# proof that 26 is the only number between a cubed and a squared number

• April 6th 2008, 09:52 AM
teh_ninjaness
proof that 26 is the only number between a cubed and a squared number
I was reading Singh's Fermats Last Theorem, and it mentioned that fermat was the first person to prove that 26 is the only number between a cubed and a squared number. I have been unable to prove this, so i thought I'd ask, apologies if its been asked before.... cheers
• April 6th 2008, 09:58 AM
ThePerfectHacker
It comes done to proving that $x^2 + 2 = y^3$ has only $x=5$ and $y=3$ as its solutions. By far the best way to prove this is over the unique factorization domain $\mathbb{Z}[\sqrt{-2}] = \{ a+bi\sqrt{2}|a,b\in\mathbb{Z} \}$. Because you can factor the left hand side as $(x+i\sqrt{2})(x-i\sqrt{2}) = y^3$.

Of course Fermat did not in any way prove this using the idea above. Unique factorization was only developed about 100 years ago by Kummer/Kronecker. I have no idea how Fermat, or Euler who later solved it also, did it by using elementary results.
• April 6th 2008, 10:08 AM
galactus
Here's a link on the matter. Interesting little algebraic number theory problem.

Math Forum - Ask Dr. Math
• April 7th 2008, 09:20 AM
teh_ninjaness
that maths is beyond me, i think i was decieved by the seeminly simple nature of the problem. But i will look into some of the areas I don't understand, thanks