Question

which number is known as Ramanujan number

Answer 1

1729 is known as Ramanujan number.

Answer 2

1729 because of this statement
Ramanujan replied that 1729 was not a boring number at all: it was a very interesting one. He explained that it was the smallest number that could be expressed by the sum of two cubes in two different ways. This story is very famous among mathematicians. 1729 is sometimes called the “Hardy-Ramanujan number”.