I have a very simple question in geometry of numbers. (It is a slight modification of Counting points on the intersection of a box and a lattice .) There's a bound I can easily prove, and it's good enough for my purposes, but it may be embarrassingly suboptimal.

Let $S = (N_1,2 N_1] \times \dotsb \times (N_n,2 N_n]$, where $N_i\geq M\geq 1$. Define the lattice $L$ as the preimage of $r_1 \mathbb{Z} \times \dotsb r_n \mathbb{Z}$ under an affine linear map $\vec{v} \mapsto A \vec{v} + \vec{b}$, where $r_i\geq M$ are integers and $A=\{a_{i,j}\}$ is a non-singular $n$-by-$n$ matrix such that $a_{i,j}\in \mathbb{Z}$, $|a_{i,j}|\leq C$. How do you bound the number of points $|S\cap L|$ in $S\cap L$?

It is simple to show (chopping $A S$ into hypercubes of side $M$) that $$|S\cap L| \leq (4 C n)^n \prod_{i=1}^n \frac{N_i}{M}.$$ How much better can one do? Can one replace $(4 n)^n$ by $2^n n!$, say? Or (much more ambitiously) $\prod_{i=1}^n N_i/M$ by $\prod_{i=1}^n N_i/r_i$?

(It would be interesting, for starters, to combine the argument above with Davenport's Lemma (as in Counting number of points on a lattice in a hypercube), but doing so in such a way as to obtain a real improvement doesn't seem obvious.)


It does seem to me that one can do better in some circumstances by using Davenport's Lemma after all. We recall (see Davenport's On a principle of Lipschitz): for $B$ convex, $$ \bigl(|B\cap\mathbb{Z}^n|-\mathrm{vol}(B)\bigr) \leq \sum_{m=0}^{n-1} V_m,$$ where $V_m$ is the sum of the $\mathrm{vol}(\pi(B))$ under all projections $\pi$ obtained by setting $n-m$ coordinates to $0$.

Let $B = R (A S + \vec{b})$, where $R(x_1,\dotsc,x_n) = (x_1/r_1,\dotsc,x_n/r_n)$. Then $|S\cap L| = |B\cap \mathbb{Z}^n|$, and $$\mathrm{vol}(B) = \frac{\det(A)\cdot \mathrm{vol}(S)}{\prod_{i=1}^n r_i}\leq C^n n! \prod_{i=1}^n \frac{N_i}{r_i}.$$

Now, the ($m$-dimensional) volume of $\pi(B)$ for $\pi$ a projection obtained by setting $n-m$ coordinates to $0$ is at most $2^{m-n}$ times the sum of the $\mathrm{vol}(\pi(R P))$ over all $m$-dimensional sides $P$ of the parallelepiped $AS$. It is clear that $\mathrm{vol}(\pi(R P)) =\mathrm{vol}(\pi(P))/\prod_{i\in I} r_i$, where $I$ is the set of coordinates that $\pi$ does not set equal to $0$. We know that $\mathrm{vol}(\pi(P)) \leq \mathrm{vol}(P)$ because, in general, (orthogonal) projections do not increase volumes. We also know that $\mathrm{vol}(P) \leq \prod_{i\in I'} C n N_i$, where $I'$ is the set of indices that vary as we traverse the side $P$. (Here $C n N_i$ is an upper bound on the length of the image under $A$ of the side $(N_i,2 N_i]$.) Hence $$\textrm{vol}(\pi(R P)) \leq (C n)^m \frac{\prod_{i\in I'} N_i}{\prod_{i\in I} r_i}.$$ We are summing over $\binom{n}{m}$ projections and $\binom{n}{m}$ sides. Thus, the contribution of a given $m$ to the right side of Davenport's Lemma is at most $$2^{-n} (2 C n)^m \binom{n}{m}^2 \cdot \frac{\prod_{i=1}^m N_i}{\prod_{i\in I} r_i},$$ where we assume that $N_1,N_2,\dotsc$ are sorted in decreasing order, and $I$ is the set of $m$ indices corresponding to the $m$ smallest values of $r_i$. We sum over all $m\leq n$, and, obtain a total of $$\begin{aligned} 2^{-n} \sum_{m=0}^{n-1} (2 C n)^m \binom{n}{m}^2 \cdot \max_{|I|=|I'|=m} \frac{\prod_{i\in I'} N_i}{\prod_{i\in I} r_i}&\leq \frac{(C n)^{n-1}}{2} \binom{2n}{n} \cdot \max_{|I|=|I'|<n} \frac{\prod_{i\in I'} N_i}{\prod_{i\in I} r_i}\\ \lesssim \frac{(2 C n)^{n-1}}{\sqrt{2 \pi}} \cdot \max_{|I|=|I'|<n} \frac{\prod_{i\in I'} N_i}{\prod_{i\in I} r_i}. \end{aligned} $$

We conclude that $$|S\cap L| \leq C^n n! \prod_{i=1}^n \frac{N_i}{r_i} + O\left( (2 C n)^{n-1} \max_{|I|=|I'|<n} \frac{\prod_{i\in I'} N_i}{\prod_{i\in I} r_i}\right).$$ That doesn't improve on the original bound in full generality, however (except in so far as it replaces $(4 C n)^n$ by $C^n n! + O((2 C n)^{n-1})$.


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