MYSTERY OF RAMANUJAN NUMBER

Ramanujan explained that 1729 is the only number that is the sum of cubes of two different pairs of numbers: 123 + 13, and 103 + 93.

It was not a sudden calculation for Ramanujan. According to his biography, "Years before, he had observed this little arithmetic morsel, recorded it in his notebook and, with that easy intimacy with numbers that was his trademark, remembered it."

The unique number later came to be known as the Hardy-Ramanujan number.