# 9.6 CardinalNumber¶

The CardinalNumber domain can be used for values indicating the cardinality of sets, both finite and infinite. For example, the dimensiondimensionVectorSpace operation in the category VectorSpace returns a cardinal number.

The non-negative integers have a natural construction as cardinals

0 = #{ }, 1 = {0}, 2 = {0, 1}, ..., n = {i | 0 <= i < n}.


The fact that 0 acts as a zero for the multiplication of cardinals is equivalent to the axiom of choice.

Cardinal numbers can be created by conversion from non-negative integers.

c0 := 0 :: CardinalNumber


 0

Type: CardinalNumber

c1 := 1 :: CardinalNumber


 1

Type: CardinalNumber

c2 := 2 :: CardinalNumber


 2

Type: CardinalNumber

c3 := 3 :: CardinalNumber


 3

Type: CardinalNumber

They can also be obtained as the named cardinal Aleph(n).

A0 := Aleph 0


 Aleph(0)

Type: CardinalNumber

A1 := Aleph 1


 Aleph(1)

Type: CardinalNumber

The finite?finite?CardinalNumber operation tests whether a value is a finite cardinal, that is, a non-negative integer.

finite? c2


 true

Type: Boolean

finite? A0


 false

Type: Boolean

Similarly, the countable?countable?CardinalNumber operation determines whether a value is a countable cardinal, that is, finite or Aleph(0).

countable? c2


 true

Type: Boolean

countable? A0


 true

Type: Boolean

countable? A1


 false

Type: Boolean

Arithmetic operations are defined on cardinal numbers as follows: If x = #X and y = #Y then

x+y= #(X+Y)cardinalityofthedisjointunionx-y= #(X-Y)cardinalityoftherelativecomplementx*y= #(X*Y)cardinalityoftheCartesianproductx^y= #(X^Y)cardinalityofthesetofmapsfromYtoX

Here are some arithmetic examples.

[c2 + c2, c2 + A1]


 [4,Aleph(1)]

Type: List CardinalNumber

[c0*c2, c1*c2, c2*c2, c0*A1, c1*A1, c2*A1, A0*A1]


 [0,2,4,0,Aleph(1),Aleph(1),Aleph(1)]

Type: List CardinalNumber

[c2^c0, c2^c1, c2^c2, A1^c0, A1^c1, A1^c2]


 [1,2,4,1,Aleph(1),Aleph(1)]

Type: List CardinalNumber

Subtraction is a partial operation: it is not defined when subtracting a larger cardinal from a smaller one, nor when subtracting two equal infinite cardinals.

[c2-c1, c2-c2, c2-c3, A1-c2, A1-A0, A1-A1]


 [1,0,”failed”,Aleph(1),Aleph(1),”failed”]

Type: List Union(CardinalNumber,”failed”)

The generalized continuum hypothesis asserts that

2^Aleph i = Aleph(i+1)


and is independent of the axioms of set theory. Goedel, The consistency of the continuum hypothesis, Ann. Math. Studies, Princeton Univ. Press, 1940.

The CardinalNumber domain provides an operation to assert whether the hypothesis is to be assumed.

generalizedContinuumHypothesisAssumed true


When the generalized continuum hypothesis is assumed, exponentiation to a transfinite power is allowed.

[c0^A0, c1^A0, c2^A0, A0^A0, A0^A1, A1^A0, A1^A1]


 [0,1,Aleph(1),Aleph(1),Aleph(2),Aleph(1),Aleph(2)]

Type: List CardinalNumber

Three commonly encountered cardinal numbers are

a= #Zcountableinfinityc= #Rthecontinuumf= #{g|g:[0,1]->R}

In this domain, these values are obtained under the generalized continuum hypothesis in this way.

a := Aleph 0


 Aleph(0)

Type: CardinalNumber

c := 2^a


 Aleph(1)

Type: CardinalNumber

f := 2^c


 Aleph(2)

Type: CardinalNumber