Primitive Representations of Integers by $x^3+y^3+2z^3$

A well-known open problem is to show that the cubic form $x^3+y^3+2z^3$ represents all integers. An obvious variant of this problem is whether every integer can be {\em primitively} represented by $x^3+y^3+2z^3$. In other words, given an integer $n$, are there coprime integers $x$, $y$, $z$ such that $x^3+y^3+2z^3=n$? In this note we answer this variant question negatively. Indeed, we use cubic reciprocity to show that for every integral solution to $x^3+y^3+2z^3=8^m$,the unknowns $x$, $y$, $z$ are divisible by $2^m$.

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