Cantor's proof is as brilliant as it is simple.# Proof: What does countability mean?

(related to Proposition: Real Numbers are Uncountable)

We want to show that the set $\mathbb R$ of real numbers is uncountable. We will show that this is true even for the real interval $[0,1]\subset\mathbb R.$ * Suppose, $[0,1]$ is countable. * Therefore, by definition of countability, there is an injective function $f:[0,1]\to\mathbb N$. * Suppose that we can list the values of this function in some hypothetical way: \[ \begin{array}{rcl} (\underline{0}.0379357232\ldots)&\rightarrow&1\\ (0.\underline{7}112223358\ldots)&\rightarrow&2\\ (0.9\underline{4}80483777\ldots)&\rightarrow&3\\ (0.00\underline{8}4973921\ldots)&\rightarrow&4\\ (0.009\underline{5}857262\ldots)&\rightarrow&5\\ (0.0399\underline{0}39551\ldots)&\rightarrow&6\\ (0.23964\underline{3}9345\ldots)&\rightarrow&7\\ \vdots \end{array} \] * Now, consider the real number $r$ built by the underlined digits in the diagonal.1 * Now, define a new real number $s$ by changing in $r$ every digit after the comma in an arbitrary way.2 * Note that we cannot find any value \(n\in\mathbb N\) such that $f(s)=n$. * Thus, although we have found a new real number, $f$ is not injective.
* This is a contradiction to the hypothesis. * Thus the number of real numbers $0\le r\le 1$ is uncountable.

Parts: 1

Thank you to the contributors under CC BY-SA 4.0!




  1. Hoffmann, Dirk W.: "Grenzen der Mathematik - Eine Reise durch die Kerngebiete der mathematischen Logik", Spektrum Akademischer Verlag, 2011


  1. In our example, \(r=3.748503\ldots\). 

  2. For instance, change after the point \(7\) to \(8\) , \(4\) to \(7\), \(8\) to \(1\), \(5\) to \(6\), \(0\) to \(9\), \(3\) to \(1\), etc. After this procedure, we will get a new real number, which is, in our case \(s=0.871691\ldots\).