Ramanujan Number is the smallest number that can be expressed as the sum of cubes of two different positive integers, in two different ways.
1729 = 7 * 13 * 19 = 1³ + 12³ = 9³ + 10³ = 19 * 91
There are many other properties attached to the Ramanujan number.
Actually, when Ramanujan was in a hospital, Hardy visited him and mentioned the number of th e cab he arrived by. That was 1729. Then Ramanujan gave its property on the spot.