In other words, $x(x+1)(2x+1)=6y^2$ has no nontrivial integral solutions. I thought this is a well-known result, but surprisingly could not find a recorded (easy) proof.
Can someone provide a proof here?
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It only takes a minute to sign up.Sign up to join this community
Counterexamples: ($x=1$, $\;$ $y=1$) ($x=24$, $\;$ $y=70$)
This is Lucas' problem of the square pyramid. A proof with elliptic curves was given by G. N. Watson in 1919. An "easier" proof was given in 1952 by W. Ljungren. I found this out in Mordell's book, he does not reproduce either proof.
There is a proof on the easier side by Anglin, 1990.
If you look up "Lucas square pyramid" you can get a link to a paper by Bennett from Acta Arithmetica, 2002.
There is no truly easy proof of this, there are just degrees of background required. Some of the proofs are called "elementary." My reaction is that elementary is in the eye of the beholder.