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If $r_2(n)$ denotes the number of integer solutions to $a^2+b^2=n$, we have the "divisor bound" $r_2(n) = O(n^{\epsilon})$ for any $\epsilon>0$. Another way to state this is that the number of lattice points inside an annulus of inner radius $R$ and area $1$ centered at the origin is $O(R^{\epsilon})$.

Is this bound still true if the annulus is not centered at the origin?

This seems harder to prove since when we are off the origin there isn't "much number theory" to help.

I am aware of averaged bounds for points inside an annuli off the origin, but I have not yet seen anything "pointwise". I would appreciate any known approaches or results on how to deal with that.

Thanks in advance

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  • $\begingroup$ Changed answer to comment: With rational center, $r_2(n)=O(n^{c/\lg\lg n})$ for a positive constant $c$. See the comments at the other question posted [here][1] (and Iwaniec's [book][2] which is referred to for the proof). The other case of irrational center (as Lucia pointed out earlier in a comment) has an elementary upper bound for $r_2(n)$ of two points (see [Putnam exam 2008][3]). [1]: mathoverflow.net/questions/218888/… [2]: ams.org/books/gsm/017/gsm017.pdf [3]: kskedlaya.org/putnam-archive/2008s.pdf $\endgroup$ Dec 21, 2018 at 21:31
  • $\begingroup$ This is a beautiful question $\endgroup$ Dec 21, 2018 at 22:42

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