Journal of Integer Sequences, Vol. 25 (2022), Article 22.4.4

A Closed Form for Representing Integers as Sums and Differences of Cubes


Michael Nyblom
School of Science
RMIT University
Melbourne, Victoria 3001
Australia

Abstract:

By applying an existing characterization for a positive integer to be represented as a sum of two cubes of positive integers, we construct an elementary proof of Ramanujan's famous result—namely, that the number 1729 is the smallest positive integer represented as a sum of two cubes in two different ways. Similarly, by applying an existing characterization for a positive integer to be represented as a difference of cubes of two positive integers, we also apply this characterization to construct the generating function for a sequence of integer ordered pairs (an, bn) ≠ (a'n, b'n) satisfying an3 + bn3 = a'3n + b'3n, which are distinct from Ramanujan's "near integer" solutions to Fermat's equation—namely, those satisfying an3 + bn3 = an3 + (–1)n.


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(Concerned with sequences A001235 A004999 A011541 A343708.)


Received December 24 2021; revised versions received January 2 2022; April 8 2022. Published in Journal of Integer Sequences, April 10 2022.


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