Ramanujam’s Number

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It is the smallest number which can be written as the sum of two cubes in two different ways. It is a very famous number, so if you dare to proclaim maths as your hobby, you must know it.
Number is 1729=10^3+9^3=1^3+12^3.

Ramanujam Number is 1729.

But allowing negative perfect cubes (the cube of a negative integer) gives the smallest solution as 91 (which is a factor of 1729):

91 = 6^3 + (−5)^3 = 4^3 + 3^3

There is one interesting story on this. Do google search if you have enough time.