Download MATH 337 Cardinality

Dr. Neal, WKU
MATH 337
Cardinality
We now shall prove that the rational numbers are a countable set while ℜ is
uncountable. This result shows that there are two different magnitudes of infinity. But
we will show that there are, in fact, an infinite number of infinities.
Definition 5.1. The cardinality of set A is the number of elements in A and is denoted
by either n(A) or A .
Finite Sets
Definition 5.2. (a) For all n ≥ 1, the first n natural numbers is the set Nn = {1, . . . , n }
and N n = n .
(b) A non-empty set A is called finite and A = n if and only if there exists a bijection
f : Nn → A . The empty set is also finite and ∅ = 0.
Note: If A and B are finite sets, then A × B is finite and A × B = A × B .
Theorem 5.1. Two non-empty finite sets have the same cardinality if and only if they are
equivalent.
Proof. Suppose A = n = B . Then there exist bijections f : Nn → A and g: Nn → B .
Then f −1: A → Nn is a bijection; hence, the composition g  f −1: A → B is also a bijection.
Therefore, A ~ B .
Now suppose A ~ B where A and B are finite sets with A = n and B = m . Then
there exist bijections f : A → B , g: Nn → A , and h: Nm → B .
Then the function
h −1  f  g: Nn → Nm is also a bijection. Therefore, N m = n ; i.e., m = n . Hence, A = B .
QED
Finite sets of different sizes cannot be equivalent. In fact, suppose A and B are
finite sets and f : A → B is a function (with the domain being all of A ).
(i) If A < B , then f cannot be onto.
(ii) If A > B , then f cannot be one-to-one.
(iii) f is a bijection if and only if A = B .
Note: Whenever A ~ Nn , then there is a bijection f : Nn → A .
f (i) = ai ∈ A for 1 ≤ i ≤ n . Then A can be written as A = {a1 ,. . .,an }.
Theorem 5.2. A finite union of disjoint, non-empty finite sets is finite.
We then can let
Dr. Neal, WKU
Proof. Let A1 , A2 , . . . , Am be disjoint, finite sets denoted by
A1 = {a1,1 , . . ., a1,n1 }
A2 = {a2,1 , . . ., a2,n2 } . . . Am = {am,1 , . . ., am,nm }
m
m
m
Then  Ai has M =
i =1
∑ ni elements. We define a bijection from f :
i =1
 Ai → {1, . . ., M} by
i=1

j
if i = 1
f (ai, j ) = 
 n1 + . . . + ni −1 + j €if i > 1
€
m
Thus,  Ai ~ N M .
QED
i =1
Infinite Sets
Definition 5.3. Set A is called infinite if it is not finite.
Some infinite sets are: the natural numbers ℵ, the integers Z , the Rationals Q , the
Reals ℜ , the even integers E , the odd integers O , the prime numbers P , and the
irrationals I (because p is irrational for every prime p ).
Denumerable and Countable Sets
Definition 5.4. (a) Set A is called denumerable (or countably infinite) if there exists a
bijection between the natural numbers ℵ and set A . In other words, a denumerable set
is equivalent to the natural numbers ℵ.
(b) If set A is denumerable ( A ~ ℵ), then A is said to have cardinality “Aleph Naught”
denoted by A = ℵ0 .
(c) Set A is called countable if it is either finite or denumerable.
Example 5.1. The set of all odd numbers O is denumerable.
 n
if n is odd
Proof. Define f :ℵ→ O by f (n) = 
. Then f is a bijection; so the odd
−n + 1 if n is even
numbers are countably infinite and O = ℵ0 .
ℵ
O
1
↓
1
2
↓
–1
3
↓
3
4
↓
–3
5
↓
5
6
↓
–5
.
.
.
.
.
.
Dr. Neal, WKU
Theorem 5.3. All denumerable sets are equivalent and have cardinality ℵ0 .
Proof. Any two denumerable sets are equivalent to the natural numbers. Because set
equivalence is an equivalence relation, any two denumerable sets are equivalent to each
other. Moreover, by being denumerable, each has cardinality ℵ0 .
QED
Note: Whenever A is denumerable, then there is a bijection f :ℵ→ A . We then can let
f (i) = ai ∈ A for i ≥ 1. Then A can be written as A = {a1 , a2 ,. . .}.
