially, the sum of a sequence of cardinals is strictly less than the product of a sequence of larger cardinals. (The reason for the term "colloquially" is that the sum or product of a "sequence" of cardinals cannot itself be defined without some aspect of the axiom of choice.)
- Well-ordering theorem
: Every set can be well-ordered. Consequently, every
cardinal
has an
initial ordinal
.
- Every element of a partially ordered set
S
is the minimal element of a well-ordered subset having no strict upper bound in
S
.
[
citation needed
]
- Zorn's lemma
: Every non-empty partially ordered set in which every chain (
i.e.
, totally ordered subset) has an upper bound contains at least one maximal element.
- Hausdorff maximal principle
: Every partially ordered set has a maximal chain. Equivalently, in any partially ordered set, every chain can be extended to a maximal chain.
- Tukey's lemma
: Every non-empty collection of
finite character
has a maximal element with respect to inclusion.
- Antichain
principle: Every partially ordered set has a maximal
antichain
. Equivalently, in any partially ordered set, every antichain can be extended to a maximal antichain.
- Abstract algebra
- Functional analysis
- Point-set topology
- Mathematical logic
- If
S
is a set of sentences of
first-order logic
and
B
is a consistent subset of
S
, then
B
is included in a set that is maximal among consistent subsets of
S
. The special case where
S
is the set of
all
first-order sentences in a given
signature
is weaker, equivalent to the
Boolean prime ideal theorem
; see the section "Weaker forms" below.
- Graph theory
Category theory
[
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]
Several results in
category theory
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. On the other hand, other foundational descriptions of category theory are considerably stronger, and an identical category-theoretic statement of choice may be stronger than the standard formulation, a la class theory, mentioned above.
Examples of category-theoretic statements which require choice include:
- Every small
category
has a
skeleton
.
- If two small categories are weakly equivalent, then they are
equivalent
.
- Every continuous functor on a small-complete category which satisfies the appropriate solution set condition has a
left-adjoint
(the Freyd adjoint functor theorem).
Weaker forms
[
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]
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). A still weaker example is the
axiom of countable choice
(AC
ω
or CC), which states that a choice function exists for any countable set of nonempty sets. 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.
Given an ordinal parameter α ≥ ω+2 — for every set
S
with rank less than α,
S
is well-orderable. Given an ordinal parameter α ≥ 1 — for every set
S
with
Hartogs number
less than ω
α
,
S
is well-orderable. As the ordinal parameter is increased, these approximate the full axiom of choice more and more closely.
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
Tarski
's 1930
ultrafilter lemma
: every
filter
is a subset of some
ultrafilter
.
Results requiring AC (or weaker forms) but weaker than it
[
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]
One of the most interesting aspects of the axiom of choice is the large number of places in mathematics where it shows up. Here are some statements that require the axiom of choice in the sense that they are not provable from ZF but are provable from ZFC (ZF plus AC). Equivalently, these statements are true in all models of ZFC but false in some models of ZF.
Possibly equivalent implications of AC
[
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]
There are several historically important set-theoretic statements implied by AC whose equivalence to AC is open. Zermelo cited the partition principle, which was formulated before AC itself, as a justification for believing AC. In 1906, Russell declared PP to be equivalent, but whether the partition principle implies AC is the oldest open problem in set theory,
[34]
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 whether they can hold without choice.
- Set theory
- Partition principle: if there is a
surjection
from
A
to
B
, there is an
injection
from
B
to
A
. Equivalently, every
partition
P
of a set
S
is less than or equal to
S
in size.
- Converse
Schroder?Bernstein theorem
: if two sets have surjections to each other, they are equinumerous.
- Weak partition principle: if there is a
injection
and a
surjection
from
A
to
B
, then
A
and
B
are equinumerous. Equivalently, a partition of a set
S
cannot be strictly larger than
S
. If WPP holds, this already implies the existence of a non-measurable set. Each of the previous three statements is implied by the preceding one, but it is unknown if any of these implications can be reversed.
- There is no infinite decreasing sequence of cardinals. The equivalence was conjectured by Schoenflies in 1905.
- Abstract algebra
- Hahn embedding theorem
: Every ordered abelian group
G
order-embeds as a subgroup of the additive group
endowed with a
lexicographical order
, where Ω is the set of Archimedean equivalence classes of
G
. This equivalence was conjectured by Hahn in 1907.
