Real numbers are uncountable

One of the slickest proofs of all time is using Georg Cantor‘s 1891 diagonal argument in proving that the real numbers \mathbb{R}  constitute an uncountable set, that is, they cannot be put in a bijective correspondence with \mathbb{N}. Strictly speaking, Cantor original paper only proved that the set of all 0,1-sequences is uncountable. However since it is easy to argue that the set of all such sequences and \mathbb{R} have the same cardinality, indirectly we get a proof for the uncountable nature of the reals. Moreover, the argument given by Cantor in his proof can be easily modified to give a direct proof for the uncountability of the reals. This is what we do presently.

Theorem: \mathbb{R} is uncountable.

Proof: We will accept that every real number x has a decimal expansion, x=N.x_1x_2\cdots and it is uniquely determined by x if one agrees never to terminate the expansion with an infinite string of 9‘s. The proof is based on contradiction, and so we suppose that \mathbb{R} is not uncountable. Then it must be that \mathbb{R} is countable and we can make a list of all the elements of \mathbb{R} along with their decimal expansion as follows:


We now consider the real number 0.y_1y_2y_3\cdots which is defined by y_i=1 if x_{ii}\ne 1 and y_i=2 otherwise. It is clear that this number differs from every x_i in the list in at least the i‘th position. Hence this list is incomplete, a contradiction.\Box


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Filed under Real Analysis, Set theory

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