Lemma 5.1. (a) If A is non-empty and finite, and B is denumerable with A ∩ B = ∅ ,
then A ∪ B is denumerable.
(b) If A and B are denumerable with A ∩ B = ∅ , then A ∪ B is denumerable.
(c) Any infinite subset S of a denumerable set A is denumerable.
Proof.
(a) Let A = {a1 ,. . .,an } and B = {b1 , b2 ,. . .} . Define a bijection f : A ∪ B →ℵ by
f (ai ) = i and f (b j ) = n + j . Note that f −1:ℵ→ A ∪ B is also a bijection given by
 ai
f −1(i) = 
bi−n
(b)
if 1 ≤ i ≤ n
.
if i > n
Let A = {a1 , a2 ,. . .} and B = {b1 , b2 ,. . .} .
f (ai ) = 2i and f (b j ) = 2 j − 1 . Note that f
−1
Define a bijection f : A ∪ B →ℵ by
:ℵ→ A ∪ B is also a bijection given by
 ai / 2
f −1(i) = 
b(i+1)/2
if i is even
if i is odd
.
(c) Let A = {a1 , a2 ,. . .}. Subset S is actually a subsequence S = {an1 , an2 ,. . .} where n1 is
the first index such that an1 ∈ S , n2 is the next index such that an2 ∈ S , etc. So we can
define a bijection f : ℵ→ S by f (k ) = ank .
QED
Corollary 5.1. The set integers Z is denumerable.
Proof. First, the whole numbers W = {0} ∪ℵ are denumerable by Lemma 1(a). The
negative integers −ℵ are clearly in one-to-one correspondence with ℵ, so they are
denumerable. Thus, Z = −ℵ∪ W is denumerable Lemma 5.1 (b).
Exercise: Find a direct one-to-one correspondence between ℵ and Z .
Dr. Neal, WKU
Corollary 5.2. (a) The prime numbers are denumerable. (b) The odd numbers are
denumerable. (c) The set of all multiples of a fixed natural number is denumerable.
Proof. All are infinite subsets of the denumerable set Z .
Lemma 5.2. Let A be an infinite set and let f : A → ℵ be one-to-one.
denumerable.
Then A is
Proof. Redefine the function as f : A → Range f . Then f is both one-to-one and onto.
Thus, A is equivalent to Range f which must be infinite because A is infinite. But then
Range f is an infinite subset of N ; hence, Range f is denumerable. Therefore A is
denumerable because it is equivalent to the denumerable set Range f .
QED
Theorem 5.4. A countable union of disjoint denumerable sets is denumerable.
Proof. Let A1 , A2 , . . . , An , . . . be disjoint denumerable sets and let A be their union.
We can enumerate set each as follows:
A1 = { a11 , a12 , a13 , . . . }
A2 = { a21 , a22 , a23 , . . . }
...
An = { an1 , an2 , an3 , . . . }
etc.
Because all the An are disjoint, each ai j is distinct. Now define f : A → ℵ by
f (ai j ) = 2 i 3 j .
By the uniqueness of prime factorizations, each ai j gives a different natural number.
Thus, f is one-to-one. By Lemma 2, the infinite set A is denumerable.
QED
Recall: Countable means finite or denumerable. So the countable collection of sets may
be finite A1 , A2 , . . . , An or denumerable A1 , A2 , . . . , An , . . . which does not affect the
preceding proof.
Corollary 5.3. A countable union of disjoint countable sets is countable.
To prove this corollary, simply apply the preceding proof where each set An could
be either finite or denumerable.
Theorem 5.5. The set of positive rational numbers Q+ and the set of negative rational
numbers Q− are denumerable.
Dr. Neal, WKU
Proof. We can write Q+ as the set {a / b a,b ∈ℵ, gcd(a,b) = 1} . Define f :Q+ →ℵ by
f (a / b) = 2 a 3b .
By the uniqueness of prime factorizations, each positive rational a / b gives a different
natural number. Thus, f is one-to-one. By Lemma 2, the infinite set Q+ is
denumerable. Because Q− ~ Q+ , we have that Q− is denumerable also.
QED
Corollary 5.4. The rational numbers Q are denumerable.
Proof. Q = Q− ∪{0} ∪ Q+ is denumerable by Lemma 5.1.
QED
Uncountable Sets
Definition 5.5. Set A is called uncountable if it is infinite but not denumerable.
