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Consider the function $x^m +/- y^n +/- z^p$, where $x, y, z, m, n, p$ are integers such that $m, n, p >= 2$. The question is, are all numbers expressable using this function? Are there any exceptions? (I know about numbers conjectured to be not expressable as $x^m - y^n$, so, I am asking an extension question). Is this formulation something new, or, is it known before?

edited Dec 22 at 4:17

asked Dec 22 at 3:14

Salahuddin559
21●2

	Every integer is of the form $x^2-y^2$ or $x^2-y^2+1$, and thus of the form $x^2-y^2+z^p$ for any $p$ (where we can take $z=0$ or $z=1$), so you need to rule out this situation. – Richard Stanley Dec 22 at 4:01
	You are right, we will exclude that solution. – Salahuddin559 Dec 22 at 4:19
	Particularly, $z$ can be zero, but none of $x, y$ and $z$ can be $1$. – Salahuddin559 Dec 22 at 5:05

joro · Answer 1 · 2012-12-22 08:16:37Z UTC

Your equation has infinitely many efficiently computable solutions $(m,n,p)=(2,2,p)$ and maybe infinitely many $(m,n,p)=(3,3,2)$ for all integers $a$ s.t. $x^m - y^n \pm z^p =a$.

Every odd integer $b$ is the difference of two squares $(1+b)/2,(b-1)/2$ so take any $p$, any $z$ the opposite parity of $a$ and express $a \mp z^p$ as difference of two squares.

For the $(3,3,2)$ case, pick random $c$ and set $x=u+c,y=u$ and pick the minus sign. Your equation is $x^3-y^3-z^2=3u^2c + 3uc^2 + c^3 -z^2=a$ leading to $ 3 u^{2} c + 3 u c^{2} + c^{3} - z^{2} - a=0$. The last equation is genus $0$ curve which might have infinitely many integral points unless I am missing some obstruction and you can try another $c$. For $c=3 c'^2$ the curve has explicit rational parametrization.

Any non-conforming numbers?

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