Stronger forms of the negation of AC
[
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]
If we abbreviate by BP the claim that every set of real numbers has the
property of Baire
, then BP is stronger than ¬AC, which asserts the nonexistence of any choice function on perhaps only a single set of nonempty sets. Strengthened negations may be compatible with weakened forms of AC. For example, ZF + DC
[35]
+ BP is consistent, if ZF is.
It is also consistent with ZF + DC that every set of reals is
Lebesgue measurable
, but this consistency result, due to
Robert M. 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). ZF + DC + AD is consistent provided that a sufficiently strong large cardinal axiom is consistent (the existence of infinitely many
Woodin cardinals
).
Quine
's system of axiomatic set theory,
New Foundations
(NF), takes its name from the title ("New Foundations for Mathematical Logic") of the 1937 article that introduced it. In the NF axiomatic system, the axiom of choice can be disproved.
[36]
Statements implying the negation of AC
[
edit
]
There are models of Zermelo-Fraenkel set theory in which the axiom of choice is false. We shall abbreviate "Zermelo-Fraenkel set theory plus the negation of the axiom of choice" by ZF¬C. For certain models of ZF¬C, it is possible to validate the negation of some standard ZFC theorems. As any model of ZF¬C is also a model of ZF, it is the case that for each of the following statements, there exists a model of ZF in which that statement is true.
- The negation of the
weak partition principle
: There is a set that can be partitioned into strictly more equivalence classes than the original set has elements, and a function whose domain is strictly smaller than its range. In fact, this is the case in all
known
models.
- There is a function
f
from the real numbers to the real numbers such that
f
is not continuous at
a
, but
f
is
sequentially continuous
at
a
, i.e., for any sequence {
x
n
} converging to
a
, lim
n
f(
x
n
)=f(a).
- There is an infinite set of real numbers without a countably infinite subset.
- The real numbers are a countable union of countable sets.
[37]
This does not imply that the real numbers are countable: As pointed out above, to show that a countable union of countable sets is itself countable requires the
Axiom of countable choice
.
- There is a field with no algebraic closure.
- In all models of ZF¬C there is a vector space with no basis.
- There is a vector space with two bases of different cardinalities.
- There is a free
complete Boolean algebra
on countably many generators.
[38]
- There is
a set that cannot be linearly ordered
.
- There exists a model of ZF¬C in which every set in R
n
is
measurable
. Thus it is possible to exclude counterintuitive results like the
Banach?Tarski paradox
which are provable in ZFC. Furthermore, this is possible whilst assuming the
Axiom of dependent choice
, which is weaker than AC but sufficient to develop most of
real analysis
.
- In all models of ZF¬C, the
generalized continuum hypothesis
does not hold.
For proofs, see
Jech (2008)
.
Additionally, by imposing definability conditions on sets (in the sense of
descriptive set theory
) one can often prove restricted versions of the axiom of choice from axioms incompatible with general choice. This appears, for example, in the
Moschovakis coding lemma
.
Axiom of choice in type theory
[
edit
]
In
type theory
, a different kind of statement is known as the axiom of choice. This form begins with two types, σ and τ, and a relation
R
between objects of type σ and objects of type τ. The axiom of choice states that if for each
x
of type σ there exists a
y
of type τ such that
R
(
x
,
y
), then there is a function
f
from objects of type σ to objects of type τ such that
R
(
x
,
f
(
x
)) holds for all
x
of type σ:
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.
Quotations
[
edit
]
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 mathematically equivalent, many mathematicians find the axiom of choice intuitive, the well-ordering principle counterintuitive, and Zorn's lemma 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, for example by choosing the left shoe from each pair. 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.
Tarski tried to publish his theorem [the equivalence between AC and "every infinite set
A
has the same cardinality as
A
×
A
", see above] in
Comptes Rendus
, but
Frechet
and
Lebesgue
refused to present it. Frechet wrote that an implication between two well known [true] propositions is not a new result, and Lebesgue wrote that an implication between two false propositions is of no interest.
Polish-American mathematician
Jan Mycielski
relates this anecdote in a 2006 article in the Notices of the AMS.
[41]
The axiom gets its name not because mathematicians prefer it to other axioms.
This quote comes from the famous
April Fools' Day
article in the
computer recreations
column of the
Scientific American
, April 1989.