All finite sets can be counted. Denumerable sets are infinite, but are equivalent to
the natural numbers 1, 2, 3, . . . , so they too can be counted.
We aim to prove that the irrational numbers and the real numbers are both
uncountable sets. For now, we can express the relationship between all types of infinite
and countable sets with the following Venn diagram:
Uncountable
Finite
∅
{1, . . ., n}
Denumerable
ℜ
ℵ
Countable
Infinite
Theorem 5.6. The interval (0, 1) is uncountable.
Proof. Because (0, 1) contains all fractions of the form 1 / n for n ≥ 2, the interval is at
least infinite. Suppose though that (0, 1) is denumerable (i.e., countably infinite). We
shall obtain a contradiction.
Dr. Neal, WKU
If (0, 1) is denumerable, then there exists a bijection f :ℵ→ (0, 1) . Thus, for every
natural number n , 0 < f (n) < 1. So we consider the list f (1) , f (2) , f (3) , . . . Each such
value in (0, 1) has a unique decimal expansion of the form 0.d1 d2 d3 . . . =
d1 d2
d
+
+ 3 + . . . , where di ∈ {0, 1, 2, . . ., 9} and there is no ending string of all 9.
10 100 1000
(For example, we write 1/2 as 0.5000000 . . . rather than 0.4999999 . . . ) So the list of
values in (0, 1) becomes
€
f (1) = 0.d11 d12 d13 d14 d15 . . . (e.g., 0.48732 . . . )
f (2) = 0.d21 d22 d23 d24 d25 . . . (e.g., 0.11111 . . . )
f (3) = 0.d31 d32 d33 d34 d35 .. . (e.g., 0.50000 . . . )
f (4) = 0.d41 d42 d43 d 44 d45 . . . (e.g., 0.678678678 . . . )
etc.
Next, consider the terms d11 in f (1) , d22 in f (2) , d33 in f (3) , d44 in f (4) , etc. We
then create a new element x = 0.e1 e2 e3 . .. in (0, 1) by letting ei = 5 if di i = 6 and letting
ei = 6 if di i ≠ 6. (The choice of 5 and 6 is arbitrary.) For the example values above, the
first four digits in x are 0.6665 . . . Because the i th decimal digit in x is different than
the i th decimal digit in f (i) , x cannot equal f (i) for any i . So x is not in the range of
f . Thus, f is not onto, which is a contradiction to it being a bijection. Thus, (0, 1)
cannot be denumerable.
QED
Corollary 5.5. The real numbers are uncountable.
We can see this result in a couple of ways: (i) The interval (0, 1) is a proper subset
of ℜ ; hence, ℜ must have at least as large a cardinality as (0, 1). (ii) We have seen
previously that the interval (0, 1) is set equivalent to the real line. So if the real line were
denumerable (equivalent to the naturals), then (0, 1) would be denumerable by
transitivity.
Corollary 5.6. The irrational numbers I are uncountable.
Proof. We know that the Rationals Q are denumerable and the Reals ℜ are
uncountable. If I were denumerable, then the disjoint union ℜ = Q ∪ I would be
denumerable by Lemma 5.1, which is a contradiction.
QED
Definition 5.6. We denote the cardinality of the real numbers by ℜ = c . If set A is
equivalent to the real line, then A = c also.
Corollary 5.7. Any open interval ( a , b ) on the real line has cardinality c .
Dr. Neal, WKU
Continuum Hypothesis
So far, we have three types of cardinalities: finite, denumerable, uncountable. These
cardinalities create different numbers: 0, 1, 2, 3, . . ., n , . . . , ℵ0 , c .
The symbol ℵ0 represents the countably infinite cardinality of the natural numbers
and is also called first infinite ordinal. The symbol c represents the uncountably infinite
cardinality of the real numbers. So we have two different types of infinity here, and we
designate 0 < n < ℵ0 < c .
The continuum hypothesis states that there are no other cardinalities between ℵ0 and
c . Georg Cantor (1845–1918) believed but could not prove this result. In 1940, Kurt
Godel proved that the continuum hypothesis is consistent with the axioms of set theory.
In other words, accepting the continuum hypothesis as true causes no contradictions
and set theory cannot disprove the hypothesis. In 1963, Paul Cohen proved that the
continuum hypothesis is independent of the axioms of set theory. In other words, set
theory also cannot prove the hypothesis.
