What is special about the number 1729?

The number 1729 is known as the Hardy–Ramanujan number after a famous visit by Hardy to see Ramanujan at a hospital. In Hardy's words:[117]

I remember once going to see him when he was ill at Putney. I had ridden in taxi cab number 1729 and remarked that the number seemed to me rather a dull one, and that I hoped it was not an unfavorable omen. "No", he replied, "it is a very interesting number; it is the smallest number expressible as the sum of two cubes in two different ways."

Immediately before this anecdote, Hardy quoted Littlewood as saying, "Every positive integer was one of [Ramanujan's] personal friends."[118]

The two different ways are:

1729
=
1
3
+
12
3
=
9
3
+
10
3
.
{\displaystyle 1729=1^{3}+12^{3}=9^{3}+10^{3}.}
Generalisations of this idea have created the notion of "taxicab numbers".

1729, known as the Hardy–Ramanujan number, is the smallest integer that can be expressed as the sum of the cubes of two unique pairs of integers.