Notes
[
edit
]
- ^
Zermelo 1904
.
- ^
Jech 1977
, p. 351
- ^
Jech, 1977, p. 348
ff
; Martin-Lof 2008, p. 210. According to
Mendelson 1964
, p. 201:
- The status of the Axiom of Choice has become less controversial in recent years. To most mathematicians it seems quite plausible and it has so many important applications in practically all branches of mathematics that not to accept it would seem to be a wilful hobbling of the practicing mathematician.
- ^
Rosenberg, Steven (21 December 2021).
An Invitation to Abstract Algebra
. CRC Press.
ISBN
9781000516333
.
- ^
Herrlich 2006
, p. 9. According to
Suppes 1972
, p. 243, this was the formulation of the axiom of choice which was originally given by
Zermelo 1904
. See also
Halmos 1960
, p. 60 for this formulation.
- ^
Suppes 1972
, p. 240.
- ^
Tourlakis (2003), pp. 209?210, 215?216.
- ^
Fraenkel, Abraham A.
;
Bar-Hillel, Yehoshua
;
Levy, Azriel
(1973),
Foundations of set theory
(2nd ed.), Amsterdam-London: North-Holland Publishing Co., pp. 69?70,
ISBN
9780080887050
,
MR
0345816
.
- ^
Rosenbloom, Paul C. (2005),
The Elements of Mathematical Logic
, Courier Dover Publications, p. 147,
ISBN
9780486446172
.
- ^
Dawson, J. W. (August 2006), "Shaken Foundations or Groundbreaking Realignment? A Centennial Assessment of Kurt Godel's Impact on Logic, Mathematics, and Computer Science",
Proc. 21st Annual IEEE Symposium on Logic in Computer Science (LICS 2006)
, pp. 339?341,
doi
:
10.1109/LICS.2006.47
,
ISBN
978-0-7695-2631-7
,
S2CID
15526447
,
The axiom of choice, though it had been employed unconsciously in many arguments in analysis, became controversial once made explicit, not only because of its non-constructive character, but because it implied such extremely unintuitive consequences as the Banach?Tarski paradox.
.
- ^
Baumeler, A., Daki?, B. and Del Santo, F., 2022.
The Axiom of Choice and the No-Signaling Principle
, arXiv preprint ? arXiv:2206.08467.
- ^
Per Martin-Lof
,
Intuitionistic type theory
, 1980.
Anne Sjerp Troelstra
,
Metamathematical investigation of intuitionistic arithmetic and analysis
, Springer, 1973.
- ^
Martin-Lof, Per
(2006). "100 Years of Zermelo's Axiom of Choice: What was the Problem with It?".
The Computer Journal
.
49
(3): 345?350.
Bibcode
:
1980CompJ..23..262L
.
doi
:
10.1093/comjnl/bxh162
.
- ^
Errett Bishop
and
Douglas S. Bridges
,
Constructive analysis
, Springer-Verlag, 1985.
- ^
Fred Richman, "Constructive mathematics without choice", in: Reuniting the Antipodes?Constructive and Nonstandard Views of the Continuum (P. Schuster et al., eds), Synthese Library 306, 199?205, Kluwer Academic Publishers, Amsterdam, 2001.
- ^
Godel, Kurt (9 November 1938).
"The Consistency of the Axiom of Choice and of the Generalized Continuum-Hypothesis"
.
Proceedings of the National Academy of Sciences of the United States of America
.
24
(12): 556?557.
Bibcode
:
1938PNAS...24..556G
.
doi
:
10.1073/pnas.24.12.556
.
PMC
1077160
.
PMID
16577857
.
- ^
Cohen, Paul (2019).
"The Independence of the Axiom of Choice"
(PDF)
.
Stanford University Libraries
.
Archived
(PDF)
from the original on 9 October 2022
. Retrieved
22 March
2019
.
- ^
This is because arithmetical statements are
absolute
to the
constructible universe
L
.
Shoenfield's absoluteness theorem
gives a more general result.
- ^
See
Moore 2013
, pp. 330?334, for a structured list of 74 equivalents. See
Howard & Rubin 1998
, pp. 11?16, for 86 equivalents with source references.
- ^
Blass, Andreas (1984). "Existence of bases implies the axiom of choice".