Therefore, the continuum hypothesis can be accepted as true or it can be accepted as
false without causing any other contradictions.
Convention. As of now, we have three types of cardinalities: finite, denumerable, and
uncountable ( n , ℵ0 , and c ). For sets of other possible types of cardinalities, we say that
the sets have the same cardinality if and only if they are equivalent (which is already
the case for cardinalities n , ℵ0 , and c ).
The Power Set
Definition 5.7. Let A be any set. The power set of A , denoted P( A ), is the collection of
all subsets of A .
n
If A is finite with n elements, then there are 2 subsets of A . For example if A =
∅ , then P( A ) = { ∅ }, which is the set containing the null set. So for A = 0, then P(A)
0
= 2 = 1. Also, if A = { a , b , c }, then the power set of A is
P( A ) = { ∅ , { a }, { b }, { c }, { a , b }, { b , c }, { a , c }, { a , b , c }}
3
So if A = 3, then P(A) = 2 = 8.
Cantor showed how to use the power set of infinite sets to create more types of
infinities beyond ℵ0 and c .
Cantor’s Theorem. For every set A , the cardinality of A is strictly less than the
cardinality of its power set: A < P(A) .
Dr. Neal, WKU
Proof. First, if A = ∅ , then A = 0 and P(A) = {∅} = 1. So the theorem holds. Next,
assume A is finite with A = n > 0. To create a subset of A , we have two choices for
n
every element in A : “in the subset” or “not in the subset.” Thus, there are 2
n
n
possibilities, which makes P(A) = 2 . Because n < 2 for all n ≥ 0 (which can be
proved by induction), we have A < P(A) for a finite set A .
Now suppose that A is infinite. Let f : A → P( A ) be defined by f (x) = { x }. If
f (x) = f (y) , then { x } = { y } which implies that x = y . Thus, f is one-to-one. Hence, A
is equivalent to the subset of P( A ) consisting of all singleton sets { x }. Because there are
more elements in P( A ), we have A ≤ P(A) . We now must show that equality cannot
hold.
So suppose that A = P(A) . Then because of equal cardinalities, there would be a
bijection g : A → P( A ). We shall show that this assumption leads to a contradiction.
Let B be the subset of A defined by
B = { x ∈ A | x ∉ g(x) }.
The set B can be understood as follows: For every element x in A , the function value
g(x) is a subset of A . So g(x) contains some of the elements of A . Either g(x) contains
x or it does not contain x . €
If g(x) does not contain x , then we set aside x to belong to
subset B .
Because B is a subset of A , it belongs to P( A ). Then because g is onto, there must
exist some x ∈ A such that g(x) = B .
Question: Is x ∈ B ?
If €x ∈ B , then by definition of B , we have that x ∉ g(x) = B , a contradiction.
If x ∉ B = g(x) , then we set aside x to€belong to B , another contradiction.
€
Therefore the onto function g cannot exist. So A ≤ P(A) but A ≠ P(A) . We
conclude that A < P(A) for infinite sets A .
How Many Infinities Are There?
We know that the natural numbers ℵ are infinite. So by Cantor’s Theorem, the power
set of ℵ has a larger cardinality. Then the power set of the power set has a still larger
cardinality. Then the power set of the power set of the power set has an even larger
cardinality.
Denoting the cardinality of the natural numbers by ℵ0 , we obtain
ℵ0 = ℵ < P(ℵ) < P( P(ℵ) ) < P( P (P(ℵ) ) ) < . . .
Dr. Neal, WKU
As written in the above inequality, these increasing cardinalities are in one-to-one
correspondence with the denumerable set {0, 1, 2, 3, . . . }. Thus we have a countably
infinite number of infinities!
However the real numbers ℜ are also infinite with a larger cardinality than ℵ.
Denoting the cardinality of the real numbers by c , we obtain
c = ℜ < P(ℜ ) < P( P(ℜ) ) < P( P (P(ℜ) ) ) < . . .
So it appears that we have another countably infinite set of infinite cardinalities.
However, it has been proven that P(ℵ) = c . Hence, we really have just one countable
string of infinities listed:
ℵ0 = ℵ < P(ℵ) = c = ℜ < P(ℜ ) < P( P(ℜ) ) < P( P (P(ℜ) ) ) < . . .
