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my intuition tells me that C,D,E and F all the cardinality of the continuum and A and B have cardinality aleph null. Is this correct? I wouldn't know how to show it though and that's what I'm interested in. Here is the question:

Consider the following sets:

A = set of all infinite arithmetic progressions in $\mathbb{Z}$,

B = set of all infinite arithmetic progressions in $\mathbb{Q}$,

C = set of all infinite arithmetic progressions in $\mathbb{R}$,

D = set of all infinite sequences in $\mathbb{R}$,

E = set of all subsets of $\mathbb{R}$,

F = set of all closed balls in $\mathbb{R^2}$.

For each pair of these sets determine which one has larger cardinality or if they have equal cardinality.

PS: I have tried looking through the set theory section but couldn't find a similar Q and A.

Andrés E. Caicedo
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TommySmith97
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  • Are you aware that there are cardinalities strictly greater than the cardinality of the continuum? Have you seen the result that $x<2^x$ even for infinite cardinal numbers? – JMoravitz Apr 26 '19 at 13:37

1 Answers1

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First, recall that for infinite sets you have $X$ is the same cardinality as $X^n$ for any positive integer $n$. Here I will be using $\simeq$ to denote "has the same cardinality as."

Arithmetic progressions can be uniquely determined by two pieces of information: the starting value and the common difference.

We see then:

  • $A\simeq \Bbb Z^2\simeq \Bbb N$
  • $B\simeq \Bbb Q^2\simeq \Bbb N$
  • $C\simeq \Bbb R^2\simeq \Bbb R$

Next, the set of infinite sequences in $\Bbb R$ can be a little tricky, but the answers to this question show

  • $D\simeq \{f~\mid~f~:\Bbb N\times \Bbb N \to \{0,1\}\}\simeq \Bbb R$

Now, the set of all subsets of $\Bbb R$ is the Power Set of $\Bbb R$ and by Cantor's theorem we know that the power set of any set is of strictly greater cardinality.

  • $E = \mathcal{P}(\Bbb R)$

Finally, the set of all closed balls in $\Bbb R^2$ can be described using three pieces of information. The $x$-coordinate of the center of the ball, the $y$-coordinate of the center of the ball, and the radius of the ball. We see then that

  • $F \simeq \Bbb R^3\simeq \Bbb R$

Or, if you prefer, these can be written as having cardinality $\aleph_0,\mathfrak{c}, 2^{\mathfrak{c}}$ respectively

JMoravitz
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  • Thank you great answer. This will help me a lot. – TommySmith97 Apr 26 '19 at 14:01
  • I have one question about D: in the answers linked what does the notation {} mean? It's been a while. – TommySmith97 Apr 26 '19 at 14:05
  • @TommySmith97 a technique called currying lets you redescribe a function which takes multiple arguments and gives an output instead as a function which takes one argument and returns a function which is then evaluated with the second argument giving the final output. It can be applied in reverse as well, a function which takes input and returns a function which later takes other input and returns a value can be thought of as a single concise function that takes both arguments as input. – JMoravitz Apr 26 '19 at 14:10
  • Ah thank you I think I understand. – TommySmith97 Apr 26 '19 at 14:11
  • @TommySmith97 as for what ${f_n}_n$ means, it is a sequence of functions. $(f_1,f_2,f_3,\dots)$. In this case, each function itself represents a real number and this is a sequence of real numbers, i.e. a sequence of functions. Each function is itself a function from $\Bbb N\to{0,1}$. – JMoravitz Apr 26 '19 at 14:12