Axiomatic set theory (Boulder, Colo., 1983)
. Contemporary Mathematics. Vol. 31. Providence, RI: American Mathematical Society. pp. 31?33.
doi
:
10.1090/conm/031/763890
.
MR
0763890
.
- ^
A. Hajnal
, A. Kertesz: Some new algebraic equivalents of the axiom of choice,
Publ. Math. Debrecen
,
19
(1972), 339?340, see also H. Rubin,
J. Rubin
,
Equivalents of the axiom of choice, II
,
North-Holland
, 1985, p. 111.
- ^
a
b
Blass, Andreas (1979).
"Injectivity, Projectivity and the Axiom of Choice"
.
Transactions of the American Mathematical Society
.
255
: 31?59.
doi
:
10.2307/1998165
.
JSTOR
1998165
.
- ^
Awodey, Steve (2010).
Category theory
(2nd ed.). Oxford: Oxford University Press. pp.
20
?24.
ISBN
978-0199237180
.
OCLC
740446073
.
- ^
projective object
at the
n
Lab
- ^
Serre, Jean-Pierre
(2003),
Trees
, Springer Monographs in Mathematics, Springer, p. 23
;
Soukup, Lajos (2008), "Infinite combinatorics: from finite to infinite",
Horizons of combinatorics
, Bolyai Society Mathematical Studies, vol. 17, Berlin: Springer, pp. 189?213,
CiteSeerX
10.1.1.222.5699
,
doi
:
10.1007/978-3-540-77200-2_10
,
ISBN
978-3-540-77199-9
,
MR
2432534
. See in particular Theorem 2.1,
pp. 192?193
.
- ^
Muger, Michael (2020).
Topology for the Working Mathematician
.
- ^
It is shown by
Jech 2008
, pp. 119?131, that the axiom of countable choice implies the equivalence of infinite and Dedekind-infinite sets, but that the equivalence of infinite and Dedekind-infinite sets does not imply the axiom of countable choice in ZF.
- ^
It was shown by
Levy 1958
and others using Mostowski models that eight definitions of a finite set are independent in ZF without AC, although they are equivalent when AC is assumed. The definitions are I-finite, Ia-finite, II-finite, III-finite, IV-finite, V-finite, VI-finite and VII-finite. I-finiteness is the same as normal finiteness. IV-finiteness is the same as Dedekind-finiteness.
- ^
"[FOM] Are (C,+) and (R,+) isomorphic"
. 21 February 2006.
- ^
Ash, C. J. (1975).
"A consequence of the axiom of choice"
.
Journal of the Australian Mathematical Society
.
19
(3): 306?308.
doi
:
10.1017/S1446788700031505
.
S2CID
122334025
.
- ^
"On the Partition Principle"
.
- ^
Axiom of dependent choice
- ^
"Quine's New Foundations"
.
Stanford Encyclopedia of Philosophy
. Retrieved
10 November
2017
.
- ^
Jech 2008
, pp. 142?144, Theorem 10.6 with proof.
- ^
Stavi, Jonathan (1974). "A model of ZF with an infinite free complete Boolean algebra".
Israel Journal of Mathematics
.
20
(2): 149?163.
doi
:
10.1007/BF02757883
.
S2CID
119543439
.
- ^
Krantz, Steven G.
(2002), "The axiom of choice",
Handbook of Logic and Proof Techniques for Computer Science
, Springer, pp. 121?126,
doi
:
10.1007/978-1-4612-0115-1_9
,
ISBN
978-1-4612-6619-8
.
- ^
The boots-and-socks metaphor was given in 1919 by
Russell 1993
, pp. 125?127. He suggested that a millionaire might have ?
0
pairs of boots and ?
0
pairs of socks.
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. 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. The
pairs
are given as forming an ?
0
, and therefore as the field of a progression. 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. Unless we can find a
rule
for selecting, i.e. a relation which is a selector, we do not know that a selection is even theoretically possible.
Russell then suggests using the location of the centre of mass of each sock as a selector.
- ^
Mycielski, Jan
(2006),
"A system of axioms of set theory for the rationalists"
(PDF)
,
Notices of the American Mathematical Society
,
53
(2): 206?213,
MR
2208445
.
References
[
edit
]
- Fraenkel, Abraham
(1922), "Der Begriff "definit" und die Unabhangigkeit des Auswahlaxioms",
Sitzungsberichte der Koniglich Preussischen Akademie der Wissenschaften
: 253?257,
JFM
48.0199.02
- Halmos, Paul R.
