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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