# Ramanujan and sum of four cubes

This is more a question on History than proof itself. About a decade ago, a college professor and a Math coach told us about this beautiful theorem:

Every multiple of 6 can be written as a sum of four cubes

The proof of the theorem is elementary as well as elegant.

Consider $(n+1)^3 + (n-1)^3 = 2n^3 + 6n$

Thus,

$6n = (n+1)^3 + (n-1)^3 + (-n)^3 + (-n)^3$

Effectively proving the theorem and also giving the required four numbers. The professor also made a remark that a proof is due to Ramanujan. I recently found this scribbled in my notebook and have since not been able to find any reference to this on the web. As far as I know, Kennigel's biography, 'The Man Who Knew the Infinity' does not mention it. There are tons of references to Taxicab numbers, Sum of four square proofs etc..

Does anyone know of any reference to the theorem and the proof? Is it a part of a more general theory?

• You might want to look at mathworld.wolfram.com/CubicNumber.html it mentions this identity along with some more about writing whole numbers as the sum of 4 signed cubes. – Vincent Pfenninger Jun 27 '13 at 16:04
• Cool, I missed the Wolfram page. The page does provide a general set of theorems for representing numbers as sums of cubes. The identity in question does appear in on the page, so does the Taxicab number. But it does not mention the source of the identity. Maybe it did not originate from Ramanujan and my notes were erroneous. – Sudeep Jun 27 '13 at 16:32
• the book Hardy wright theory of numbers has the equation you stated.author is probably Mordell and Hammond. – user242371 May 21 '15 at 10:20
• It follows that all primes $\ge5$ are the sum of five cubes. – Lucian Sep 20 '16 at 12:24