Hi! I’m Yen. I remember Georg Cantor once said that “The essence of mathematics lies in its freedom”

Cantor’s diagonal argument: The best known example of an uncountable set is the set R of all real numbers, Cantor’s diagonal argument shows that this set is uncountable.

In his 1891 article, Cantor considered the set T of all infinite sequences of binary digits (i.e. consisting only of 0es and 1s). First, he constructively shows the following theorem: If s1, s2, … , sn, … is any enumeration of elements from T, then there is always an element s of T which corresponds to no sn in the enumeration. To prove this, given an enumeration of arbitrary members from T, like e.g.

- s
_{1}=(0,0,0,0,0,0,0,…) - s
_{2}=(1,1,1,1,1,1,1,…) - s
_{3}=(0,1,0,1,0,1,0,…) - s
_{4}=(1,0,1,0,1,0,1,…) - s
_{5}=(1,1,0,1,0,1,1,…) - s
_{6}=(0,0,1,1,0,1,1,…) - s
_{7}=(1,0,0,0,1,0,0,…) - …

He constructs the sequence s by choosing its nth digit as complementary to the nth digit of sn, for every n. In the example, this yields:

- s
_{1}=(0,0,0,0,0,0,0,…) - s
_{2}=(1,1,1,1,1,1,1,…) - s
_{3}=(0,1,0,1,0,1,0,…) - s
_{4}=(1,0,1,0,1,0,1,…) - s
_{5}=(1,1,0,1,0,1,1,…) - s
_{6}=(0,0,1,1,0,1,1,…) - s
_{7}=(1,0,0,0,1,0,0,…) - …
- s =(1,0,1,1,1,0,1,…)

By construction, s differs from each sn, since their nth digits differ (highlighted in the example). Hence, s cannot occur in the enumeration. Based on this theorem, Cantor then uses an indirect argument to show that: The set T is uncountable. He assumes for contradiction that T was countable. Then (all) its elements could be written as an enumeration s1, s2, … , sn, … .

Applying the above theorem to this enumeration would produce a sequence s not belonging to the enumeration. This contradicts the assumption, so T must be uncountable.