To prove that P(ℵ) = c , we first state a result (without proof) that was first
postulated by Cantor. He was unable to prove it, but it was proven independently by
Ernst Schroeder in 1896 and Felix Bernstein in 1898:
Cantor-Schroeder-Bernstein Theorem. Let A and B be sets. If A ≤ B and B ≤ A ,
then A = B .
This proof of this result is trivial for finite sets; however it is non-trivial for infinite
sets. We demonstrate its usefulness though with our last result.
Theorem 5.9. P(ℵ) = c .
Proof. We first note that every number x ∈ (0, 1] can be in a unique infinite binary
expansion that does not end in an infinite string of zeros. For example, 1/2 can be
−1
written as 0.10000 . or as 0.01111. In the first case, 0.1 = 1× 2 = 1/2. In the second
∞
(1 / 2)2 €
case, 0.01111 = ∑ 2−k =
= 1/2. So we write 1/2 as 0.01111 in binary.
1−
1
/
2
k =2
Now define f : (0, 1] → P(ℵ) as follows: Let x =
∞
∑ ak 2 −k
, where ak = 0 or 1 for
k =1
all k where the binary expansion does not end in an infinite string of zeros. Let f (x) be
the set of indices k in the expansion of x for which ak are non-zero. Then f (x) is a
subset of ℵ (i.e., an element of P(ℵ) ).
By uniqueness of the binary expansion, f is well-defined and one-to-one. For if
f (x1) = f (x2 ) , then x1 = ∑ 2 −k =
∑ 2 −k = x2 . By Theorem 5.6, since f is a onek ∈f (x1 )
k ∈f (x 2 )
to-one function (but not necessarily onto), then c = (0, 1] ≤ P(ℵ) .
Next let Ω be the set of denumerable subsets of ℵ. We now define g : P(ℵ) → ℜ by
Dr. Neal, WKU

2−k
 ∑
k ∈ A
g(A) = 
2 + ∑ 2−k
 k∈ A

if A ∈ Ω
.
if A ∉ Ω
Then g( A) gives a real number such that 0 < g( A) ≤ 1 or 2 < g( A) ≤ 3. If 0 < g( A) = g(B)
≤ 1, then ∑ 2 −k = ∑ 2 −k where A and B are infinite sets. Thus the sums cannot end
€
k ∈A
k ∈B
in an infinite string of zeros since the index sets are infinite. Thus by uniqueness of
binary expansion, the index sets A and B must be equal.
If 2 < g( A) = g(B) ≤ 3, then by subtracting 2,
∑ 2 −k
=
k ∈A
∑ 2 −k where A and B are
k ∈B
finite sets. The sums here do end in an infinite string of zeros since the index sets are
finite. Thus g( A) and g(B) must be rational numbers, and again by uniqueness of
binary expansion, the index sets A and B must be equal.
Thus g is one-to-one (but not necessarily onto); hence, P(ℵ) ≤ ℜ = c . Since c ≤
QED
P(ℵ) and P(ℵ) ≤ c , we may conclude that P(ℵ) = c .
Addendum
The Schroeder-Bernstein Theorem states that if A1 ⊆ A2 ⊆ A3 and A1 is equivalent to A3 ,
then A2 is equivalent to A3 . We can apply the result to show that the open interval
We simply note that
(0, 1) is equivalent to the half-open interval [0,1) .
[1 / 2,1) ⊆ (0, 1) ⊆ [0, 1) and [1 / 2,1) ~ [0,1) because [a,b) ~ [c, d) by a direct linear
d−c
function f :[a, b) → [c, d) defined by f ( x) =
( x − a) + c .
b−a
Thus, (0, 1) ~ [0, 1) , and in general (a, b) ~ [a,b) , although it is difficult to define a
direct one-to-one correspondence between the sets.
Exercises
1. For each problem, prove the result by defining a bijection f from ℵ into the set and
verifying that f is a bijection. Also give the function formula for f −1 .
–
(a) Prove that the negative odd numbers O = {–1, –3, –5, –7, . . . } are denumerable.
(b) Prove that the even numbers E = { . . ., –6, –4, –2, 0, 2, 4, 6, . . . } are denumerable.
2. Prove that the intervals [0, 1) and [0, 1] are uncountable.