(1960).
Naive Set Theory
. The University Series in Undergraduate Mathematics. Princeton, NJ: van Nostrand Company.
Zbl
0087.04403
.
- Herrlich, Horst
(2006).
Axiom of Choice
. Lecture Notes in Math. 1876. Berlin:
Springer-Verlag
.
ISBN
978-3-540-30989-5
.
- Howard, Paul;
Rubin, Jean E.
(1998).
Consequences of the axiom of choice
. Mathematical Surveys and Monographs. Vol. 59. Providence, Rhode Island:
American Mathematical Society
.
ISBN
9780821809778
.
- Jech, Thomas
(2008) [1973].
The axiom of choice
. Mineola, New York:
Dover Publications
.
ISBN
978-0-486-46624-8
.
- Jech, Thomas
(1977). "About the Axiom of Choice". In John Barwise (ed.).
Handbook of Mathematical Logic
.
- Levy, Azriel
(1958).
"The independence of various definitions of finiteness"
(PDF)
.
Fundamenta Mathematicae
.
46
: 1?13.
doi
:
10.4064/fm-46-1-1-13
.
Archived
(PDF)
from the original on 9 October 2022.
- Per Martin-Lof, "100 years of Zermelo's axiom of choice: What was the problem with it?", in
Logicism, Intuitionism, and Formalism: What Has Become of Them?
, Sten Lindstrom, Erik Palmgren, Krister Segerberg, and Viggo Stoltenberg-Hansen, editors (2008).
ISBN
1-4020-8925-2
- Mendelson, Elliott
(1964).
Introduction to Mathematical Logic
. New York: Van Nostrand Reinhold.
- Moore, Gregory H. (1982).
Zermelo's axiom of choice, Its origins, development and influence
.
Springer
.
ISBN
978-0-387-90670-6
.
, available as a
Dover Publications
reprint, 2013,
ISBN
0-486-48841-1
.
- Mostowski, Andrzej
(1938), "Uber den Begriff einer Endlichen Menge",
Comptes Rendus des Seances de la Societe des Sciences et des Lettres de Varsovie, Classe III
,
31
(8): 13?20
- Moore, Gregory H (2013) [1982].
Zermelo's axiom of choice: Its origins, development & influence
. Mineola, New York: Dover Publications.
ISBN
978-0-486-48841-7
.
- Herman Rubin
,
Jean E. Rubin
: Equivalents of the axiom of choice. North Holland, 1963. Reissued by
Elsevier
, April 1970.
ISBN
0-7204-2225-6
.
- Herman Rubin, Jean E. Rubin: Equivalents of the Axiom of Choice II. North Holland/Elsevier, July 1985,
ISBN
0-444-87708-8
.
- Russell, Bertrand
(1993) [1919].
Introduction to mathematical philosophy
. New York: Dover Publications.
ISBN
978-0-486-27724-0
.
- Schechter, Eric
(1996).
Handbook of Analysis and Its Foundations
. San Diego, CA: Academic Press.
ISBN
978-0-12-622760-4
.
OCLC
175294365
.
- Suppes, Patrick
(1972) [1960].
Axiomatic set theory
. Mineola, New York: Dover.
ISBN
978-0-486-61630-8
.
- George Tourlakis,
Lectures in Logic and Set Theory. Vol. II: Set Theory
,
Cambridge University Press
, 2003.
ISBN
0-511-06659-7
- Zermelo, Ernst (1904).
"Beweis, dass jede Menge wohlgeordnet werden kann"
(reprint)
.
Mathematische Annalen
.
59
(4): 514?16.
doi
:
10.1007/BF01445300
.
S2CID
124189935
.
- Ernst Zermelo
, "Untersuchungen uber die Grundlagen der Mengenlehre I,"
Mathematische Annalen 65
: (1908) pp. 261?81.
PDF download via digizeitschriften.de
- Translated in:
Jean van Heijenoort
, 2002.
From Frege to Godel: A Source Book in Mathematical Logic, 1879?1931
. New edition.
Harvard University Press
.
ISBN
0-674-32449-8
- 1904. "Proof that every set can be well-ordered," 139-41.
- 1908. "Investigations in the foundations of set theory I," 199?215.
External links
[
edit
]