# Real algebraic sets bounded away from integer points

A subset $S$ of $\mathbb{R}^n$ is "bounded away from integer points" if for some positive $\epsilon$ every point in $S$ lies at a distance of at least $\epsilon$ from $\mathbb{Z}^n$. For example the line $x+y=1/2$ in $\mathbb{R}^2$ is bounded away from integer points, but the curve $x^2+y=1/2$ is not, because the points $(n+\frac{1}{4n},-n^2-\frac{1}{16n^2})$ for $n=1,2,\ldots$ lie on this curve.

Question: Can anyone give an algorithm to determine whether a system of polynomial equations with real algebraic coefficients cuts out a subset $S$ of $\mathbb{R}^n$ that is bounded away from integer points? Is there a simple description of all such subsets $S$?

Remark: I have a vague notion that if $S$ is bounded away from integer points then this must be trivially'' verifiable, perhaps because $S$ projects on a linear affine subset of $\mathbb{R}^n$ that is obviously bounded away from integer points, but this is little more than guesswork. I don't actually know that the problem is decidable, but I would be surprised if it were not.

UPDATE: (I'll use this section to collect my latest thoughts on the problem.)

The situation is fairly transparent in $\mathbb{R}^2$, and the real problem is how things generalize to higher dimensions. Let $S$ be as above. Let $\lfloor\cdot\rfloor$ be the floor function, which will be applied to points coordinatewise. Then I propose the following conjecture:

There exists some translate of $S$ bounded away from integer points if and only if the set of all points $\lfloor p\rfloor$ for $p\in S$ is contained in a finite union of linear-affine subspaces of $\mathbb{R}^n$ (which will be defined over the rationals).

• There are examples other than hyperplanes: consider the hyperbola in $\mathbb{R}^2$ whose asymptotes are lines $x=1/2$ and $y=1/2$. – Boris Bukh Jan 28 '11 at 13:43
• Interesting question, but I'm not sure I share your optimism. Is there any reason why this problem should be considered over the reals (versus, say, bounded away from the Gaussian integers over the complexes)? – Thierry Zell Jan 28 '11 at 16:09
• @Thierry: I suspect that the problems is decidable only because I can't find any hard-to-verify examples of sets bounded away from integer points. For example in the plane we have certain lines with rational slope, and examples like Boris mentioned in his comment, such as $(x-1/2)(y-1/2)=1$, where the branches at infinity are asymptotic to certain lines with rational slope. I don't know any more subtle examples. Does the problem get harder in higher dimensions? I don't know. As for the Gaussians and the complexes, maybe this boils down to the same problem... I'll have to think about that. – Sidney Raffer Jan 28 '11 at 16:42
• A trivial but rather large set of examples that I think has gone unmentioned would be bounded sets, e.g., that determined by $x^2+y^2=3$. – Gerry Myerson Jan 29 '11 at 5:28
• @Gerry: Yes, and by quantifier elimination over real closed fields, we can recognize all such examples algorithmically. So only unbounded sets pose a problem – Sidney Raffer Jan 29 '11 at 6:08