The Hardy-Ramanujan Number is 1729. G. H. Hardy comments about this number:
"Once, in the taxi from London, Hardy noticed its number, 1729. He must have thought about it a little because he entered the room where Ramanujan lay in bed and, with scarcely a hello, blurted out his disappointment with it. It was, he declared, 'rather a dull number,' adding that he hoped that wasn't a bad omen. 'No, Hardy,' said Ramanujan, 'it is a very interesting number. It is the smallest number expressible as the sum of two cubes in two different ways.' "
This is because 1729 = 103 + 93 = 123 + 13.
Ta(n), the nth taxicab number, is the smallest number expressible as the sum of two cubes in n different ways.